Wear Behaviour of Human Enamel Opposing Lithium Disilicate

Wear Behaviour of Human Enamel
Opposing Lithium Disilicate Glass Ceramic
and Type ІІІ Gold
Ahreum Lee
BDS (Otago)
A thesis submitted in partial fulfillment for the degree of
Doctor of Clinical Dentistry in Prosthodontics
Department of Oral Rehabilitation
University of Otago, Dunedin
New Zealand
2012
PUBLICATION AND PRESENTATION
Publication arising from this thesis:
Lee A., He L.H., Lyons K. and Swain M. (2011), Tooth wear and wear
investigation in dentistry, J Oral Rehabil, 39, 217-225.
Oral presentation:
Lee A., He L.H., Lyons K., Swain M. In vitro investigation of human
enamel against lithium disilicate glass ceramic and type ІІІ gold The
Academy of Australian and New Zealand Prosthodontists, Biennial
Meeting, Cairns, QLD, July 11th – 14th 2012
ii
ABSTRACT
Background
Wear of teeth and restorative materials is a complex and multifactorial
phenomenon with the interplay of biological, mechanical, and chemical factors.
Numerous in vitro and in vivo wear studies have been conducted that have
improved our understanding of the wear process, and that have assisted the
development of restorative materials with excellent wear resistance. Due to the
lack of standardised experimental methods, however, the wear behaviour of
human enamel opposing different restorative materials is still not clear. Currently,
concepts established in mechanical engineering (tribology) and physics have been
applied in dental research to better understand the underlying wear mechanisms
and the controlling factors involved in the wear process of dental biomaterials.
Aim
The aim of this research was to investigate the friction and wear behaviour of
human tooth enamel opposing two different indirect restorative materials, namely
heat pressable lithium disilicate glass ceramic (IPS e.max Press, Ivoclar Vivadent,
Schaan, Liechtenstein) and type ІІІ gold (Maxigold, Ivoclar Vivadent).
Methodology
Friction-wear tests on human enamel, opposing IPS e.max Press and type ІІІ gold,
were conducted in a ball-on-flat configuration using a reciprocating horizontal
wear testing apparatus. The wear pairs were subjected to two typical wear
conditions; two-body (with distilled water lubrication) and three-body (with
PMMA beads to simulate food slurry). A normal load of 9.8N with a
reciprocating displacement of ~200 m and frequency ~1.6 Hz were used for
~1,100 cycles per test. The frictional force of each cycle was recorded and the
iii
corresponding friction coefficient () for different wear pairs was calculated.
Following completion of the wear testing, the wear scars on the enamel specimens
were examined under light microscopy (Alphaphot YS2, Nikon, Tokyo, Japan)
and SEM (S360, Cambridge Instruments, Cambridge, UK).
Results
The results showed that type ІІІ gold had a lower  and caused less wear damage
on enamel than IPS e.max Press in both two- and three-body wear conditions. The
enamel surface opposing the IPS e.max Press specimen exhibited significant
cracks, plough furrows and surface loss, indicating abrasive wear as the
predominant wear mechanism. In comparison, the enamel surface opposing type
ІІІ gold showed little wear scar with small patches of gold smear adhering to the
surface, indicating the adhesive wear mechanism.
Conclusions
Within the limits of this in vitro research, the following conclusions were drawn:
1. Type ІІІ gold has a lower  and causes less wear damage on the enamel than
IPS e.max Press.
2. Abrasive wear was the main wear mechanism for the IPS e.max Press and
enamel wear pair, while adhesive wear occurred predominantly for the type ІІІ
gold and enamel wear pair.
Further in vitro research is required to characterise and predict the wear behaviour
of human enamel against a range of dental restorative materials.
iv
ACKNOWLEDGEMENTS
I would like to express my greatest gratitude to all my research supervisors Prof.
Michael Swain, Dr. Lihong (Chris) He and Assoc Prof. Karl Lyons for their
tremendous support and patient guidance in conducting and writing this thesis. I
feel sincerely fortunate to have them as my supervisors and mentors for my
Doctorate research. I would also like to thank Dr Basil Al-Amleh for his
stimulating ideas and challenging questions during the early stage of my research.
I am greatly indebted to a myriad of other people who shared their expertise and
experiences for my project: Suyoung Han, my dearest friend and PhD student in
Physiology, for the statistical analyses; Liz Girvan for her assistance with the
scanning electron microscope; Leo van Rens from Emtech Engineering for his
technical support with the wear testing machine; Steve Swindells for equipment
and materials during sample preparation; Bohwan Seo for the IT support; as well
as Sir John Walsh Research Institute, University of Otago for the research grant.
I must acknowledge all my friends and colleagues who have made my life in
Dunedin more prosperous and enjoyable. My special thanks goes to my ‘best
mates’, Suyoung Han and Erin Park, for standing by my side and putting up with
my ups and downs over the past three years. I am also grateful to many friends
and colleagues at the Dental School: Jennifer Lee, Yeen Lim, Sunyoung Ma, Reza
Shah, Leanne Hou, Ellie Knight, Latfiya Al-Harthi and many more, for their
friendship and encouragement.
Last but not the least, I wish to thank my family in Korea: my parents Jong-Gu
Lee and Keum-Hee Hwang, my sister Rira Lee and my brother-in-law Wol Kim
for their undivided support, love and prayer.
Thank you God for giving me the strength and courage to plod on this path
despite my constitution wanting to give up and throw in the towel.
v
DEDICATION
This thesis is lovingly dedicated to my parents and my soul mate, Jong-Min Baek
who have been my constant source of inspiration and motivation. Without their
unconditional love and support, this thesis would not have been made possible.
vi
TABLE OF CONTENTS
PUBLICATION AND PRESENTATION .................................................................................. II
ABSTRACT .............................................................................................................................. III
ACKNOWLEDGEMENTS ......................................................................................................... V
DEDICATION .......................................................................................................................... VI
TABLE OF CONTENTS .......................................................................................................... VII
LIST OF FIGURES .................................................................................................................... X
LIST OF TABLES .................................................................................................................... XII
LIST OF ACRONYMS & ABBREVIATIONS .......................................................................XIII
1
LITERATURE REVIEW .................................................................................................... 1
1.1
INTRODUCTION ........................................................................................................................................ 1
1.2
TOOTH WEAR ........................................................................................................................................... 5
1.2.1
Prevalence .................................................................................................................................. 5
1.2.2
Aetiology .................................................................................................................................... 7
1.2.3
Clinical management of tooth wear ........................................................................... 14
1.3
WEAR INVESTIGATION IN DENTISTRY ................................................................................................ 15
1.3.1
In vitro wear testing ........................................................................................................... 15
1.3.2
In vivo wear testing ............................................................................................................ 19
1.4
TRIBOLOGY ............................................................................................................................................ 22
1.4.1
Friction ...................................................................................................................................... 22
1.4.2
Wear mechanisms ............................................................................................................... 24
1.4.3
Sliding wear............................................................................................................................ 28
1.4.3.1
1.5
Sliding wear model ....................................................................................................... 29
TOOTH ENAMEL .................................................................................................................................... 32
1.5.1
Histological background .................................................................................................. 32
1.5.2
Mechanical properties of enamel ................................................................................ 35
vii
1.5.3
1.6
RESTORATIVE MATERIALS .................................................................................................................... 43
1.6.1
Amalgam alloys .............................................................................................................. 44
1.6.1.2
Gold based alloys .......................................................................................................... 45
1.6.2
Composite resin ................................................................................................................... 46
1.6.3
Dental ceramics .................................................................................................................... 48
1.7
3
Metal alloys ............................................................................................................................ 43
1.6.1.1
1.6.3.1
2
Wear behaviour of enamel ............................................................................................. 39
Wear behaviour of dental ceramics ...................................................................... 50
SIGNIFICANCE OF THE RESEARCH ....................................................................................................... 56
MATERIALS AND METHODS..................................................................................... 58
2.1
COLLECTION OF TEETH ........................................................................................................................ 58
2.2
ENAMEL SPECIMEN PREPARATION ..................................................................................................... 60
2.3
HARDNESS AND ELASTIC MODULUS MEASUREMENT ...................................................................... 62
2.4
RESTORATIVE SPECIMEN PREPARATION ............................................................................................ 63
2.4.1
IPS e.max Press specimen preparation ..................................................................... 64
2.4.2
Type ІІІ gold specimen preparation .......................................................................... 65
2.5
DESCRIPTION OF TOOTH WEAR MACHINE ........................................................................................ 66
2.6
METHODS OF ASSESSMENT ................................................................................................................ 68
2.6.1
Scanning electron microscopy (SEM) observations ............................................ 68
2.6.2
Statistical analysis ................................................................................................................ 68
RESULTS ........................................................................................................................ 69
3.1
MICROHARDNESS CHARACTERISATION ............................................................................................. 69
3.2
FRICTION BEHAVIOUR OF ENAMEL OPPOSING IPS E.MAX PRESS AND TYPE ІІІ GOLD IN
DIFFERENT WEAR CONDITIONS ...................................................................................................................... 72
3.3
4
SEM OBSERVATION OF WEAR SCARS ............................................................................................... 77
DISCUSSION ................................................................................................................. 87
4.1
WEAR TESTING METHODOLOGY ......................................................................................................... 87
4.1.1
Wear testing device ........................................................................................................... 87
4.1.2
Specimen preparation ....................................................................................................... 88
viii
4.2
RESTORATIVE MATERIALS .................................................................................................................... 91
4.2.1
IPS e.max Press ..................................................................................................................... 91
4.2.2
Type ІІІ gold .......................................................................................................................... 92
4.3
INFLUENCE OF MATERIAL ON THE FRICTION AND WEAR BEHAVIOURS OF ENAMEL .................. 93
4.4
INFLUENCE OF WEAR CONDITION ON THE FRICTION AND WEAR BEHAVIOURS OF ENAMEL .... 97
4.5
CONCLUSIONS .................................................................................................................................... 100
4.6
FUTURE RESEARCH.............................................................................................................................. 101
REFERENCES ........................................................................................................................ 102
APPENDIX ............................................................................................................................ 118
ix
LIST OF FIGURES
FIG. 1-1 CLINICAL MANIFESTATION OF ATTRITIONAL WEAR ON THE MAXILLARY AND MANDIBULAR
ANTERIOR TEETH............................................................................................................................................ 7
FIG. 1-2 CLINICAL MANIFESTATION OF TOOTHBRUSH ABRASION ON THE CERVICAL ASPECTS OF THE
MAXILLARY ANTERIOR TEETH (INDICATED BY THE ARROWS) CAUSED BY OVERZEALOUS
HORIZONTAL TOOTHBRUSHING .................................................................................................................. 9
FIG. 1-3 CLINICAL MANIFESTATION OF EROSIVE LESIONS ON THE PALATAL ASPECTS OF MAXILLARY
ANTERIOR TEETH CAUSED BY REGURGITATION OF ACIDIC GASTRIC CONTENTS; THE PATIENT HAD A
HISTORY OF BULIMIA NERVOSA ............................................................................................................... 10
FIG. 1-4 TOOTH WEAR MECHANISMS AND THEIR INTERACTIONS ............................................................... 12
FIG. 1-5 TYPICAL DENTAL WEAR TEST CONFIGURATIONS ............................................................................. 15
FIG. 1-6 FRICTIONAL FORCE (F) AND THE NORMAL LOAD (N) ................................................................... 22
FIG. 1-7 MAGNIFIED VIEW OF THE CONTACT AREA BETWEEN THE SLIDING SURFACES ........................... 24
FIG. 1-8 FUNDAMENTAL MECHANISMS OF WEAR (A) ADHESION (B) ABRASION (C) FATIGUE (D)
CORROSION................................................................................................................................................. 25
FIG. 1-9 LINEAR RELATIONSHIP BETWEEN THE WEAR VOLUME AND THE FRICTIONAL ENERGY
DISSIPATED DURING SLIDING MOVEMENT FOR THE THREE TESTED MATERIALS ............................... 30
FIG. 1-10 SEM IMAGES SHOWING (A) INTERROD ENAMEL SURROUNDING EACH ROD; IR, INTERROD; R,
ROD AND (B) FISH SCALE STRUCTURE OF MATURE ENAMEL ............................................................... 33
FIG. 2-1 DIAGRAM SHOWING TOOTH GROUPING FOR EXPERIMENTS ........................................................ 61
FIG. 2-2 (A) SPHERE ENDED TITANIUM RODS, (B) SILICONE MOULD MADE WITH POLYVINYLSILOXANE
PUTTY MATERIAL (C) SPHERE ENDED WAX PATTERNS WITH DIAMETER OF
4MM ............................ 64
FIG. 2-3 REPRESENTATIVE IMAGES OF (A) A POLISHED GOLD SPECIMEN (B) A GLAZED IPS E.MAX PRESS
SPECIMEN .................................................................................................................................................... 65
FIG. 2-4 ILLUSTRATION OF METHODS AND MATERIALS: (A) SCHEMATIC VIEW OF THE WEAR TESTING
DEVICE; (B)
A VIEW OF THE WEAR TESTING MACHINE ......................................................................... 67
FIG. 3-1 TYPICAL FORCE-DISPLACEMENT CURVE FOR A NANO-INDENTATION TESTING IN AN ENAMEL
SPECIMEN AS SHOWN ON THE
IBIS IMAGE SOFTWARE PROGRAMME .............................................. 70
FIG. 3-2 REPRESENTATIVE MICROSCOPIC IMAGE OF INDENTATIONS ON THE ENAMEL SPECIMEN ......... 70
x
FIG. 3-3 EVOLUTION OF FRICTION COEFFICIENT () AS A FUNCTION OF NUMBER OF CYCLES FOR THE
TWO WEAR PAIRS UNDER (A) THE TWO-BODY AND (B) THE THREE-BODY WEAR CONDITIONS ... 74
FIG. 3-4 LIGHT MICROSCOPE IMAGES OF THE WORN ENAMEL SURFACE OPPOSING (A) IPS E.MAX
PRESS AND (B) TYPE ІІІ GOLD SPECIMENS UNDER THE TWO-BODY WEAR CONDITION
(MAGNIFICATION X10) (ARROWS INDICATE THE SLIDING DIRECTION) ............................................. 78
FIG. 3-5 WEAR SCAR ON THE ENAMEL SURFACE OPPOSING IPS E.MAX PRESS SPECIMEN IN TWO-BODY
WEAR UNDER (A)
MAGNIFICATION X115 (THE SLIDING DIRECTION INDICATED BY THE DOUBLE
POINTED ARROW) (B) X500 (C) X1500 ................................................................................................. 80
FIG. 3-6 WEAR SCAR ON THE ENAMEL SURFACE OPPOSING TYPE ІІІ GOLD SPECIMEN IN TWO-BODY
WEAR UNDER (A)
MAGNIFICATION X115 (B) X500 (SLIDING DIRECTION INDICATED BY THE
DOUBLE POINTED ARROW) (C) X1500 ................................................................................................... 81
FIG. 3-7 LIGHT MICROSCOPE IMAGES OF THE ENAMEL SURFACE OPPOSING (A) IPS E.MAX PRESS AND
(B) TYPE ІІІ GOLD SPECIMENS FOLLOWING THREE-BODY WEAR TESTING (MAGNIFICATION X10)
...................................................................................................................................................................... 83
FIG. 3-8 WEAR SCAR ON THE ENAMEL SURFACE OPPOSING THE IPS E.MAX PRESS SPECIMEN
FOLLOWING THREE-BODY WEAR TESTING (A)
MAGNIFICATION X115 (SLIDING DIRECTION
INDICATED BY THE DOUBLE POINTED ARROW) (B) X500 (C) X1500 ................................................ 85
FIG. 3-9 WEAR SCAR ON THE ENAMEL SURFACE OPPOSING TYPE ІІІ GOLD SPECIMEN IN THREE-BODY
WEAR TESTING (A)
POINTED ARROW
MAGNIFICATION X115 (SLIDING DIRECTION INDICATED BY THE DOUBLE
) (B) X500 (C) X1500................................................................................................ 86
FIG. 4-1 RELATIONSHIP BETWEEN THE AVERAGE STEADY-STATE FRICTION COEFFICIENT () OF THE
WEAR PAIRS AND (A) HARDNESS OR (B) ELASTIC MODULUS OF ENAMEL SPECIMENS.................... 90
FIG. 4-2 ILLUSTRATION OF THE MECHANISM OF WEAR ON THE ENAMEL SURFACE OPPOSING IPS E.MAX
PRESS ........................................................................................................................................................... 95
xi
LIST OF TABLES
TABLE 1-1 SMITH AND KNIGHT TOOTH WEAR INDEX .................................................................................. 20
TABLE 1-2 ELASTIC MODULUS OF ENAMEL FROM DIFFERENT MEASUREMENT METHODS ....................... 37
TABLE 1-3 CLASSIFICATION OF METAL ALLOYS BASED ON PHYSICAL PROPERTIES AND THEIR
APPLICATION IN DENTISTRY ...................................................................................................................... 44
TABLE 1-4 DENTAL CERAMICS FOR ALL-CERAMIC SYSTEMS ........................................................................ 49
TABLE 1-5 STUDIES INCLUDED IN THE REVIEW............................................................................................... 54
TABLE 2-1 INCLUSION AND EXCLUSION CRITERIA FOR CONDITION OF EXTRACTED HUMAN TEETH ..... 58
TABLE 2-2 COMPOSITION OF HBSS USED FOR STORAGE OF THE EXTRACTED TEETH............................. 59
TABLE 2-3 TEST PARAMETERS APPLIED FOR THE NANO-INDENTATION TESTING...................................... 62
TABLE 2-4 BASIC PROPERTIES OF THE RESTORATIVE MATERIALS USED FOR WEAR TESTING ................... 63
TABLE 3-1 AVERAGE HARDNESS (GPA) AND ELASTIC MODULUS (GPA) VALUES FOR EACH ENAMEL
SPECIMEN .................................................................................................................................................... 71
TABLE 3-2 AVERAGE STEADY-STATE  VALUES AFTER 500 CYCLES FOR THE TWO WEAR PAIRS IN THE
DIFFERENT WEAR CONDITIONS................................................................................................................. 75
TABLE 3-3 RESULTS OF TWO-WAY ANOVA OF MAIN FACTORS (MATERIAL AND WEAR CONDITION)
AND THEIR INTERACTION FOR THE STEADY-STATE
 VALUE ............................................................... 76
TABLE 3-4 OUTCOME OF POWER ANALYSIS ................................................................................................... 76
xii
LIST OF ACRONYMS & ABBREVIATIONS
BEWE
Basic erosive wear examination
CEJ
Cementum enamel junction
Cp4Ti
Commercially pure grade 4 titanium
FDP
Fixed dental prosthesis
FESEM
Field emission scanning electron microscopy
FGC
Fluorapatite glass ceramic
FIB
Focused ion beam
GERD
Gastro-esophageal reflux disease
HBSS
Hanks’ balanced salt solution
ISO
International standard organisation
LEU
Leucite-reinforced glass ceramic
LS2
Lithium disilicate glass ceramic
LVDT
Linear variable differential transformer
MCC
Metal ceramic crown
NCCL
Non-carious cervical lesion
NFGC
Nanofluorapatite glass ceramic
PMMA
Polymethyl methacrylate
xiii
SEM
Scanning electron microscopy
TIFF
Tagged image file format
TMJ
Temporomandibular joint
UMIS
Ultra micro indentation system
Y-TZP
Yttria-stabilised tetragonal zirconia polycrystal
E
Elastic modulus
F
Frictional force
H
Hardness
K
Wear coefficient
KIC
Fracture toughness

Friction coefficient
N
Normal (applied) load
S
Sliding distance
V
Wear volume
xiv
1. LITERATURE REVIEW
1.1
Introduction
Wear is defined as a progressive loss of material from its surface as a consequence
of relative motion (Mair et al. 1996; Hahnel et al. 2011). This topic has been a
subject of interest in a number of areas of engineering. Wear testing is common
practice for predicting the service time of a component under simulated operating
condition and for providing data during material development (Reid et al. 1990).
Wear has also been the centre of discussion in dentistry for many years with
regards to management and prevention of tooth wear and also to the development
of restorative materials with good wear resistance (Turner and Missirlian 1984;
Lewis and Dwyer-Joyce 2005). Wear of teeth and restorative materials is the
result of complex and multifactorial phenomena with the interplay of biological,
mechanical, and chemical factors (Eisenburger and Addy 2002). The rate of wear
may be affected by various factors such as neuromuscular forces, lubricants,
patient habit, and the type of the restorative material used in the oral environment
(Mahalick et al. 1971; Kim et al. 2001). This underlines the complex nature of the
wear process and consequent difficulties we encounter in conducting in vivo and
in vitro wear studies.
The main advantage of an in vivo study is that we can obtain and observe wear
behaviour of enamel and restorative materials resulting from real oral
environment and biomechanics (Mair et al. 1996; Zhou and Zheng 2008).
However, in vivo wear studies are time-consuming, expensive (Sulong and Aziz
1990) and there can be a wide variation in results due to patient- and operatorrelated factors (Heintze 2006). Most of all, the fundamental problem with the in
1
vivo wear model is that it is impossible to isolate and vary key factors that may
influence the wear process (Lewis and Dwyer-Joyce 2005).
In comparison, an in vitro approach allows precise control of the environment and
variables, which may influence the wear process of dental biomaterials
(Lambrechts et al. 2006). However, there is no universally acceptable standard
wear testing method for dental biomaterials at present (Heintze 2006). Numerous
wear simulation devices, developed for research purposes employ different wear
testing concepts (i.e. forces, movement, contact profile etc), rendering comparison
of wide-ranging wear data difficult.
Although the in vitro model cannot simulate the oral environment with all the
biological variables, with a well controlled experimental design, the factors that
lead to a certain wear mechanism of the material can be identified (Lambrechts et
al. 2006). The wear propensity of a dental restorative material demonstrated in
vitro may help researchers and clinicians understand and predict the response of a
particular material in a clinical setting (Wang et al. 1998).
Appropriate selection of a restorative material is crucial for preserving occlusal
harmony and normal masticatory function. The wear rate for sound enamel under
friction from mastication has been reported to range between 20 to 40 m per
annum (Lambrechts et al. 1989). Ideally, the wear rate of a dental restorative
material should be compatible with that of natural tooth enamel (Seghi et al. 1991;
Hudson et al. 1995). However, the rate of wear may be affected by the
introduction of a restoration with wear properties differing from those of the
natural teeth (Mahalick et al. 1971; Joshi et al. 2010). The consequences of
introducing abrasive restorative materials can be detrimental, resulting in
unacceptable damage to the occluding surfaces, alteration of the functional path of
masticatory movement, and loss of anterior guidance and aesthetics (Mahalick et
al. 1971; Joshi et al. 2010).
2
Gold has long been a preferred indirect restorative material in stress bearing areas,
due to its biocompatibility, durability, and low abrasiveness against natural teeth
(Jacobi et al. 1991). However, with the increasing demand for aesthetically
pleasing, natural looking restorations, the use of all-ceramic restorations has
increased remarkably (Raigrodski 2004; Conrad et al. 2007). All-ceramic crowns
with high strength ceramic cores such as alumina, zirconia or lithium disilicate
glass ceramic have been developed to eliminate a metal framework traditionally
used for metal-ceramic crowns (MCC), to improve the optical translucency and
provide natural appearance (Etman and Woolford 2010). Although, such ceramic
systems fulfill biomechanical requirements and their clinical performance has
been successful for single crowns (McLaren and White 2000) and short-span
fixed dental prostheses (FDPs) (Vult von Steyern et al. 2001; Wolfart et al. 2009),
their wear behaviour and effects on opposing enamel have not been studied in
detail.
Previous in vitro wear studies concentrated on establishing a relative ranking of
different dental ceramics based simply on wear volume or depth (Jacobi et al.
1991; Ramp et al. 1999). However, studies rarely provided detailed information
on the underlying wear mechanism and the controlling factors involved in the
wear process. Some in vitro studies demonstrated a positive correlation between
the mechanical properties of dental ceramics (e.g. hardness (H), fracture
toughness (KIC)) and their wear resistance (DeLong et al. 1989; Heintze et al.
2007). Consequently, numerous wear models and numerical equations have been
formulated to justify and/or predict their wear behaviour. No single wear model or
equation, however can be applied universally for different ceramic materials
(Seghi et al. 1991).
The following review provides an appraisal of the literature concerning tooth wear
and wear investigations conducted in dentistry, with a particular focus on
understanding the wear behaviour of dental enamel and restorative materials
3
currently used in dentistry. Relevant concepts in tribology will be reviewed to
help understand the underlying mechanisms and controlling factors involved in
the wear process of dental biomaterials.
4
1.2
Tooth wear
1.2.1
Prevalence
Tooth wear is a clinical problem that is becoming increasingly important in the
aging population (Mair et al. 1996; Zhou and Zheng 2008). This may be due to an
increase in dental awareness, coupled to increased interest in retaining teeth as
opposed to having them extracted (Jagger and Harrison 1994). Data from
prevalence studies have demonstrated high levels of tooth wear in adults (Smith
and Robb 1996), adolescents (Bartlett et al. 1998) and children (Dugmore and
Rock 2004), suggesting that tooth wear is a clinical finding in all age groups.
Smith and Robb (1996), in a cross-sectional study observed that tooth wear is
common in adults, with up to 97% of the study cohort (1,007 adults)
experiencing some degree of tooth wear (Smith and Robb 1996). However, only
5-7% of the study cohort exhibited severe tooth wear, for which interventive
restorative treatment was justified.
The retrospective study by Bartlett (2003) investigated the progress of tooth wear
by examining the study models over a median time of 26 months (Bartlett 2003).
The authors reported slow progression of tooth wear in the study cohort,
suggesting that progression of tooth wear is not inevitable. However, the
systematic review by Van't Spijker et al. (2009) reported that the predicted
percentage of adults presenting with severe tooth wear increased from 3% at age
20 years to 17% at age 70 years, indicating a tendency for accumulative wear with
age (Van't Spijker et al. 2009).
The cross-sectional study by Ayers et al. (2002) investigated the prevalence and
severity of tooth wear in the primary dentition of New Zealand school children
aged between 5 and 8 years old (Ayers et al. 2002). A high percentage (82%) of
5
children had at least one primary tooth with dentine exposure. The severity of
tooth wear was not significantly associated with dietary factors, but appeared to
be related to early weaning from the breast. In a study based in the United
Kingdom, the prevalence of tooth wear was high (57%) in adolescents aged
between 11 and 14 years old, but dentine involvement was rare (Bartlett et al.
1998). The systemic review by Kreulen et al. (2010) reported that the prevalence
of tooth wear leading to dentine exposure in deciduous teeth increased with age,
while wear of permanent teeth in adolescents did not correlate with age (Kreulen
et al. 2010). However, a cross-sectional survey conducted in Brazil found that
children (7-10 years old) with tooth wear in primary teeth were more likely to
develop tooth wear in the permanent dentition (Sales-Peres et al. 2011). The
results from the longitudinal study by Harding et al. (2010) also demonstrated an
association between tooth wear recorded at age 5 and molar tooth wear recorded
at age 12 (Harding et al. 2010). The authors emphasised that tooth wear is a
lifelong cumulative process and should be recorded in both the primary and
permanent dentitions.
Although numerous epidemiological studies seem to indicate that tooth wear is
prevalent and increasing in the general population, the results are not easily
comparable due to the wide range of tooth wear indices used and the variation in
diagnostic criteria (Bardsley et al. 2004). Currently, there is no agreed consensus
on a universally acceptable tooth wear index for quantifying tooth wear (Bartlett
and Dugmore 2008). The lack of standardisation in epidemiological studies
complicates the evaluation of whether a true increase in prevalence is being
reported (Bardsley 2008) and therefore, conclusion from prevalence studies
should be considered with caution (Shaw 1997).
6
1.2.2
Aetiology
The terms attrition, abrasion, and erosion have been used interchangeably in the
past to describe the loss of tooth structure and dental biomaterials (Mair et al.
1996). These terms, however, are not in themselves descriptive of the wear
process, nor do they imply the causative factor, but instead describe clinical
manifestations of a number of underlying events (Johansson et al. 2008).
Attrition is defined as a gradual loss of hard tooth substance from occlusal
contacts with an opposing dentition or restorations (Hattab and Yassin 2000). It is
related to aging, but may be accelerated by intrinsic factors such as parafunctional
habits of bruxism, traumatic occlusion in the partially edentulous dentition, and
malocclusion (Hattab and Yassin 2000; Grippo et al. 2004). Clinically, occlusal
wear attributable to attrition will produce matching wear facets on opposing teeth
as shown in Fig. 1-1. In early stages, there appears a small polished facet on a
cusp tip or slight flattening on an incisal edge. However, advanced attrition leads
to dentine exposure, which may result in an increased rate of wear (Kelleher and
Bishop 1999; Litonjua et al. 2003).
[From Kelleher et al. 2012]
Fig. 1-1 Clinical manifestation of attritional wear on the maxillary and mandibular
anterior teeth
7
It has been suggested that progressively greater loss of tooth structure occurs
towards the anterior teeth, due to leverage changes produced by eccentric
posterior interferences (Abrahamsen 2005; Spear 2008). Instead of occurring at
the temporomandibular joints (TMJs), posterior occlusal contacts become the
fulcrum point with greater forces applied to the anterior teeth. Spear argued that
the steepness of the condylar eminence has a significant effect on the development
and occurrence of posterior interferences during mandibular movement (Spear
2008; Spear 2009). He observed that patients with a flat condylar eminence tend
to have significant posterior interferences, causing flattening of posterior teeth. On
the other hand, patients with steep condylar eminences have minimal posterior
interferences and hence little or no posterior wear. However, there have been no
experimental studies that have confirmed the relationship between the angle of the
condylar eminence and posterior teeth contact. Moreover, controversy remains
regarding the relationship between functional occlusal contact and tooth wear
(Abdullah et al. 1994; Bishop et al. 1997).
Abrasion is the loss of tooth substance through mechanical means, independent
of occlusal contact (Mair 1992). The site and pattern of abrasion wear can be
diagnostic as different foreign objects produce different patterns of abrasion wear
(Hattab and Yassin 2000). Some forms of abrasion may be associated with habit
or occupation, such as a rounded ditch on the cervical aspects of teeth due to
vigorous horizontal toothbrushing (Fig. 1-2) or incisal notching caused by pipe
smoking or nail biting (Mair 1992; Bishop et al. 1997; Grippo et al. 2004). The
most common cause of dental abrasion found in the cervical areas is
toothbrushing and the severity and distribution of toothbrushing abrasion may be
related to brushing technique, time, frequency, bristle design and the abrasiveness
of the dentifrice (Bishop et al. 1997; Hattab and Yassin 2000).
8
Fig. 1-2 Clinical manifestation of toothbrush abrasion on the cervical aspects of
the maxillary anterior teeth (indicated by the arrows) caused by overzealous
horizontal toothbrushing
Abfraction is a relatively new term that describes loss of hard tooth substance in
the cervical region as a result of crack formation during tooth flexure (Kelleher
and Bishop 1999; Lewis and Dwyer-Joyce 2005). Some authors proposed that
tensile and compressive stresses from mastication and malocclusion play a major
role in the formation and progression of wedge-shaped abfraction lesions (Lee and
Eakle 1984). However, the true aetiology of abfraction lesions has been
controversial as numerous studies found evidence of a common association of
toothbrush abrasion and erosion in the development of these lesions (Levitch et al.
1994; Borcic et al. 2004; Nguyen et al. 2008; Wood et al. 2008; He et al. 2011).
The aetiology is multifactorial and the term, non-carious cervical lesion (NCCL)
is preferred to describe the loss of tooth substance at the cementum-enamel
junction (CEJ) without bacterial involvement.
Erosion is defined as the loss of tooth structure by a nonbacterial chemical
process (Mair 1992; Litonjua et al. 2003). Some authors however, disagree with
the term erosion due to its remarkably different meaning between dentistry and
engineering tribology (Zhou and Zheng 2008). The term ‘corrosion’ has been
advocated to correctly describe the process of tooth surface loss due to chemical
9
or electrochemical action (Grippo et al. 2004; He et al. 2011). In this review,
however, the term erosion will be used to denote chemical dissolution of teeth or
restorations.
Erosive lesions initially present as “silky-glazed” dull enamel surfaces, which lack
developmental ridges and stain lines (Hattab and Yassin 2000; Lussi 2006). In the
advanced stage, restorations may project above the occlusal surface and the cusps
of posterior teeth exhibit concavities known as “cupping” (Hattab and Yassin
2000; Litonjua et al. 2003). The distribution and wear pattern of erosion are
specifically associated with the origin of the acid and the posture of the head when
the acid is present (Spear 2008; Spear 2009). As intrinsic acid enters the oral
cavity from the eosophagus, it tends to produce a significant tooth surface loss on
the lingual and/or occlusal surfaces of teeth (Fig. 1-3). In comparison, extrinsic
acid often results in erosive wear on facial (labial) surfaces of teeth by its nature
of entering the oral cavity from the anterior aspect.
Fig. 1-3 Clinical manifestation of erosive lesions on the palatal aspects of
maxillary anterior teeth caused by regurgitation of acidic gastric contents; the
patient had a history of bulimia nervosa
10
Tooth wear may involve the entire dentition (generalised) or be localised to
anterior or posterior teeth, depending on the causative factor (Spear 2008). For
instance, for patients with gastro-esophageal reflux disease (GERD), the palatal
surfaces of the maxillary anterior teeth are severely affected, while the mandibular
teeth are protected from the erosive effect by the tongue and saliva. For wear by
attrition, the occlusal condition influences the quantity and distribution of the
tooth wear pattern. One longitudinal study reported that increased incisal wear
correlates with horizontal overjet and vertical overbite (Silness et al. 1993). The
authors claimed that the anterior guidance as determined by the overbite and
overjet, and the ratio between these, can be used as predictors of attrition wear of
the maxillary and mandibular incisors. In the case of the anterior open bite, where
no occlusal contact exists between the maxillary and mandibular anterior teeth,
greater wear is anticipated on the posterior teeth than on the anterior teeth (Spear
2009). However, a correlation between attrition and other occlusal parameters has
not been reported, and thus no single occlusion-based treatment protocol can be
recommended in the management of attrition (Van't Spijker et al. 2007).
Although research into tooth wear has grown considerably over recent years, our
understanding of its aetiology and pathogensis is still lacking (Bartlett 2010).
Differentiation among attrition, abrasion, erosion and abfraction is difficult, since
these aetiological factors may act synchronically or additively with other entities
masking the true nature of tooth wear (Litonjua et al. 2003). The occurrence and
pattern of tooth wear is closely associated with educational, cultural, dietary,
occupational and geographic factors in the population (Hattab and Yassin 2000).
It has been suggested that the aetiology of clinical wear may be considered in
terms of site, timing and underlying wear mechanism rather than nomenclature
(Fig. 1-4) (Mair et al. 1996).
11
[From Mair et al. 1996]
Fig. 1-4 Tooth wear mechanisms and their interactions
Thegosis is defined as the action of sliding teeth laterally and it has been proposed
that this is a genetically determined habit, originally established to sharpen teeth
(Every and Kuhne 1971). Bruxism is the action of grinding teeth without the
presence of food (Lewis and Dwyer-Joyce 2005). Mastication is the action of
chewing food and is composed of two phases; the open phase and the closed
phase (Mair et al. 1996). Initially, the teeth approximate from an open position to
a position of a near contact (open phase), and the abrasive particles are suspended
and free to move in the food (slurry). This is followed by a closed phase, in which
the teeth are brought close together and the food particles become trapped
between the tooth surfaces. Entrapment of food particles is largely influenced by
textural characteristics of the surfaces; rougher surfaces are more likely to trap
food particles than smooth tooth surfaces. Following compression and crushing of
12
the food bolus, grinding occurs either with tooth-food-tooth contact (indirect) or
direct tooth-tooth contact of the opposing teeth surfaces (DeLong 2006).
Masticatory parameters such as the magnitude of the force and duration of the
masticatory cycle vary widely among individuals and depend largely on the food
type, size of food bolus and chemical and physical action of saliva (Sajewicz and
Kulesza 2007; Zhou and Zheng 2008). The total duration of the masticatory cycle
was reported to be approximately 0.70s (Peyron et al. 1996; Bhatka et al. 2004),
whilst the mean duration of the occlusion is about 0.10s (Lewis and Dwyer-Joyce
2005; Zhou and Zheng 2008). These periods equal to 15-30 minutes of contact
loading each day (Lewis and Dwyer-Joyce 2005).
13
1.2.3
Clinical management of tooth wear
Clinicians need to have a good appreciation of the multifactorial nature of the
tooth wear process, and should carry out a thorough clinical examination
including a medical and dental history, occupation, diet and parafunctional habits
of the patient (Hattab and Yassin 2000).
The successful management of any case of tooth wear is based on deriving an
accurate diagnosis, having a clear understanding of the basic principles of
occlusion, and a good working knowledge of available material and techniques
(Mehta et al. 2012). The quantity and positional wear pattern are pathognomonic
of the causative factor and thus, the clinician should carefully observe the wear
patterns in order to differentiate various causes and to confirm the diagnosis
(Abrahamsen 2005).
It is important to identify, and when possible prevent the aetiological factor(s)
from further inflicting damage on the tooth or on dental restorations (Mehta et al.
2012). Long-term monitoring is essential for assessing the effectiveness of
preventive measures taken and any further progression of the wear before
embarking on interventive treatment. Early diagnosis and appropriate prevention
measures can prevent complicated restorative treatment in the future (Bartlett
2003). Restorative treatment decisions should be based on the patients’ need,
severity of wear and potential for progression of wear (Bartlett 2005). For
instance, restorative treatment would be indicated when the patient presents with
clinical symptoms such as tooth sensitivity or pain that cannot be controlled
conservatively, or uncontrolled tooth wear is occurring, that is altering the
occlusal vertical dimension with functional and aesthetic deficit (Smith et al.
1997).
14
1.3
Wear investigation in dentistry
1.3.1
In vitro wear testing
Numerous wear testing devices have been developed and used to predict the
clinical performance of various dental biomaterials. They differ in the degree of
complexity and use different variables including force, contact geometry,
displacement, lubricant, antagonist and cycles (Lambrechts et al. 2006; Zhou and
Zheng 2008). Fig. 1-5 shows the geometric arrangements used in several common
types of tooth wear testing devices.
[From Lewis and Dwyer-Joyce 2005]
Fig. 1-5 Typical dental wear test configurations
15
Most of the wear testing devices are used for two-body wear evaluation, in which
two surfaces move against each other in direct contact. These conditions occur
during non-masticatory movement in the mouth (Lewis and Dwyer-Joyce 2005;
Lambrechts et al. 2006). During mastication, food particles present in the mouth
play an important role in the wear of teeth and dental biomaterials, and some
simulation devices include abrasive slurries to replicate the three-body wear
condition (Lambrechts et al. 2006; Zheng and Zhou 2007). The composition of
food simulation slurry varies widely in different studies, such as ground rice in
phosphate buffer (Zheng and Zhou 2007), cornmeal grit and wholemeal flour in
distilled water (Al-Hiyasat et al. 1999) or polymethyl methacrylate (PMMA)
beads in water (Imai et al. 2000).
Among the many geometric designs, a pin-on-disk wear-test rig has been very
popular to simulate two-body wear between the sample and the antagonist
(Mueller et al. 1985; Hahnel et al. 2010). This method uses a simple relative
movement between the wear pair and gives relatively quick results (Mueller et al.
1985). However, it does not properly simulate the oral environment (Sajewicz and
Kulesza 2007) and repeatability of results using the same condition (i.e. load,
contact pressure, sliding speed) was reported to be poor (Bianchi et al. 2002).
More complex in vitro wear testing devices have been developed to provide a
more accurate simulation of the masticatory movement (Kaidonis et al. 1998; Li
and Zhou 2002; Zheng et al. 2003). Some testing devices incorporate
unidirectional sliding movement of mastication, where the specimen slides in one
direction for a specified duration, after which it is repositioned to its original
position (Kaidonis et al. 1998; Burak et al. 1999).
DeLong and Douglas in the early 1980s developed an “artificial oral
environment” which simulated the physiological movement of the oral cavity
through two servo-hydraulic units that control horizontal and vertical movements
(DeLong and Douglas 1983). Physiological conditions of the oral cavity were
16
reproduced by controlled setting of the biting force, temperature and artificial
saliva. They compared their simulative wear data on amalgam, composite resin
and dental porcelain with clinical data and demonstrated a good correlation
between the in vitro and in vivo data (DeLong et al. 1985; DeLong et al. 1986;
Sakaguchi et al. 1986).
Heintze et al. (2005) investigated the wear resistance of 10 restorative dental
materials (8 composite resins, one amalgam (AmalcapPlus, Vivadent, Ellwangen,
Germany) and one ceramic (IPS Empress TC1, Ivoclar, Schaan, Liechtenstein))
using five different wear simulation methods in order to validate the compatibility
of different wear simulation devices (Heintze et al. 2005). The relative ranking of
the tested materials varied significantly between the different wear testing
methods. The authors concluded that variations in the wear simulation settings
resulted in measuring different wear mechanisms and therefore, care must be
taken when interpreting and comparing the results of in vitro wear data.
Despite the development of sophisticated and complex wear testing devices, a
clear correlation between in vitro and clinical data has not been established (Lewis
and Dwyer-Joyce 2005) and the clinical performance of dental biomaterials
cannot be precisely predicted. Sajewicz and Kulesza (2007) argued that the
emphasis in previous in vitro wear studies has been on the development of a wear
simulator, that produces physiological movements or a force pattern similar to the
oral environment, however there are no standard oral conditions (Sajewicz and
Kulesza 2007). In addition, material wear can be influenced by various factors
including load, contact area and geometry. Therefore, it would be inappropriate to
rank the wear resistance from the wear loss only, without verifying the validity of
the testing system in respect to the wear conditions, simulation domain and wear
mechanism (Hu et al. 2002).
17
In 2001, the International Standard Organisation (ISO) published a technical
specification termed “wear by two- and/or three-body contact”, and in the
specification, eight different wear testing methods were described (Heintze 2006).
However, the specification did not provide any information about validity or
accuracy of the testing methods and whether the testing devices with which the
methods were conducted were qualified for that purpose. The lack of an
internationally acceptable in vitro method for evaluating wear behaviour of dental
biomaterials, combined with the various methods used in the past makes it
difficult or even impossible to compare some in vitro wear data from different
reports.
The limitations and issues of in vitro wear studies in dentistry have been
addressed elsewhere (Heintze 2006; Lambrechts et al. 2006). The current trend
with in vitro wear studies has shifted from developing a physiological wear
simulator to identifying the underlying wear mechanisms from mechanical
engineering (tribology) and physics points of view (Sajewicz 2006; Ramalho and
Antunes 2007). For example, using the Hertz theory in contact mechanics, the
ball-on-disk experimental design is commonly used to investigate and compare
the wear mechanisms of dental biomaterials (Peterson et al. 1998; Yu et al. 2006).
Understanding the in vitro wear propensity of a dental restorative material will
help researchers and clinicians predict the wear response of a particular material
in a clinical setting (Wang et al. 1998).
18
1.3.2
In vivo wear testing
An in vivo wear investigation of dental biomaterials usually encompasses two
parts; subjective performance assessment of the material and quantitative
measurement of wear (Sulong and Aziz 1990). The clinical performance of a
restoration is assessed based on specified criteria such as marginal adaptation,
gingival health, structural integrity and patient satisfaction after a certain period of
use (Schmalz and Ryge 2005). This is accompanied with quantitative wear
measurement using various methods including study casts, intraoral photographs
and tooth wear indices that can be used alone or in combination to identify
morphological changes of teeth over time (Donachie and Walls 1995; Bartlett et al.
2005).
Tooth wear indices are the most popular method of quantifying wear over a long
period of time as they are readily available and do not require special equipment
(Al-Omiri et al. 2010). Numerous indices have been developed for use in clinical
studies and most are based on numerical grades to quantify the amount of hard
tissue loss (Bardsley 2008). The Smith and Knight tooth wear index is the most
frequently used index in the dental literature (Van't Spijker et al. 2009) and it
records wear on all four surfaces (buccal, cervical, lingual and incisal-occlusal),
irrespective of the aetiology of tooth wear as shown in Table 1-1 (Smith and
Knight 1984). Some indices such as the Basic Erosive Wear Examination
(BEWE) or the classification for dental attrition are designed to examine one
aetiological factor, specific for erosion, attrition or abrasion (Eccles 1979;
Seligman et al. 1988; Bartlett et al. 2008).
However, a universally acceptable tooth wear index has yet to be found (Bartlett
and Dugmore 2008) and new indices are continually being designed and applied
in clinical studies (Fares et al. 2009). Bardsley (2008), in his review article
claimed that there are too many indices with a lack of standardisation in
19
terminology (Bardsley 2008). This results in difficulty in interpreting and
comparing the results of many epidemiological studies.
Table 1-1 Smith and Knight Tooth wear index
Grade
Criteria
0
No loss of enamel surface characteristics
1
Loss of enamel surface characteristics
2
Buccal, lingual, and occlusal loss of enamel, exposing
dentine for less than 1⁄3 of the surface; incisal loss of enamel;
minimal dentine exposure
3
Buccal, lingual, and occlusal loss of enamel, exposing
dentine for more than 1⁄3 of the surface; incisal loss of
enamel; substantial loss of dentine
4
Buccal, lingual, and occlusal complete loss of enamel, pulp
exposure, or exposure of secondary dentine; incisal pulp
exposure or exposure of secondary dentine
[Adapted from Smith and Knight 1984]
Study casts are a valuable diagnostic tool for monitoring progression of tooth
wear and quantifying the amount of wear (Bartlett 2003). Silicone impressions of
teeth or restorations are taken at regular intervals to make replica models in stone
or epoxy resin, which are then compared for quantitative analysis (Sulong and
Aziz 1990). Measurements can be recorded using a number of methods including
stylus or laser profilometry (Glentworth et al. 1984) or stereomicroscopy images
and computerised image fitting (Lambrechts et al. 1989). With advancement in
measuring techniques, 3D laser scanning device has become available to scan the
surface of a replica to construct a 3D image for quantifying the wear more
accurately (Peters et al. 1999). Al-Omiri et al. (2010) compared the reliability of
20
three different methods; CAD-CAM laser scanning machine, a tool maker
microscope, and a conventional tooth wear index (Smith and Knight 1984) to
detect incisal wear over a 6 month period. The tooth wear index was the least
sensitive for tooth wear quantification as it was unable to identify as much tooth
wear as detected with the other methods. However, the authors cautioned that
despite improved accuracy and reliability, new sophisticated measuring tools are
costly and require specialised equipments, restricting their use in everyday dental
practice and large epidemiological studies (Al-Omiri et al. 2010).
Nevertheless, the fundamental problem with in vivo wear studies is the inherent
patient factor (Kim et al. 2001). Although, measurements can be taken to
standardise the testing conditions among the participants, such as the dietary
regime and toothbrushing, each individual represents a variable and this
confounds the interpretation of wear results (Lewis and Dwyer-Joyce 2005). In
addition, the sensitivity of measurement and replica techniques are an important
consideration (Lewis and Dwyer-Joyce 2005). Many of the deviations in the
results occur due to an inaccurate replica technique, repositioning problems, and
restrictions of the measuring devices (Lambrechts et al. 1984). Therefore,
appropriate training and calibration are important to minimise subjective errors
and a combination of methods should be used for a more reliable quantitative
analysis.
21
1.4
Tribology
The application of tribology in dentistry is a growing and rapidly expanding field
(Zhou and Zheng 2008). Tribology is defined as ‘the science and technology of
interacting surfaces in relative motion’ and encompasses the study of friction,
lubrication and wear (Hutchings 1992). The oral cavity presents a unique and
dynamic tribological system composed of four elements: the opposing teeth
and/or restorative materials, solid (food) or liquid (saliva) lubricant and an
environment represented by air (d’Incau et al. 2012). Fundamentals of friction and
sliding wear are reviewed in the following to achieve a better understanding of the
underlying wear mechanisms of dental biomaterials in the oral environment.
1.4.1
Friction
When two surfaces slide over each other, resistance to the sliding motion is
encountered. This resistance is known as friction, and frictional force can be
defined as the resistive force that acts in the opposite direction to motion (Halling
1976). As illustrated in Fig. 1-6, a tangential force (F) is required to move the
upper body (A) over the stationary counter-surface. The ratio between the
frictional force (F) and the applied load (N) is known as the coefficient of friction,
and is usually represented by the symbol The magnitude of the frictional force
dissipated is conveniently described by the value of 

F/N
[1]
Fig. 1-6 Frictional force (F) and the normal load (N)
22
In the early experimental work by the French engineer Amontons, it was observed
that the  for a given pair of materials seemed to be constant over a range of
conditions (Hutchings 1992). This observation led to the formulation of two
empirical ‘Laws of Sliding Friction’, as follows:
(a)
The friction force is directly proportional to the normal load
(b)
The friction force is independent of the apparent area of contact
Based on these laws of sliding friction, the average value of various engineering
materials have been determined over a broad spectrum of test conditions (Kogut
and Etsion 2004). These values have been used to provide a general guideline of
the sensitivity of the to the materials in contact. However, it has been recognised
that the value is both material- and system-dependant and is not definitely an
intrinsic property of two sliding materials. Therefore, it is important to reflect on
the environmental system in which the sliding movement occurs, when
determining or comparing the  of different materials. During wear testing, a
continuous friction record provides a numerical value of  and moreover, it allows
changes in sliding behaviour to be monitored (Hutchings 1992). These changes
often indicate alterations in surface nature or topography, or in the mechanism of
wear. Knowledge of such changes is sometimes more useful than measurement of
an absolute wear rate.
23
1.4.2
Wear mechanisms
The surfaces of all materials are rough at a microscopic level with sharp rugged
projections called asperities, which have a surface profile of peaks and valleys
(Hutchings 1992). When two surfaces (materials) are placed in contact, they will
touch at the tips of asperities as shown in Fig. 1-7 (Halling 1976).
Fig. 1-7 Magnified view of the contact area between the sliding surfaces
The materials are supported on the summits of the highest of these surface
asperities, so that the area of intimate contact is very small (Bowden and Tabor
2001). The real area of contact is almost independent of the size of the surfaces
and is determined by the load. As the normal load is increased, the surface
asperities will deform, initially elastically, and finally plastically, such that the
real contact area increases. Understanding of the way in which the asperities of
two materials interact during motion is therefore important to study friction and
wear behaviour of a material (Hutchings 1992).
24
Four different wear mechanisms have been described in tribology: abrasive,
adhesive, corrosive and fatigue wear (Fig. 1-8) (Mair 1992; Mair et al. 1996).
These mechanisms may operate alone or in combination to cause surface loss of
interacting materials.
[From Mair et al., 1996]
Fig. 1-8 Fundamental mechanisms of wear (a) adhesion (b) abrasion (c) fatigue
(d) corrosion
25
Abrasive wear describes the deformation and removal of a softer material by
harder asperities of a counter-surface or by hard third particles entering the
contact zone (Dowson 1981; Mair et al. 1996). A distinction is made between
two-body and three-body abrasive wear, based on whether the abrasive medium
is integrated or separate to the contact surfaces.
Two-body abrasion occurs when the surfaces are rubbed away by direct contact
(Lambrechts et al. 2006). When two surfaces have very different levels of
hardness, microasperities on the harder surface move across the softer surface
with a micro-ploughing mechanism (d’Incau et al. 2012). In the mouth, these
conditions occur predominantly during ‘non-masticatory tooth movement’ and are
particularly prevalent in ‘bruxism’ (Lambrechts et al. 2006). Attrition is a form of
two-body abrasion wear, which is the result of physiological or pathological
proximal and occlusal inter-dental friction.
Three-body abrasion describes the surface wear by an intervening slurry of
abrasive particles (Lambrechts et al. 2006). In the chewing cycle, the opposing
teeth trap a layer of food and grind it as the teeth slide over one another
(Lambrechts et al. 2006). The abrasive food particles act as a slurry and abrade the
surface of a tooth or a restoration. An important feature of three-body abrasion is
that the surfaces do not match as there is no direct contact between them (Mair
2000). This process tends to hollow out softer areas on a surface. For instance, in
composite restorative material, the slurry preferentially abrades the softer polymer
matrix, exposing the harder filler particles (Lambrechts et al. 2006).
Adhesive wear occurs when there is a high attraction between the sliding surfaces
such that ‘cold welds’ occur between the asperities (Hutchings 1992; Lambrechts
et al. 2006). Further movements of the surfaces fracture these welds and may
eventually lead to transfer of material from one surface to another (Mair 1992).
The amount of material transfer depends on various factors including the distance
26
separating the materials, material properties and the environmental conditions
such as temperature or pressure (Hutchings 1992). Although this type of wear is
generally associated with metallic materials such as amalgam (Wassell et al.
1994) or gold alloys (Hacker et al. 1996), it has been demonstrated to occur
between two surfaces of polymethyl-methacrylate (Vaziri et al. 1988; Mair 2000).
Corrosive or tribochemical wear is caused when chemicals weaken the intermolecular bonds of the surface and thus potentiate the other mechanical wear
processes (Mair 1992). In the mouth, this effect is usually caused by acids, which
may originate from extrinsic or intrinsic sources (Lambrechts et al. 2006). The
most important feature in corrosive wear is that as soon as the weakened surface
molecules have been removed, the underlying (previously unaffected) surface is
exposed to chemical attack (Mair 2000). Chemical dissolution and mechanical
wear process act simultaneously to cause a significant amount of surface loss.
Conditions that may affect corrosive wear of teeth and restorations in the mouth
include the degree of occlusal loading, the lubricating and buffering capacity of
saliva and the chemical nature of the oral environment (Sulong and Aziz 1990).
Fatigue wear occurs when movement of surface molecules are transferred to the
subsurface causing rupture of intermolecular bonds and a zone of ‘subsurface
damage’ (Mair 2000). The sliding movement causes a zone of compression ahead
of motion and a zone of tension behind the motion in the subsurface of the
material (Mair et al. 1996). Eventually, microcracks form within the subsurface
and, if these propagate and coalesce to the surface, there can be loss of surface
fragment, inducing fatigue wear (Lambrechts et al. 2006). The displaced fragment
can become interposed between the two surfaces in contact, giving rise to threebody abrasion.
27
1.4.3
Sliding wear
When two materials in contact slide over each other, one or both of the materials
will suffer wear on the surface (Hutchings 1992). Sliding surfaces are often
lubricated to lower the frictional force and reduce the wear damage. Lubricants
function by introducing a layer of material with low shear strength between the
sliding surfaces. This results in partial or complete separation of the sliding
surfaces, preventing asperity contact and junction formation between the sliding
surfaces. However, under very high contact pressures, or at very low sliding
velocity, the lubricating fluid will be rapidly forced out, leading to direct contact
between the asperities (Persson 1999). This region is known as boundary
lubrication and high friction and wear rates will prevail in this region.
In the oral cavity, saliva serves as an important lubricant, protecting the hard
(enamel) and soft (mucosal) tissues from mechanical and chemical irritants. The
main lubricating components of saliva are mucins, which are complex protein
molecules formed by polypeptide chains that join together (Humphrey and
Williamson 2001). These mucins possess the properties of low solubility, high
viscosity, high elasticity and strong adhesiveness, rendering them ideal as
lubricants. The buffering capacity of saliva is important in minimising the erosive
effects of gastric or dietary acids (Jarvinen et al. 1991). In vitro studies have
shown that artificial saliva can have both cooling and lubricant effect during the
tooth wear process, and hence the risk of burn on tooth surface and the wear rate
are reduced under artificial saliva conditions compared to those under dry
conditions (Li and Zhou 2002). Therefore, rapid wear on teeth and restorative
materials will occur in the absence of saliva, such as in xerostomia, salivary gland
tumour and irradiation therapy (Gan et al. 2012).
28
1.4.3.1 Sliding wear model
The most common sliding wear model proposed in tribology is the Archard model,
which relates the wear volume (V) to the product of the sliding distance (S) with
the applied force (N) (Archard 1953). The wear coefficient K establishes the
proportionality and provides an important means of comparing the severity of
wear processes in different systems (Hutchings 1992). The Archard model is
based on the assumption that the wear rate depends on the normal load and the
hardness or yield strength of the material.
V=K
[2]
V= Wear volume, S= Sliding distance, N= Normal applied load, K= Coefficient of
wear, H= Hardness
Although the Archard model provides a valuable means of describing the severity
of wear via the wear coefficient, it must be remembered that its validity cannot be
used to support any particular wear mechanism (Hutchings 1992). Numerous
studies have shown that for the same material, the wear coefficient strongly
depends on the wear mode, displacement amplitude, contact geometry and other
parameters (Huq and Celis 2002). Therefore, the Archard model may not be
suitable for describing the wear behaviour of all materials, especially brittle
materials like ceramics due to the complex nature of the wear process with
numerous contributing factors.
Recently, researchers in the engineering field have demonstrated a correlation
between the wear volume and the dissipated frictional energy (Huq and Celis
2002; Fouvry et al. 2003). Fig. 1-9 illustrates the experimental results by Ramalho
and Miranda (2006) who demonstrated a linear relationship between the wear
29
volume of the three tested materials (mild steel AISI1037, hard steel AISI52100
and tungsten carbide-WC) and the dissipated friction energy (Ramalho and
Miranda 2006). The researchers suggested that the wear of materials is closely
related to the frictional energy dissipated during the sliding movement and that,
the energy dissipated by friction can be used to characterise the wear resistance of
a material (Ramalho and Miranda 2006). According to this energy-wear approach,
the  is an important variable to quantify the wear rate under alternating sliding
situations. The slope of the wear volume versus dissipated energy curve has been
defined as ‘the energy wear coefficient’ and this is used to compare the wear
resistance of different materials (Fouvry et al. 2003). The slope can also be used
to compare the wear resistance of the same material in different environmental
conditions.
[From Ramalho and Miranda 2006]
Fig. 1-9 Logarithmic relationship between the wear volume and the frictional
energy dissipated during sliding movement for the three tested materials
30
This energy-wear approach has been introduced in dental tribology to evaluate the
wear resistance of various dental biomaterials. One of the advantages of
evaluating wear data using dissipated friction energy is that this approach allows
monitoring instantaneous changes in friction and wear conditions during the wear
process. The majority of dental biomaterials are composite materials with
structural inhomogeneity and they demonstrate variable tribological behaviour
during the process of friction and wear. The in vitro study by Zheng et al. (2003)
demonstrated that tribological characteristics of enamel vary during the wear
process due to differential microstructural and mechanical properties as a function
of location on enamel (Zheng et al. 2003).
Currently, the in vitro wear simulation devices developed for dental research
incorporate a sensor that records frictional force as a function of time to obtain
kinetic characteristics (Sajewicz and Kulesza 2007). This data helps researchers
interpret wear mechanisms and utilise energy criteria to evaluate the wear
resistance of the test materials. Several in vitro studies have confirmed a positive
correlation between the volumetric wear and the friction energy dissipated during
sliding wear testing of different dental restorative materials (Sajewicz 2006;
Ramalho and Antunes 2007).
31
1.5
Tooth enamel
1.5.1
Histological background
Enamel surrounding the outer surface of a tooth is the most highly mineralised
tissue in the body, composed of 97% inorganic and 3% organic material (Lewis
and Dwyer-Joyce 2005). The main inorganic component is hydroxyapatite in the
form of elongated hexagonal crystals. Hydroxyapatite crystallites are organised
into rods separated by interrod enamel (Fig. 1-10a). The distinction between the
rod and interrod enamel is based on the crystal orientation (Simmer and Fincham
1995). The hydroxyapatite crystals within the enamel rod run parallel to the long
axis of the rod (White et al. 2001), while interrod crystallites are at an angle of 45°
to 60° to the long axis of the rods (Poole and Brooks 1961). The continuity of
enamel rods produces the characteristic ‘fish-scale’ appearance in cross-sections
under SEM observation (Fig. 1-10b).
Although most of the enamel organic matrix is removed during mineralisation and
maturation, some protein, notably ameloblastin remains at the incisal edges and
proximal sides of rod boundaries defining a rod sheath (Hu et al. 1997). The
ameloblastin remnants help to define the rounded discontinuity seen around the
top of the ‘fish scale’ on rods (Paine et al. 2001; He and Swain 2008). These
minute quantities of protein remnants have profound effects on the mechanical
properties of enamel. Firstly, they define three dimensional cleavage planes,
which will deflect cracks. This could prevent cracks propagating though enamel to
cause catastrophic macro-mechanical failure, but instead spreads the damage
laterally over a larger area (White et al. 2001). Secondly, the presence of minute
quantities of protein remnants allows limited differential movement between
adjacent rods. Limited slippage reduces stresses without crack growth and
propagation (He and Swain 2007).
32
[Adapted from He and Swain 2008]
Fig. 1-10 SEM images showing (a) interrod enamel surrounding each rod; IR,
interrod; R, rod and (b) fish scale structure of mature enamel
Precise regulation of biomineralisation procedure gives enamel its unique
compositional and structural characteristics. Its hierarchical structure and the
presence of minute quantities of proteins contribute to favorable mechanical and
wear properties, required to bear a wide range of functional loads and to resist
chemical and mechanical attacks in the oral environment.
33
However, unlike other hard tissues in the body such as bone, cartilage and dentine,
dental enamel cannot regenerate or repair apart from limited remineralisation,
following loss such as by dental caries or wear (He and Swain 2008), since the
enamel forming cells (ameloblasts) degenerate after completion of enamel
formation (amelogenesis). Therefore, pathological or excessive tooth wear results
in permanent loss of dental enamel and often exposure of the dentinal tissues
underneath, when the enamel layer is breached (Lewis and Dwyer-Joyce 2005).
Therefore, the use of restorative dental materials is inevitable to restore the shape
and function of the teeth.
34
1.5.2
Mechanical properties of enamel
Understanding the mechanical properties of human enamel is essential to clinical
tooth preparation and to the development of novel artificial restorative materials
(Xu et al. 1998). Hence, the mechanical behaviour of human enamel has been the
subject of numerous studies. Earlier mechanical studies assumed that enamel was
a homogenous solid with uniform mechanical properties (Remizov et al. 1991;
Cuy et al. 2002). However, now it is known that human enamel is an anisotropic
material and the mechanical behaviour of enamel depends largely on the location,
chemical composition and enamel rod orientation (Cuy et al. 2002; Roy and Basu
2008).
Various macroscopic tests, such as flexural, tensile and compression testing, have
been used to measure the hardness (H), elastic modulus (E) and toughness of
dental enamel (Craig et al. 1961; Rasmussen et al. 1976). Rasmussen et al. (1976)
used a three-point flexural test to study the fracture property of enamel. It was
found that fracture in enamel was highly anisotropic, with the weakest path of
fracture parallel to the enamel rods. Results from microhardness testing and
compression tests have demonstrated that the H and E of the outer enamel are
higher than those of the inner enamel (Craig et al. 1961; Meredith et al. 1996).
Vickers indentation measurements have shown that the H and E obtained from the
occlusal section are higher than those obtained from an axial section (Xu et al.
1998). Cuy et al. (2002) used the nano-indentation technique to map out the
mechanical properties of dental enamel over the axial cross-section of a maxillary
second molar. The authors found that local variations in mechanical properties
were strongly correlated with changes in chemical content and microstructure
across the entire depth of the enamel sample (Cuy et al. 2002). The outer enamel
surface had H > 6 GPa and E > 115 GPa, while at the enamel-dentine junction, the
H and E were measured < 3 GPa and < 70 GPa respectively.
35
While macroscopic tests provide results that reflect the overall mechanical
response of the enamel, micro- to nano-scale tests reveal information on the
hierarchical micromechanics (Fong et al. 2000). If the measured area is smaller
than the rod diameter, the elastic response would be dominated by the rod elastic
properties. However, if the measured area is larger than the rod diameter, both the
rods and the sheath structure play a role in the mechanical behaviour (He and
Swain 2008). Data discrepancy between macro- and micro/nano-indentation
testing on E is shown in Table 1-2.
Local microstructural features such as rod orientation also contribute to the
mechanical properties of dental enamel (Roy and Basu 2008). Staines et al. (1981)
found that enamel H was higher when the indentation direction was perpendicular
to the c-axis of the hexagonal hydroxyapatite crystals (parallel to the basal plane)
than when the indentation direction was coaxial. The study by Xu et al. (1998)
demonstrated that the fracture toughness of enamel can differ by a factor of three,
depending on the alignment of rods relative to the orientation of the crack.
Habelitz et al. (2001) reported that the H and E were lower in the tail area and
interrod enamel, compared to those in the head area of the enamel rod. The
authors suggested that the changes in crystal orientation and content of soft
organic tissues resulted in variations in the mechanical properties in these areas.
The mechanical anisotropy coupled to the natural variation in the structure of
enamel within an individual tooth make it impossible to quote a single value for
the H or E (Dickinson et al. 2007). Variations in mechanical properties are also
seen between teeth from different individuals due to a developmental cause or
exposure to different oral environments (e.g. fluoridation of water) (Vieira et al.
2005). Due to these complications, many studies examining the mechanical
properties of healthy enamel have considered only the average values.
36
Table 1-2 Elastic modulus of enamel from different measurement methods
Author(s)
Surface and site
Test method
Test load
Compressive
/
Occlusal surface
Craig et al. (1961)
84.1 ± 6.2
Cross section
Macro-scale tests
E (GPa)
77.9 ± 54.8
Staines et al. (1981)
Occlusal surface
Spherical indentation
Up to 800N
83 ± 8.0
Xu et al. (1998)
Occlusal surface
Vickers indentation
2-50N
/
Occlusal surface
Modified Vickers
indentation
1.9N
94 ± 5.0
Cuy et al. (2002)
Micro- and nanoscale tests
Cross section
Cross section
80 ± 4.0
Nano-indentation
Depth control (400
&800nm)
Outer-enamel
Fong et al. (2000)
EDJ
> 115
Nano-indentation
0.3-2.5mL
Occlusal surface
Habelitz et al. (2001)
Head of rod
47 - 120
< 70
98.3 ± 5.9
Nano-indentation
1.5mN
88.0 ± 8.6
37
Tail of rod
Barbour et al. (2003)
Occlusal surface
80.3 ± 7.2
Nano-indentation
3mN
99.6 ± 1.8
5mN
101.9 ± 1.6
7mN
105.2 ± 1.3
[Adapted from He et al. 2008]
38
1.5.3
Wear behaviour of enamel
Despite extensive knowledge of the histology and mechanical properties of
enamel, little is known about the wear behaviour of human enamel. Reviewing the
existing literature, it is found that limited detailed research has been conducted to
evaluate the friction and wear behaviour of human enamel.
Li and Zhou (2001) investigated the influence of the antagonist and lubrication on
the wear behaviour of human enamel, using a reciprocating wear test apparatus.
The enamel samples were opposed by two different antagonists (steel and pure
titanium) in dry and artificial saliva conditions. It was found that the depth and
severity of the wear scar were much smaller with artificial saliva lubrication than
in dry conditions and therefore, concluded that saliva plays an important lubricant
effect during the wear process of enamel in the oral environment. The enamel and
pure titanium friction pair showed a better tribological behaviour than the steel
and enamel pair, based on lower  and less severe wear scar observed with SEM.
This indicates that the antagonist material opposing a natural tooth affects the
wear behaviour of enamel and thus, an appropriate restorative material should be
carefully selected to minimise the wear of natural dentition.
Zheng and Zhou (2007) further explored the friction and wear behaviour of
human enamel at varying loads (10N, 20N and 40N) under two- and three-body
wear conditions. The results demonstrated that human tooth enamel exhibits lower
friction and smaller wear volume under three-body wear conditions than under
two-body wear conditions. Although increasing normal load resulted in a
progressive increase in the wear volume of enamel, this effect was lessened under
three-body wear condition. The enamel wear scar under three-body wear was
characterised by scratches accompanied by small pits, while ploughing and
delamination were dominant on two-body wear scar. This suggests that tooth
enamel experiences different wear mechanisms under different wear conditions.
39
The in vitro study by Zheng et al. (2011) investigated the wear behaviour of
human enamel in acidic environment, using a reciprocating wear testing apparatus.
The friction-wear tests were conducted in two mediums, citric acid solution
(pH=3.20) and artificial saliva, under varying loads (10N, 20N and 40N). In citric
acid solution, severe dissolution of the enamel rods was observed and the surface
hardness decreased. Under a low loading, the surface softening of enamel caused
by erosion played a significant role in its wear behaviour, and the wear
mechanism was dominant by adhesion delamination. Therefore, the enamel wear
in the citric acid solution was considerably higher than in the artificial saliva.
However, at high loading (40N) both the wear morphology and wear loss of
enamel in the two mediums were similar. The authors claimed that with the load
increasing, brittle fracture by the external loading aggravated and enamel wear
tended to be characterised mainly by mechanical removal due to its inherent
brittleness.
Arsecularatne and Hoffman (2010) used the techniques of focused ion beam (FIB)
milling and field emission scanning electron microscopy (FESEM) to examine the
process occurring below the wear scar. These techniques were considered to
expose the subsurface profile without inducing further cracking. In vitro
reciprocating wear tests with enamel cusp sliding on flat enamel specimen were
conducted under hydrated condition at varying loads (2N, 4N and 8N). Subsurface
cracking under the wear scar and formation of wear particles were observed
during enamel wear. The intensity of subsurface cracking increased with
increasing load (Arsecularatne and Hoffman 2010). From an analysis of the
experimental results available in the literature, the authors argued that the process
of enamel wear by fracture is similar to that taking place during wear of structural
ceramics. In a recently published paper, the authors further investigated the
subsurface wear process of enamel and claimed that the path followed by these
40
cracks seems to be dictated either by histological structure or by the contact stress
field (Arsecularatne and Hoffman 2012).
The in vitro studies evaluating the wear behaviour of enamel have shown that
high mineral content and corresponding H of enamel result in relatively low wear
rates at lower loads (Zheng and Zhou 2007; Zheng et al. 2011). However, the
brittle nature of the enamel contributes to higher wear rate at higher loads. In
contrast, dentine has a much higher organic content than enamel, composed of
70% (w/w) inorganic (hydroxyapatite crystallites), 18% (w/w) organic material
and the remaining 10% (w/w) being water (Lewis and Dwyer-Joyce 2005). Higher
organic content and relative softness of dentine make it less prone to fracture
under high loading conditions (Burak et al. 1999).
Zheng et al. (2003) investigated the differences in the friction and wear behaviour
of different human tooth structures (enamel, dentinoenamel junction-DEJ and
dentine), using a reciprocating wear testing apparatus with an artificial saliva
solution (Zheng et al. 2003). The results showed that the enamel zone exhibited a
lower  and better wear resistance compared to the dentine zone. The H decreased
as the test locations moved from the outer enamel to the inner dentine. The
enamel wear resulted from the microcracking induced mechanism and was
characterised by delamination, while dentine wear was caused by plastic
deformation and strong ploughing appeared on the worn surface. Therefore, the
authors concluded that the wear mechanisms differ between layers in the tooth
structure due to significant variations in H. Furthermore, tribological behaviour of
various tooth tissues is strongly influenced by microstructure orientations, such as
the course of enamel rods and dentinal tubules.
From the results of in vitro investigations on the wear behaviour of dental enamel,
it can be inferred that enamel is a unique, biocomposite that wears by brittle
fracture and/or surface delamination under sliding motion (Zheng and Zhou 2007;
41
Arsecularatne and Hoffman 2010). The tribological characteristics of enamel can
vary considerably as the process of enamel wear is taking place, due to differential
microstructure and mechanical properties in different locations within enamel
(Zheng et al. 2003).
42
1.6
Restorative materials
The wear resistance of dental restorative materials is crucial, as good wear
resistance contributes to stable and long-term restoration of occlusion (Ohkubo et
al. 2002). It is important to understand the underlying wear mechanism between
different wear pairs, as restorations can be in contact with the same or different
materials in clinical situations.
Dental restorative materials can be categorised into 4 groups: metal alloys,
composites, polymers and ceramics. In vitro wear studies on dental restorative
materials have established relative ranking of the wear resistance of different
restorative materials based on wear rate. In general, dental ceramics possess
excellent wear resistance, but may produce higher antagonist wear compared to
other restorative materials (Jacobi et al. 1991; Hudson et al. 1995; Jagger and
Harrison 1995; Olivera and Marques 2008).
1.6.1
Metal alloys
An alloy is a metallic solid formed by the combination of two or more metals
(Craig 1997; Wataha 2000). Alloys are used since pure metals do not posses
appropriate physical, biological and corrosion properties to serve adequately as a
restorative material in the oral environment (Wataha 2002). Dental alloys usually
contain at least 4 metals or more and thus are complex metallurgically (Wataha
2000). Based on microstructure (phase structure), alloys can be described as
single-phase or multi-phase. Single-phase alloys have essentially the same
composition throughout their structures, while multi-phase alloys have areas of
composition that vary by microstructural location (Wataha 2002). Microstructural
characteristics affect the mechanical, corrosion and etching properties of alloys.
43
The alloys used for full cast and metal-ceramic restorations can be classified into
four types based on mechanical properties, as shown in Table 1-3. The mechanical
properties of alloys are defined by yield strength and percent elongation.
Table 1-3 Classification of metal alloys based on physical properties and their
application in dentistry
Type
Hardness
Yield Strength
(MPa)
Elongation
(%)
Application
І
Soft
<140
18
Inlays for non stress-bearing
areas
ІІ
Medium
140-200
18
Inlays or onlays in stressbearing areas
ІІІ
Hard
201-340
12
Full or partial crowns, shortspan FPDs
ІV
Extrahard
> 340
10
Full crowns, long-span FPDs,
removable partial dentures
[Adapted from ANSI/ADA specification no. 5 for dental casting alloys]
Generally, most metal alloys possess sufficient strength to withstand maximum
masticatory force in the oral cavity. Dental amalgam and cast gold, are the
restorative material of choice for stress-bearing restorations due to their strength
and durability (Sulong and Aziz 1990).
1.6.1.1 Amalgam alloys
Dental amalgam is a widely used direct restorative material for stress bearing
areas due to its excellent resistance to occlusal wear (DeLong et al. 1985).
Quantitative clinical studies reported that dental amalgam typically loses 200μm
44
in contact areas and 24μm in non-contact areas after four years in the mouth
(Lambrechts et al. 1984; Lambrechts et al. 1985).
Meuller et al. (1985) investigated the wear behaviour of amalgam opposing
enamel, using a pin-on-disk wear testing device (Mueller et al. 1985). The authors
demonstrated smear of dental amalgam on the opposing enamel in the form of
silver or mercury. It was suggested that dental amalgam wears by adhesion
against enamel and that the adhesive wear mode contributes to wear facets in the
occlusal surfaces of these restorations. Subsequent in vitro studies confirmed the
adhesive wear mechanism of dental amalgam with the use of SEM and energy
dispersive analyses (DeLong et al. 1985; Ratledge et al. 1994). The in vitro study
by DeLong et al. (1985) reported the  of 0.156 for the enamel and amalgam wear
couple (DeLong et al. 1985). The low  value indicated that the friction force was
overcome by shallow slippage and shear within the amalgam.
1.6.1.2 Gold based alloys
Cast gold (high-noble metal) restorations have a long history of satisfactory
clinical performance. They are said to be kind to opposing tooth substance and
restorative materials wearing at approximately the same rate as enamel (Ramp et
al. 1997; Yip et al. 2004). Type ІІІ gold has been used as the reference material in
many in vitro studies, evaluating wear resistance of different or novel indirect
restorative materials (Jacobi et al. 1991; Olivera and Marques 2008).
The in vitro studies on the wear behaviour of gold have demonstrated that gold
smeared against its antagonist during wear experiments and thus concluded that
gold wears by adhesion (Monasky and Taylor 1971; Ramp et al. 1997). Ramp et
al. (1997) claimed that high ductility associated with gold alloys due to mobility
of line and point defects allows gold atoms to slide against each other and absorb
occlusal forces to decrease the amount of wear on the opposing enamel (Ramp et
45
al. 1997). In addition, the thin layer of gold adhered on the antagonist surface may
serve as a solid lubricant, protecting the enamel from further friction and wear
damage.
1.6.2
Composite resin
Poor wear resistance of early composite resins was the major drawback that
restricted its use in stress-bearing regions of the dental arch (Sulong and Aziz
1990). Hence, a great deal of research has been dedicated to understand and
optimise the wear resistance of composite resins. Improvements in the material
properties together with their positive clinical performances have encouraged the
use of composite resins as a viable alternative to amalgam in clinical situations
(Suzuki 2004; Lambrechts et al. 2006). Generally, the wear behaviour of
composite can be associated with either material or clinical factors (Lambrechts et
al. 2006). Material factors relate to the characteristics, content, and distribution of
filler, the degree of conversion and interfacial bond between matrix and filler. In
addition, the silane coupling, the nature of matrix and surface hardness also
influence the wear resistance of composite materials.
Condon and Ferracane (1997) investigated the effects of compositional factors
including the degree of cure, filler level and silanation level on the wear resistance
of three different experimental composites. The authors found that greater
composite wear was correlated with lower filler levels and reduced percent of
silane-treated fillers (Condon and Ferracane 1997). Correlation between degree of
cure and composite wear was not as prominent as the other factors. However, the
in vitro study by Ferracane et al. (1997) demonstrated a strong negative
correlation between the degree of cure and the wear of the same experimental
composites. It was suggested that longer curing time caused the polymer network
to be more highly crosslinked, and therefore more resistant to the forces of
abrasion. The results indicate that maximum wear resistance requires that the
composite be cured to its maximum amount (Ferracane et al. 1997).
46
The wear behaviour of dental composites is also related to wear conditions (Hu et
al. 2002; Zhou and Zheng 2008). Abrasion from food particles and chemical
degradation appear to be dominant on the contact-free areas, while fatigue and
adhesive mechanisms dominate on the occlusal contact wear (Mair et al. 1996).
Hu et al. (2002) compared the wear behaviour of three different composite resins;
P-50 (3M Dental products, St Paul, USA), Z100 (3M Dental products) and Silux
Plus (3M Dental products) under two- and three-body wear conditions. The results
showed that the volumetric wear losses had different rankings between the two
wear conditions, indicating that composites experienced different wear
mechanisms under different wear conditions (Hu et al. 2002). It was concluded
that testing conditions have a highly significant influence on deciding the relative
wear resistance and behaviour of composite materials.
The chemical environment of the oral cavity has a significant effect on the in vivo
wear behaviour of composite resins. The resin matrix can be softened and filler
particles can be leached out when composites are exposed to certain chemicals or
food simulating liquids (Yap et al. 2002). Gan et al. (2012) studied the friction
and wear behaviour of five different composites under different simulated oral
environments; distilled water, artificial saliva and a cola soft drink, using a
reciprocating horizontal wear testing apparatus. The results showed that the
artificial saliva and cola drink lowered the and wear loss of dental composites
compared to distilled water lubrication (Gan et al. 2012). The cola drink displayed
a better lubrication effect than artificial saliva and the authors suggested that
better surface wettability and higher viscosity of a cola drink contribute to a better
lubricating effect than other media. The results also demonstrated that the hybrid
resin composites with high filler weight and good filler distribution possessed
better wear resistance than nano-filled composites with relatively low filler weight.
Abrasive wear was the main wear mechanisms for the five tested composites, but
47
brittle cracks and delamination were more common for composites with relatively
lower filler weight.
In conclusion, the wear and friction behaviour of dental composites are functions
of microstructure, material property and test conditions. Design of operating
condition and selection of an appropriate composite material are key to reducing
the contact wear of dental composites (Gan et al. 2012).
1.6.3
Dental ceramics
Ceramics are key materials for dental restorations due to their high wear
resistance, biocompatibility and, above all, aesthetics (Kelly 1997). With
advancement in ceramic technology, diverse types of dental ceramics with various
fabrication methods have been developed (Table 1-4). Essentially, ceramics are a
composite material in which the glassy matrix is filled with various amounts of
crystalline or glass particles (Kelly 2008). Dental ceramics can be divided into
three main groups, based on the composition:
-
Predominantly glass
-
Particle filled glass
-
Polycrystalline
Silicate and glass ceramics are used as a veneer for metal-ceramic or all-ceramic
crowns to optimise the contour and aesthetics (Guess et al. 2011). They can also
be used in monolithic forms to fabricate small-sized restorations such as inlays,
onlays and laminate veneers. High-strength ceramics such as alumina and zirconia
were developed as a core material for crowns and fixed dental prostheses (FDP) to
extend the application to the high-load bearing areas (Conrad et al. 2007).
Particularly, yttria-stabilised tetragonal zirconia polycrystal (Y-TZP) has become
a popular core material for all-ceramic FDPs, crowns and implant abutment for
both the anterior and posterior of the mouth due to its high flexural strength (9001200 MPa) and fracture toughness (9-10 MPam1/2) (Christel et al. 1989).
48
Table 1-4 Dental ceramics for all-ceramic systems
Ceramic
Predominantly
glass
Fabrication
method
Products
Manufacturer
Strength
(MPa)
CAD/CAM
Vitablocs
Mark ІІ
Vita Zahnfabrik, Bad
Sackingen, Germany
154
Hand
layering
IPS eris
Ivoclar Vivadent
AG, Schaan,
Liechtenstein
Heat
pressing
IPS Empress
Ivoclar Vivadent AG
CAD/CAM
IPS Empress
CAD
Ivoclar Vivadent AG
Heat
pressing
IPS e.max
Press
Ivoclar Vivadent AG
360
CAD/CAM
IPS e.max
CAD
Ivoclar Vivadent AG
400
Glass infiltrated
alumina oxide
Slip casting
In-Ceram
Alumina
Vita Zahnfabrik
500
Glass infiltrated
alumina oxide with
35% zirconia
Slip casting/
Milling
In-Ceram
Zirconia
Vita Zahnfabrik
421-800
Alumina
Densely
sintered
Procera
AllCeram
Nobel Biocare AB,
Göteborg, Sweden
487-699
Zirconia
CAD/CAM
Lava,
3M ESPE, Seefeld,
Germany
900-1200
Material
Feldspathic
porcelain
Leucite reinforced
(SiO2Al2O3K2O)
Lithium disilicate
(SiO2Li2O)
Particle-filled
glass
160
Polycrystalline
[Adapted from Kelly 2008]
49
1.6.3.1 Wear behaviour of dental ceramics
Numerous in vivo and in vitro studies have reported increased wear of enamel
when opposed by dental ceramics compared to by metal alloys and composite
resin restorative materials (Ratledge et al. 1994; Jagger and Harrison 1995;
Olivera and Marques 2008). Dental clinicians are thus cautioned of the use of
ceramic occlusal surfaces in extensive full mouth rehabilitation treatment (Wiley
1989; Ratledge et al. 1994; Oh et al. 2002). Most ceramics have comparatively
higher H values than human enamel and the H of a ceramic has been used as a
predictor of its potential to abrade opposing teeth (Kelly et al. 1996). However,
scientific studies have demonstrated poor correlation between material H and the
abrasive potential of ceramic materials on human enamel (Seghi et al. 1991; Oh et
al. 2002). Therefore, the H value alone is not a reliable predictor of wear on the
opposing enamel. Instead, ceramic microstructure, surface characteristics and
condition of the oral environment appear to be strongly associated with the wear
process (Kelly et al. 1996).
Although numerous in vitro wear studies have been conducted on dental ceramics,
few studies have provided detailed information on the wear behaviour of a range
of dental ceramics used for all ceramic systems at present. Investigations for
engineering ceramics are quite advanced through establishing comprehensive
wear maps, possibly due to a requirement to define the necessary conditions for
maintaining the high wear resistance and smooth surface during operation (Adachi
et al. 1997).
Jahanmir and Dong (1995) investigated the wear behaviour of a machinable glass
ceramic (Dicor MGC, Dentsply International Inc., York, USA), using a
reciprocating tribometer against alumina ball antagonist. Scratch marks in the
direction of the motion and formation of loose wear debris were observed on the
wear scar and indicated that microfracture process along the mica-glass interface
50
was the dominant wear mechanism. The authors suggested that the wear of Dicor
MGC was controlled by short-crack toughness as well as the size and volume
fraction of the mica plates (Jahanmir and Dong 1995). Nagarajan and Jahanmir
(1996) further explored the effect of mica plate size on the wear behaviour of
Dicor MGC, using a pin-on-disk tribometer under distilled water lubrication
(Nagarajan and Jahanmir 1996). Despite different wear testing conditions, the
wear mode was dominated by microfracture process similar to that observed by
Jahanmir and Dong (1995). However, there was a clear transition of wear from
localised fracture at low load to contact or spallation mode at high loads. The
wear rate increased with the increase in the mica plate diameter and therefore it
was concluded that the mica plate diameter plays an important role in controlling
the wear mode and wear transition of glass ceramics.
Yu et al. (2006) investigated the friction and wear behaviour of two different
dental porcelains; lab processed (Vita VMK95, Vita Zahnfabrik, Bad Sackingen,
Germany) and machined feldspathic porcelains (Cerec Vitablocs Mark ІІ, Vita
Zahnfabrik), against a Si3N4 ball antagonist using a reciprocating horizontal
tribometer. The wear pairs were subjected to varying loads (10-40N),
reciprocating amplitudes (100-500mm) and frequencies (1-4Hz) under dry and
artificial saliva lubrication to evaluate the effects of these variables on the wear
behaviour of the tested porcelains. Laboratory-processed Vita VMK95 possessed
a lower  and better wear resistance. Among the test parameters, the load effect
was the most prominent. Abrasive wear was suggested as the main wear
mechanism for both porcelains, but brittle cracks and delamination were observed
on the machined Cerec Vitabloc Mark ІІ, especially under un-lubricated
conditions.
Environmental conditions also affect the wear behaviour of dental ceramics.
Acidic environment caused by dietary habit and intrinsic gastric disease may
result in the degradation of ceramic surfaces in the oral cavity (Oh et al. 2002).
51
Surface degradation increases the abrasivity of ceramics against opposing teeth or
restorations by continuously exposing a rough surface. The in vitro study by AlHiyasat et al. (1997) demonstrated that the exposure to acidic solution (a
carbonated beverage) significantly increased the enamel wear produced by
ceramic samples.
Molla et al. (2009) investigated the influence of environment on the friction and
wear behaviour of a new type of a glass ceramic (K2O–B2O3–Al2O3–SiO2–MgO–
F glass-ceramic containing about 70% crystalline content) proposed for use as a
restorative material. The authors found that lubrication with artificial saliva
significantly reduced the  (0.67) compared to that in dry condition (= 0.88).
The tribochemical wear with crystal phase pull outs was dominant in dry
conditions, while formation and brittle fracture of the tribochemical layer was
more pronounced in lubricated condition.
Surface flaws and roughness induced during ceramic processing or clinical
adjustments reduce strength and may lead to abrasive wear of opposing dentition
(Magne et al. 1999). A smooth, polished surface exhibits a lower  and reduces
antagonistic wear. Early researchers agreed that surface glazing (re-glazing after
clinical adjustments) of ceramic restorations was necessary to provide a smooth
and glossy surface, by sealing the surface pores or defects (Barghi et al. 1976).
Many dentists therefore, preferred the porcelain surface of a ceramic restoration to
be glazed (or re-glazed) prior to cementation (Newitter et al. 1982). Numerous
studies have been conducted to evaluate different finishing and polishing
techniques that would produce surfaces as smooth or smoother than glazed
porcelain (Haywood et al. 1989; Scurria and Powers 1994). Results of the studies
are inconsistent as some authors reported a glazed surface is superior to polished
surface (Chen et al. 1999), while others found otherwise (Jagger and Harrison
1994).
52
Extensive research efforts have been made to develop high-toughness dental
ceramics in order to minimise the damage by brittle fracture. Recently, yttriastabilised tetragonal zirconia polycrystal (Y-TZP) has been developed as an
alternative to metal-ceramic crowns or glass ceramics in posterior restorations
(Raigrodski 2004).
In a paper published by Albashaireh et al. (2010), wear behaviour of five different
dental ceramics (Y-TZP, lithium disilicate (LS2), leucite-reinforced (LEU),
fluorapatite (FGC), and nanofluorapatite (NFGC) glass ceramics) were
investigated using a commercial chewing (Willytec GmbH, Munich, Germany)
simulator against a standardised zirconia ball. The wear rate of the five ceramic
materials in the order of descending wear rate was: Y-TZP, LS2, LEU, FGC and
NFGC. The wear rate correlated with the difference in the flexural strength and
fracture toughness; Y-TZP with the greatest flexural strength and fracture
toughness displayed a significantly lower wear rate than the other ceramics.
Observation of the wear scars showed no cracks or flaws on Y-TZP and LEU,
indicating a mixture of attritional and adhesive wear, while the other dental
ceramics exhibited cracks and chipped particles, consistent with the fatigue wear
mechanism.
From the current literature, it is clear that the wear mechanisms of different dental
ceramics differ significantly, due to differences in microstructure, physical
properties and the in vitro testing conditions (e.g. antagonist, load, sliding speed
etc) used. Table 1-5 summarises the different wear mechanisms of various dental
ceramics discussed in this review.
53
Table 1-5 Studies included in the review
Dental Ceramic
Study
Antagonist
Wear Condition
Load: 4.9N
Dicor MGC
Jahanmir and
Dong (1995)
Conclusion
Stroke length: 5mm
 Wear mechanism: micro-fracture along micaglass interface
Cycles: 500
 Wear controlled by altering the microstructure
Alumina Ball
Humidity: 52-63%
Load: 10-40N
Feldspathic
porcelains (Vita
VMK 95 and
Cerec Vitablocs
Mark2)
Stroke length: 100-500mm
Yu et al. (2006)
Si3N4 Ball
 Wear mechanism: abrasion- brittle fracture and
delamination more frequent for Cerec Vitabloc
Mark2 than Vita VMK 95
Cycle: 10,000
 Load effect prominent
Frequency: 1-4 Hz
 Saliva: lubricating effect
Lubricated (artificial saliva)
54
K2O–B2O3–
Al2O3–SiO2–
MgO–F GC
containing about
70% crystalline
Load: 1N
Stroke: 100mm
Molla et al. (2009)
Steel Ball
Frequency: 8Hz
Dry/Lubricated (artificial saliva)
LS2
Load: 49N
FGC
 In dry condition: mica-crystal pull-out
 In saliva: formation and brittle fracture of
tribochemical layer
 Wear mechanism: abrasive and attritional wear
on Y-TZP and LEU
Y-TZP
LEU
 Wear mechanism: microfracture and
tribochemical wear
Albashaireh et al.
(2010)
Y-TZP ball
Cycles: 300,000
 Wear mechanism: fatigue wear on LS2, FGC and
NFGC
 Physical properties and microstructure influence
the wear rate
NFGC
 Greater wear on FGC and NFGC than the other 2
glass ceramic (LEU, LS2)
55
1.7
Significance of the research
It has been recognised that tooth wear is a clinical problem that is becoming
increasingly important in the aging population (Mair et al. 1996; Zhou and Zheng
2008). The management of tooth wear requires a proper understanding of the
underlying mechanisms by which it occurs and the controlling factors involved in
the wear process (Mair et al. 1996). Tribology is the study of friction, wear and
lubrication of interacting surfaces in relative motion (Hutchings 1992).
Fundamentals of tribology have been applied in dental research to broaden the
understanding of friction and wear behaviour of dental biomaterials in oral
biomechanical function (Mair 1999; Zhou and Zheng 2008).
A range of restorative materials are available in dentistry to restore the function
and aesthetics of the missing teeth or tooth structure. The proper selection of
restorative materials is vital to preserve normal masticatory function and occlusal
harmony. Gold based alloys have been considered the most ideal indirect
restorative material for stress bearing areas due to their excellent wear resistance,
durability and minimal antagonistic wear (Hacker et al. 1996; Yip et al. 2004).
Dental ceramics have become popular as an alternative to gold based alloys due to
their superior aesthetic potential and biocompatibility (Piddock and Qualtrough
1990; Elmaria et al. 2006). However, the main shortcomings of ceramics are their
propensity to abrade and fracture on opposing dentition or restorations (Oh et al.
2002). Previous in vitro studies demonstrated that antagonistic enamel wear by
dental ceramics is substantially greater than by gold or composite resin materials
(Jagger and Harrison 1995; Olivera and Marques 2008). In an attempt to optimise
the mechanical and wear properties of dental ceramics, a range of new dental
ceramics have been developed and introduced in the market. Therefore, there is a
need for continuous in vivo and in vitro studies to characterise and predict their
clinical wear behaviour more accurately.
56
This research project will evaluate and compare the friction and wear behaviour of
human tooth enamel opposing two different indirect restorative materials; lithium
disilicate glass ceramic (IPS e.max Press) and type ІІІ gold using a reciprocating
sliding wear testing apparatus. Results from this project will provide insight into
the underlying wear mechanisms of human enamel opposing the two different
materials.
57
2. MATERIALS AND METHODS
2.1
Collection of teeth
Category A ethical approval was granted from the University of Otago Ethics
Committee for the collection and testing of extracted human teeth for this study
(Reference number: 10/113). The information sheet and consent form were
distributed to the Oral Surgery clinic, to recruit donors for extracted teeth. The
operating dentists in the Oral Surgery clinic were informed about the research and
asked to provide the information sheet and the consent form to potential
participants prior to the extraction procedure. Patients agreeing to donate their
extracted tooth/teeth were asked to sign a consent form before proceeding with the
extraction procedure. Strict inclusion and exclusion criteria were specified for the
condition of the extracted teeth as shown in Table 2-1.
Table 2-1 Inclusion and exclusion criteria for condition of extracted human teeth
Criteria
Inclusion
Condition
Healthy permanent teeth (molars/premolars)
Donor age between 18 and 30 years old
Absence of restorations, enamel defects and caries
Exclusion
Previously restored with fillings
Presence of hard tissues defects (e.g. hypomineralised enamel)
Donor age > 30 years old and <18 years old
The extracted teeth were cleaned under running tap water and any teeth with
caries or obvious defects under visual inspection were discarded from the study.
Participants’ information (date of birth and gender) was recorded on the container
58
in which the extracted tooth was placed, to ensure that the specified inclusion
criteria for participants were met. The collected teeth were stored in Hanks’
balanced salt solution (HBSS) to avoid dehydration and to maintain enamel
hardness until prepared as suggested by Sajewicz (2009). The composition of the
HBSS (Gibco® Life Technologies, Ghent, Belgium) used for this study is
provided in Table 2-2. The chemical potential of HBSS for dissolving the calcium
phosphate phases in tooth enamel is low and therefore, surface demineralisation is
prevented during storage (Habelitz et al. 2002).
Table 2-2 Composition of HBSS used for storage of the extracted teeth
Components
Conc (mg/L)
mM
Potassium Chloride (KCl)
400
5.33
Potassium Phosphate Monobasic (KH2PO4)
60
0.441
Sodium Bicarbonate (NaHCO3)
350
4.17
Sodium Chloride (NaCl)
8000
137.93
48
0.338
1000
5.56
10
0.0266
Inorganic Salts
Sodium Phosphate dibasic (Na2HPO4) anhydrous
Other Components
D-Glucose (Dextrose)
Phenol Red
The collected teeth (permanent premolars and/or molars) were examined under
the light microscope (Alphaphot YS2, Nikon, Tokyo, Japan) at x10 magnification
for any enamel defects or damage that may not have been detected under visual
59
inspection. Developmental defects or damage produced during the extraction
procedure could alter the mechanical properties of enamel, possibly affecting the
experimental results. Therefore, only teeth with no obvious visible damage were
selected for testing, which was completed within three months of extraction. Six
permanent premolars obtained from different young individuals aged between 18
and 30 years old, were used in this study.
2.2
Enamel specimen preparation
The roots of the collected premolars were sectioned off along the cementumenamel junction (CEJ) using a diamond cutting wheel (33CA, Struers,
Copenhagen, Denmark) attached to a high speed cutting machine (Accutom-50,
Struers). The dental pulp was removed with a spoon excavator. The crowns were
then sectioned in half in the mesio-distal direction using the high speed cutting
machine (Accutom-50, Struers). The cutting was performed at a speed of 3000
rpm under copious water irrigation to minimise the influence of generated heat,
which could result in dehydration and changes in microstructure and chemistry of
the teeth.
The lingual and buccal halves were then embedded in cold-curing epoxy resin
(Epofix, Struers) with the enamel surface facing up. Autopolymerising resin (GC
Pattern Resin, GC Corp, Tokyo, Japan) was initially used to stabilise the crown
and to expose a 3mm x 3mm enamel surface on a clear glass slide. The enamel
specimens were ground using abrasive papers (TegraPol-21, Struers) from 1000,
1500, to 2500 grit and then polished with 6μm followed by 1μm diamond
suspensions (DiaPro, Struers). The polished enamel specimens were cleaned
ultrasonically to remove any surface debris and stored in HBSS until used.
Ten flat enamel specimens were obtained from five permanent premolars and they
were randomly assigned into two groups; two-body and three-body wear testing,
as shown in Fig. 2-1. Each enamel specimen was opposed by two different
60
restorative samples; IPS e.max Press and gold. It was ensured that the wear scars
on the same enamel specimen were 2mm apart to prevent overlapping of the wear
zones and possible fatigue effects.
Fig. 2-1 Diagram showing tooth grouping for experiments
61
2.3
Hardness and elastic modulus measurement
The H and E values of each enamel specimen was measured before wear testing.
The enamel specimens were indented using a nano-based indentation system
(Ultra Micro Indentation System, UMIS-200, CSIRO, Forestville, Australia). The
IBIS Image software (Version 001.000.716) was used to control the system and
the test parameters. The nano-indentation tests were conducted at a load of 25mN
with a calibrated Berkovich indenter (Table 2-3). The maximum load was held for
10 seconds to minimise the effect of creep on the unloading curve.
Table 2-3 Test parameters applied for the nano-indentation testing
Maximum load:
25mN
Hold @ maximum:
10.0 (s)
Start delay:
10.0 (min)
Position:
9 indents in an array of 3 x 3
50μm intervals
The resin block containing the flat enamel specimen was mounted on the stage of
the UMIS. Once the desired surface of the enamel specimen was identified by the
attached microscope, the nano-indentation testing was initiated by the software
programme. The programme calibrated the position of the indenter tip prior to
commencing the indentation process. The force (F) and displacement (D) data
were acquired over the course of the loading-unloading indentation cycle. Using
this data, the software calculated the H and E values of each enamel specimen by
Oliver-Pharr analysis method (Oliver and Pharr 1992). Images of the indented
regions on the enamel specimen were captured by the IBIS Image software
connected to the light microscope fitted to the nano-indentation machine. This
ensured that the indents were made on the enamel surface.
62
2.4
Restorative specimen preparation
Specimens (n=5 per material) were prepared from two different indirect
restorative materials; heat pressable lithium disilicate glass ceramic (IPS e.max
Press) and type ІІІ gold. The basic properties of the restorative materials tested are
summarised in Table 2-4.
Table 2-4 Basic properties of the restorative materials used for wear testing
Maxigold
IPS e.max Press
Type
Type ІІІ gold
Glass ceramic
Manufacturer
Ivoclar Vivadent Inc.
Ivoclar Vivadent Inc.
Composition
Au (59.5%), Pd (2.7%),
Ag (26.3), Cu (8.5%), Zn
(2.7), In < 1.0%,
Ir <
1.0%
SiO2 (57-80%), Li2O (1119%), K2O (0-13%), P2O5
(0-11%), ZnO (0-8%), MgO
Al2O3, and other oxides
Firing temperature (°C)
940-1000
915-920
Vickers hardness (MPa)
150
5800
Elastic modulus (GPa)
86
95
Flexural strength (MPa)
-
360
0.2% Proof stress (MPa)
310
N/A
63
2.4.1
IPS e.max Press specimen preparation
IPS e.max Press specimens were fabricated using the lost-wax technique. Sphere
ended titanium (Cp4Ti) rods with 4mm diameter were available (Fig. 2-2a) and
these were used to fabricate a mould with polyvinylsiloxane putty material
(Express STD, 3M ESPE, St. Paul, MN, USA) (Fig 2-2b). Molten blue modeling
wax (S-U-Gnatho-Wax Blue, Schuler Dental, Ulm, Germany) was poured into the
moulds to produce sphere ended wax patterns (Fig. 2-2c). These wax patterns
were invested and removed by heat. The void was filled with the IPS e.max Press
ingot and pressed at 925
in a combination furnace (Programat EP 5000; Ivoclar
Vivadent AG.).
Fig. 2-2 (a) Sphere ended titanium rods, (b) silicone mould made with
polyvinylsiloxane putty material (c) sphere ended wax patterns with diameter of
4mm
The ceramic specimens were autoglazed by heating them to the maximum glazing
temperature of 800
and holding the temperature for 1 minute in the furnace.
The finished ceramic specimens were examined under a light microscope
(Alphaphot YS2, Nikon) at x20 magnification to screen for any surface cracks and
flaws. 10 specimens were prepared for each group, of those, 5 showing no visible
defects were selected for further study.
64
2.4.2
Type ІІІ gold specimen preparation
The conventional lost wax technique was used to fabricate the gold specimens.
The sphere ended wax patterns were made from the same mould used for the
ceramic specimens. The wax patterns were invested and cast according to the
manufacturer’s instructions. The gold specimens were polished using brown and
green abrasive wheels (Shofu Dental Corporation, Menlo Park, USA) to provide a
smooth contact surface. The finished gold specimens were examined under a light
microscope at x20 magnification to screen for any surface defects. The specimens
with visible surface defects were discarded from the study.
Images of completed gold and ceramic specimens are shown in Fig. 2-3 (a) and
(b) respectively.
Fig. 2-3 Representative images of (a) a polished gold specimen (b) a glazed IPS
e.max Press specimen
65
2.5
Description of tooth wear machine
Fig. 2-4(a) schematically illustrates the wear testing apparatus used for this study.
The device consists of two main components; the fixed upper (maxillary) and the
movable lower (mandibular) compartments. The restorative specimens were
secured in the upper specimen holder by a laterally-directed screw. The spherical
end of the restorative specimens made a point contact with the opposing flat
enamel sample, establishing a ball-on-flat contact geometry. This geometric
arrangement was considered to be reasonably representative of the masticatory
function when investigating the occlusal surface wear of enamel (Arsecularatne
and Hoffman 2010). The flat enamel specimens were glued onto the aluminum
plates, which were bolted to the base of the lower compartment. The position of
the aluminum plates on the lower compartment could be adjusted, allowing
multiple wear tests on the same enamel specimen. Each enamel specimen was
opposed by two different restorative samples with the interval of 2mm between
the wear scars. The lower compartment moved back and forth on a low-friction
platform, driven through the crank-and-rocker mechanism by the gear motor. The
upper compartment was designed to support weights for applying loads to the
specimens.
The linear variable differential transformer (LVDT) was used to measure the
linear displacement of the lower compartment on the low-friction platform (Fig.
2-4a). The load cell attached to the lower compartment converted the frictional
force into a measureable electrical output. The load cell was calibrated with a
series of standard weights prior to wear testing and the baseline frictional force
from sliding of the lower compartment was measured and deducted to obtain a
true frictional force. Variations in frictional force and linear displacement were
automatically recorded with the help of a computer-based data acquisition system
(PowerLab, ADInstruments Pty Ltd., Bella Vista, Australia). This data was used
66
to calculate the friction coefficient values for different wear pairs; enamel-gold
and enamel-ceramic, as a function of cycles.
A static load of 1kg (9.8N), with a reciprocating amplitude of ~200 μm and
frequency of 1.6Hz were used for the wear testing. Tests lasting up to ~1,100
cycles were conducted. Distilled water was used as the lubricant for the two-body
wear tests, while all the three-body friction-wear tests were undertaken using a
food simulation slurry that contained PMMA beads.
Fig. 2-4 Illustration of methods and materials: (a) Schematic view of the wear
testing device; (b) A view of the wear testing machine
67
2.6
Methods of assessment
2.6.1
Scanning electron microscopy (SEM) observations
Following completion of the wear testing, the enamel wear scars were examined
under SEM (S360, Cambridge Instruments, Cambridge, UK). The enamel
specimens were carbon-coated for SEM observation. Each enamel specimen was
first observed at low magnification (x115) to give an overall picture of the wear
scar. Subsequent details of the wear scar were closely examined at different
magnification (x500, x1500). Images of the SEM scans were captured using the
Dindima Image Slave, and were 1024 x 768 pixels in size in a TIFF file format
(Tagged Image File Format).
2.6.2
Statistical analysis
The  values were presented as the mean ± standard error of the mean. Data were
analysed using statistical software (SPSS 16.0 for Windows; SPSS, Inc, Chicago,
USA). A paired t-test was performed to compare the average steady-state  values
between enamel opposing IPS e.max Press and type ІІІ gold under the same wear
condition. Two-way analysis of variance (ANOVA) was used to determine the
effects of the material and/or wear condition on the value during wear testing.
The significance level for all tests was set at p < 0.05.
68
3. RESULTS
3.1
Microhardness characterisation
An exemplar force-displacement curve generated by the nano-indenter is shown in
Fig. 3-1. The first indent was used to calibrate the position of the indenter tip and
therefore, the measurements from the first indent were excluded from subsequent
calculation of the average hardness (H) and elastic modulus (E) values. The forcedisplacement curves from the eight indents generally overlapped with each other,
indicating a high level of reproducibility. Fig. 3-2 shows a microscopic image of
the indents on the enamel surface. The first indent made for calibration appears to
be deeper and wider than the rest of the indents (red arrow in Fig. 3-2).
Table 3-1 presents the average H and E values for each enamel specimen which
was randomly assigned to two different wear testing conditions; two- and threebody wear conditions. The average H values ranged between 4 and 5.5 GPa, while
the average E values varied in the range between 90 and 130 GPa. These values
correspond to the normal range of enamel reported in the literature (Cuy et al.
2002; Zhou and Hsiung 2007) and thus confirmed that the preparation of the
enamel specimens did not breach the enamel layer.
69
Fig. 3-1 Typical force-displacement curve for a nano-indentation testing in an
enamel specimen as shown on the IBIS Image software programme
Fig. 3-2 Representative microscopic image of indentations on the enamel
specimen
70
Table 3-1 Average hardness (GPa) and elastic modulus (GPa) values for each
enamel specimen
Two-body
wear
Three-body
wear
Hardness (GPa)
Elastic Modulus (GPa)
①
5.33 ± 0.99
124.17 ± 9.96
②
4.65 ± 0.17
110.81 ± 5.22
③
5.08 ± 0.22
109.09 ± 6.78
④
4.68 ± 0.21
106.99 ± 3.98
⑤
4.92 ± 0.20
100.25 ± 7.09
①
4.25 ± 0.34
91.55 ± 7.87
②
4.71 ± 0.38
102.65 ± 6.98
③
4.89 ± 0.24
114.00 ± 4.72
④
4.69 ± 0.18
107.41 ± 10.10
⑤
4.09 ± 0.32
121.05 ± 4.47
71
3.2
Friction behaviour of enamel opposing IPS e.max
Press and type ІІІ gold in different wear conditions
Fig. 3-3(a) shows typical variations in the friction coefficient (μ) as a function of
the number of cycles for the two wear pairs under the two-body wear condition
(distilled water lubrication). The blue dots represent the enamel and IPS e.max
Press wear pair, while the red dots indicate the enamel and type ІІІ gold wear pair.
There is a large difference in the evolutionary pattern of the μ between the two
wear pairs.
For the enamel and IPS e.max Press wear pairs, the μ was usually low at the early
wear stage, but it increased sharply to 0.5 within the first 100 cycles. The increase
in the μ continued steadily up to 400 cycles and thereafter, it attained a steadystate stage. In comparison, the μ for the enamel and type ІІІ gold wear pairs
increased slowly but steadily up to 500 cycles and thereafter, it reached a
relatively steady-state stage.
The average steady-state μ value for the enamel and IPS e.max Press wear pair
after 500 cycles was calculated to be 0.64 ± 0.039, while for the enamel and type
ІІІ gold wear pair, the  value was 0.25 ± 0.052 (Table 3-2). The paired t-test
revealed that the steady-state μ value after 500 cycles was significantly higher
when the enamel was opposed by IPS e.max Press than when opposed by type ІІІ
gold (p = 0.0091).
Fig. 3-3(b) shows typical variations in the  as a function of the number of cycles
for the two wear pairs under the three-body wear condition (food slurry composed
of PMMA beads). The blue dots again represent the enamel and IPS e.max Press
wear pair, while the red dots indicate the enamel and type ІІІ gold wear pair.
72
For the enamel and IPS e.max Press wear pairs, the  was low in the initial wear
stage and then increased sharply to 0.5 within 100 cycles. Following the sharp
increase, however, the  remained relatively unchanged. The average steady-state
μ value after 500 cycles for the enamel and IPS e.max Press wear pairs was 0.56 ±
0.071 under the three-body wear condition. This was slightly lower than that
observed under the two-body wear condition (0.63 ± 0.039).
For the enamel and type ІІІ gold wear pairs, the  increased steadily up to 500
cycles and thereafter, the  reached an almost steady-state stage. The average
steady-state value for this wear pair after 500 cycles reached 0.40 ± 0.058,
which is approximately 60% higher than that (0.25 ± 0.052) measured under the
two-body wear condition.
The paired t-test revealed that the steady-state μ value after 500 cycles was not
significantly different between the enamel opposing IPS e.max Press and type ІІІ
gold (p = 0.2344) under the three-body wear condition as shown in Table 3-2.
73
(a)
(b)
Fig. 3-3 Evolution of friction coefficient () as a function of number of cycles for
the two wear pairs under (a) the two-body and (b) the three-body wear conditions
74
Table 3-2 Average steady-state  values after 500 cycles for the two wear pairs in
the different wear conditions
Wear condition
Two-body
Three-body
IPS e.max Press
0.64 ± 0.039
0.56 ± 0.071
Type ІІІ gold
0.25 ± 0.052
0.40 ± 0.058
p
0.0091
0.2344
Materials
Outcome of the two-way ANOVA and corresponding description are summarised
in Table 3-3. The results of the two-way ANOVA revealed that the type of
restorative material opposing enamel significantly affected the steady-state 
value during wear testing (p < 0.0003). However, ANOVA failed to show
significant differences for the wear condition in the value during wear testing (p
> 0.5). There was a significant interaction between the effects of wear condition
and restorative material on the  value (p < 0.001). The results of power analysis
showed that the significant effect of restorative material on the steady-state 
value was likely to be true (power = 0.99089). However, the actual power was
low (0.0876) for the wear condition, which suggests that the sample size is not
sufficient to draw a reliable conclusion on the effect of the wear condition on the
 value (Table 3-4).
75
Table 3-3 Results of two-way ANOVA of main factors (material and wear
condition) and their interaction for the steady-state  value
DF
Sum of
squares
Mean
square
F value
p value
Wear condition
1
0.00637
0.00637
0.35736
0.55785
Material
1
0.3762
0.3762
21.09721
2.58799 x 10-4
Wear condition
*Material
2
0.38257
0.19129
10.72728
9.6997 x 10-4
Error
17
0.30314
0.01783
-
-
Corrected total
19
0.68572
-
-
-
Table 3-4 Outcome of power analysis

Actual power
Wear condition
0.08726
(Two-body Vs Three-body)
0.05
Materials
(IPS e.max Press Vs Type ІІІ gold)
0.99089
76
3.3
SEM observation of wear scars
In general, worn surfaces contain invaluable information for identifying their wear
mechanisms such as tribochemical reactions, plastic deformation and brittle
fracture (Adachi et al. 1997). Therefore, detailed light and SEM examinations
were carried out to understand the nature of dominant wear mechanisms occurring
between enamel opposing IPS e.max Press or type ІІІ gold.
Light microscope images of the wear scars on the enamel surface opposing IPS
e.max Press and type ІІІ gold under the two-body wear condition are shown in Fig
3-4 (a) and (b) respectively. There are distinct differences in the morphology of
the enamel wear scar opposing the two different restorative specimens. The
enamel surface opposing the IPS e.max Press exhibits distinctive wear tracks
along the sliding direction as indicated by the red double-pointed arrow (Fig. 34a). The surface irregularities (roughness) along the wear tracks can be easily
contrasted to the surrounding unaffected enamel surface. Three discernible wear
tracks are noticeable, but the one in the middle appears to be the main wear track
spanning the full length of the sliding stroke. The two minor wear tracks along the
main one may have been caused by instability of the upper specimen holder
during the initial stage of the testing.
In contrast, the enamel specimen opposing the type ІІІ gold exhibits a barely
noticeable wear scar with minimal surface characteristics (Fig. 3-4b). Small
patches of greyish gold smear can be observed along the sliding direction, but the
bulk of the enamel surface appears to be intact.
77
(a) Opposing IPS e.max Press
(b) Opposing type ІІІ gold
Fig. 3-4 Light microscope images of the worn enamel surface opposing (a) IPS
e.max Press and (b) type ІІІ gold specimens under the two-body wear condition
(Magnification x10) (arrows indicate the sliding direction)
78
Fig. 3-5 shows the SEM images of the typical enamel wear scar opposing IPS
e.max Press under the two-body wear condition. The wear scar was first observed
at low magnification (x115) to provide an overall picture of the surface, and
subsequent details of the wear scar were then closely examined under higher
magnifications (x500 and x1500).
The enamel wear scar opposing IPS e.max Press exhibits distinct striations and
ploughing along the sliding direction (yellow arrow in Fig. 3-5b). The higher
magnification SEM images clearly show numerous cracks and delaminated
surfaces (indicated by the red arrow in Fig. 3-5b). The exposed underlying enamel
that resulted from delamination appears to be rough and irregular. The SEM
image at x1500 magnification (Fig. 3-5c) shows the margin (red arrows) of a
delaminated area inside the wear scar.
Fig. 3-6 shows the SEM observations of the typical enamel wear scar opposing
type ІІІ gold under the two-body wear condition. No remarkable surface
characteristics apart from small patches of gold smear are observed on the low
magnification image (Fig. 3-6a). However, at high magnification, fine crack lines
can be seen with gold smears in the direction perpendicular to the sliding direction.
Minute areas of enamel surface loss are detected under the highest magnification
(x1500) as indicated by red circles in Fig. 3-6(c).
79
(a)
(b)
(c)
Fig. 3-5 Wear scar on the enamel surface opposing IPS e.max Press specimen in
two-body wear under (a) Magnification x115 (the sliding direction indicated by
the double pointed arrow) (b) x500 (c) x1500
80
(a)
(b)
(c)
Fig. 3-6 Wear scar on the enamel surface opposing type ІІІ gold specimen in twobody wear under (a) Magnification x115 (b) x500 (sliding direction indicated by
the double pointed arrow) (c) x1500
81
Representative light microscope images of the worn enamel surface opposing IPS
e.max Press and type ІІІ gold under the three-body wear condition are shown in
Fig. 3-7 (a) and (b) respectively.
The enamel wear scar opposing IPS e.max Press under the three-body wear
condition reveals a rough surface with ploughing grooves running parallel to the
sliding direction (Fig. 3-7a). The wear scar appears to be more elliptical in shape
and smaller compared to that observed under the two-body wear condition (Fig. 34a). The area of surface loss or delamination appears to be darker (red arrows)
than the unaffected smooth enamel.
In comparison, the enamel wear scar opposing type ІІІ gold following the threebody wear testing (Fig. 3-7b) appears to have a larger contact surface with greater
amount of gold smear adhered onto the enamel surface than that observed
following the two-body wear testing (Fig. 3-4b). The wear scar reveals a series of
fracture lines perpendicular to the sliding direction (Fig. 3-7b). The fracture lines
are more prominent on the edges of the contact area (red arrows). The patches of
gold smear appear to be clustered around the initial contact point, but they are also
observed along the fracture lines.
82
(a) Opposing IPS e.max Press
(b) Opposing type ІІІ gold
Fig. 3-7 Light microscope images of the enamel surface opposing (a) IPS e.max
Press and (b) type ІІІ gold specimens following three-body wear testing
(Magnification x10)
83
SEM images of the enamel wear scar opposing IPS e.max Press following the
three-body wear testing are shown in Fig. 3-8. The area of contact on the enamel
wear scar appears to be smaller and more elliptical under the three-body wear
condition, compared to that observed under the two-body wear condition (Fig. 35a). High magnification SEM images reveal numerous surface cracks and
ploughing, but surface delamination is not noticeable. Instead, surface pitting and
irregularities are obvious (indicated by red arrows in Fig. 3-8c), possibly due to
the three-body abrasion from the PMMA particles in the food slurry. The degree
of cracking and the depth of ploughing appear to be less severe than those
observed under the two-body wear condition (Fig. 3-5).
Fig. 3-9 shows the SEM images of the typical enamel wear scar opposing type ІІІ
gold under three-body wear testing. Small patches of gold smear appear to be
clustered at the edge of the contact area and along the crack lines. High
magnification SEM images clearly show a series of fracture lines perpendicular to
the sliding direction (Fig. 3-9b). The fracture lines and surface cracking on the
three-body wear scar are well-defined and more pronounced (Fig. 3-9c) than those
observed under the two-body wear testing (Fig. 3-6c).
84
(a)
(b)
(c)
Fig. 3-8 Wear scar on the enamel surface opposing the IPS e.max Press specimen
following three-body wear testing (a) Magnification x115 (sliding direction
indicated by the double pointed arrow) (b) x500 (c) x1500
85
(a)
(b)
(c)
Fig. 3-9 Wear scar on the enamel surface opposing type ІІІ gold specimen in
three-body wear testing (a) Magnification x115 (sliding direction indicated by the
double pointed arrow ) (b) x500 (c) x1500
86
4
DISCUSSION
4.1
Wear testing methodology
4.1.1
Wear testing device
As reviewed in the introduction, various in vitro wear simulation devices have
been developed and used for the investigation of wear behaviour of dental
biomaterials. In this study, a reciprocating sliding wear testing device with a ballon-flat contact geometry was used to simulate the basic elements of oral
functioning. It was designed without an impact component, as it was considered
that this approach would simplify subsequent analysis of the mechanisms of wear
(Wassell et al. 1994). Moreover, it was believed that the impact component would
not contribute significantly to intraoral wear due to the marked deceleration of the
mandible immediately prior to tooth contact (Bates et al. 1975).
The in vitro friction-wear tests were conducted in ambient conditions in air with
the selected parameters of 9.8N normal load, ~1.6Hz oscillation frequency and
~200 m reciprocating amplitude. The selection of these parameters was based on
the literature concerning the physiological condition of human masticatory system
to replicate intraoral wear of human teeth (Bates et al. 1975). The magnitude of
masticatory force on teeth in the oral cavity ranges from 3 to 36N during the
chewing process of human beings (Dowson 1998). Distilled water was used as the
lubricant in the two-body wear testing, while the three-body wear tests were
undertaken under food simulation slurry, composed of unplasticized PMMA
beads in distilled water. Previous in vitro studies have used different mixtures of
food slurry to replicate the three-body wear during mastication. Some studies used
organic food products such as flour, poppy seeds and rice mixed with water (AlHiyasat et al. 1999), while others used artificial products such as PMMA beads
87
(Xu et al. 2004) or silicon carbide grits. It has been suggested that if an abrasive
medium is used for in vitro wear tests, it has to be mildly abrasive as human oral
mechanisms tend to reject gritty particles automatically (Cornell et al. 1957).
Previous studies have used PMMA beads as the third body medium as they do not
degrade in the same way as organic products and it also expedites the wear
process (Matsumura et al. 1995; Leinfelder and Suzuki 1999). The in vitro study
by Lawson et al. (2012) demonstrated that PMMA as a three-body wear testing
medium produced wear of the same order of magnitude as wear with natural seeds,
but with less variation (Lawson et al. 2012). Based on its greater ability to
discriminate between different materials, and material stability during the wear
testing, it was concluded that the PMMA is a practical and relevant material to use
as a third-body wear testing medium.
4.1.2
Specimen preparation
Hemispherical shape specimens with a 4mm of diameter were selected as this
corresponded to the average curvature of a molar crown. Although every effort
was made to standardise the dimensions of the restorative specimens, procedural
errors during processing and handling of the specimens may have resulted in
slight dimensional variations among the restorative specimens. This may have
lead to slight differences in the contact area dimensions.
Flat enamel specimens were prepared from intact permanent premolars obtained
from young individuals aged between 18 and 30 years old. Each enamel specimen
was subjected to two wear tests, one opposing an IPS e.max Press specimen and
one opposing a gold specimen. The interval of 2mm between the wear tests
ensured that no residual stress influenced the wear zones. Intact and smooth
enamel surface between the two wear scars on the same enamel specimen was
observed under light microscope and this verified an adequate distance between
the wear tests. In addition, the order in which the enamel specimen was subjected
to wear testing against the restorative specimens alternated to verify that the
88
material per se did not affect the wear results. The frictional behaviour of enamel
was consistent regardless of the order in which the enamel specimen was opposed
by the two different restorative materials.
The variability in the evolution pattern of  among the five wear pairs may be
attributed to the fact that the five tested teeth were collected from five different
individuals. The mechanical property of a tooth depends upon individual
differences such as age and sex and in addition, on the location of the contact
surface within a tooth (Roy and Basu 2008). Zheng et al. (2003) reported that the
friction and wear behaviours of human enamel are strongly dependent upon
enamel thickness and enamel rod orientation (Zheng et al. 2003). Wear
mechanisms differ between layers in the tooth structure due to a significant
variation in H.
However, the results of this study show no correlation between the average
steady-state  value and mechanical properties (H and E) of the enamel specimens
(Fig. 4-1). This indicates that the variations in H and E of the enamel specimens
did not significantly affect the friction and wear behaviours of the enamel
specimens. Differences in surface finish and other physical factors related to the
enamel as well as the antagonistic restorative material might be attributable to
variability in the steady-state  value of the same wear pairs.
89
(a)
(b)
Fig. 4-1 Relationship between the average steady-state friction coefficient () of
the wear pairs and (a) hardness or (b) elastic modulus of enamel specimens
90
4.2
Restorative materials
There are major differences in the nature of the interatomic microstructures
between the lithium disilicate glass ceramic and type ІІІ gold. In ceramic materials,
ionic or covalent bonding between atoms leads to crystal structures with a limited
number of slip systems available for dislocations (Hutchings 1992). As a result,
ceramic materials show only limited plastic flow at room temperature and they are
more inclined to respond to stress by brittle fracture. In comparison, metallic
bonding is formed by the interaction between the delocalised electrons and the
metal nuclei. When stress is applied to a metallic material, the nature of metallic
bond enables the metal atoms to slide over each other. Therefore, metallic
materials undergo plastic deformation under stress and this accounts for properties
of malleability and ductility of metallic materials (Zum Gahr 1987).
4.2.1
IPS e.max Press
IPS e.max Press (Ivoclar Vivadent) is a lithium disilicate glass ceramic (SiO2 –
Li2O), that is fabricated through a combination of the lost-wax and heat-pressed
techniques (Conrad et al. 2007). Pressable glass ceramic has become a popular
dental restorative system due to good marginal integrity, mechanical properties
and translucency (Conrad et al. 2007; Albashaireh et al. 2010). The manufacturer
claims that IPS e.max Press has been reformulated from its predecessor, Empress
2 (Ivoclar Vivadent) with improved physical properties and translucency through
a different firing process (Guess et al. 2011). However, some in vitro studies have
shown no significant difference in the phase composition and biaxial flexural
strength between the IPS e.max Press and Empress 2 (Albakry et al. 2003;
Guazzato et al. 2004). IPS e.max Press possesses a flexural strength of 360 MPa,
H of 5.5 GPa, E of 91 GPa and a fracture toughness of 3.0 MPam1/2 (Albakry et
al. 2003; Wolfart et al. 2009). IPS e.max Press can be used in monolithic
application for inlays, onlays, and full coverage crowns or as a core material for
crowns and 3-unit FDPs in the anterior region (Guess et al. 2011). Clinical studies
91
have reported good short- to medium-term survival rates of restorations made of
IPS e.max Press: This includes onlays (100% after 3 years) (Guess et al. 2009),
crowns (96.6% after 3 years) (Etman and Woolford 2010), and FDPs (93% after 8
years) (Wolfart et al. 2009).
In spite of its favorable clinical outcome and wide-spread use, there is a paucity of
information on the wear behaviour of IPS e.max Press and its effects on opposing
enamel and restorations. The clinical study by Esquivel-Upshaw et al. (2006)
evaluated the wear rate of enamel opposing FDPs made in glazed IPS e.max Press
in the posterior region. The mean occlusal wear of opposing enamel was 88.3μm
after 1 year (Esquivel-Upshaw et al. 2006). The authors reported that the wear rate
of enamel was independent of the individual's biting force. In another clinical
study, material and enamel antagonist wear of three different dental ceramics was
investigated; Procera Allceram, IPS e.max Press and the metal ceramic (IPS
Classic) (Etman et al. 2008). After two years of service, the mean amount of tooth
wear was 261μm for Procera, 215μm for IPS e.max Press and 156μm for the
metal ceramic; this difference was statistically significant. The result from clinical
studies suggests that the mean occlusal wear of enamel opposing fixed prostheses
made with IPS e.max Press is significantly higher than the wear rate measured
when opposing mature enamel, which has been reported in the range of 29μm per
year for molars and 15μm per year for premolars (Lambrechts et al. 1989).
4.2.2
Type ІІІ gold
Maxigold (Ivoclar Vivadent) which was used in this study is a type ІІІ gold,
composed of Au (59.5%), Pd (2.7%), Ag (26.3%), Cu (8.5%), and Zn (2.7%).
This material is used exclusively for full cast restorations as the solidus
temperature is relatively low for metal-ceramic application and the copper and
silver content can be problematic during ceramic application (Wataha 2002). It
possesses an elastic modulus of 86 GPa, a Vicker’s hardness of 1.47 GPa and
92
0.2% proof stress of 310 MPa. Ductility of this material is expressed as percent
elongation (20%) which represents the amount of permanent deformation an alloy
can endure before tensile fracture. Studies evaluating the clinical performance of
low-gold alloys have shown compatible clinical performance to conventional high
gold alloys in terms of tarnish resistance and surface polish (Sturdevant et al.
1987).
4.3
Influence of material on the friction and wear
behaviours of enamel
For the enamel and IPS e.max Press wear pair under the two-body wear condition,
the μ was low in the initial contact stage but increased sharply within 100 cycles.
This continued to increase steadily up to around 500 cycles and then remained
almost constant for the remaining cycles, with little fluctuations (Fig. 3-3a).
Similar evolutionary patterns of were observed in several in vitro studies
evaluating the wear behaviour of natural enamel (Zheng et al. 2003; Zheng and
Zhou 2007). However, the steady-state value varied depending on the antagonist
material and the wear testing conditions. Clinical studies of human enamel have
demonstrated slightly higher wear rates during the initial period, followed by
steady-state wear rate. Lambrecht et al. (1989) suggested that steady state wear
rate occurs when the occlusal environment reaches a dynamic balance
(Lambrechts et al. 1989).
From the tribological point of view, the reason for a sharp rise in the μ within the
initial cycles can be explained by the process of asperity smoothing in the initial
stage of contact (Roy and Basu 2008). IPS e.max Press is a brittle material with
limited capacity for plastic flow at room temperature (Hutchings 1992). This
indicates that it will respond to stress by brittle fracture rather than plastic
deformation. In the initial stage of contact, the load is concentrated over a small
contact area on the flat enamel surface. This results in brittle fracture of surface
asperities in contact, and the occurrence of fracture leads to increased friction, as
93
it provides an additional mechanism for the dissipation of energy at the sliding
contact. With time (cycles), the surface asperities of the IPS e.max Press and
enamel specimens become reduced, leading to a more conformal contact (AlHiyasat et al. 1998). In conformal contacts, where the mean contact pressure is
low, the scale of the fracture when it occurs will be small, leading to reduction in
the frictional force and wear rate.
The SEM observations of the enamel surface opposing IPS e.max Press in twobody wear (Fig. 3-5) have demonstrated noticeable surface characteristics of deep
ploughing and striations along the sliding direction, suggesting abrasive wear as
one of the predominant wear mechanisms (Zum Gahr 1987). Areas of irregular
concavities and gaps on the enamel surface result from localised bulk
delamination and surface fracture (Zheng et al. 2003). This type of wear occurs
due to repetitive loading on brittle enamel surface. As the IPS e.max Press
specimen slides over the enamel surface, sliding contact generates a frictional
force which results in tensile, compressive and shear stresses on the enamel
surface (Fig. 4-2). Microcracks develop within the subsurface and these
subsurface cracks propagate and emerge to eventually form a particle which
becomes displaced (Reid et al. 1990). The displaced enamel particles may
contribute to a three-body wear, by acting as a solid lubricant between the mating
surfaces (Zum Gahr 1987).
94
Fig. 4-2 Illustration of the mechanism of wear on the enamel surface opposing
IPS e.max Press
In vitro studies evaluating the wear performance of type ІІІ or ІV gold reported no
significant difference between gold opposing enamel and enamel opposing
enamel (Monasky and Taylor 1971). Comparative wear studies have shown that
gold was less abrasive to opposing enamel, and more resistant to wear than
ceramic and composite resin restorative materials (Al-Hiyasat et al. 1998).
Therefore, this material has been used extensively for restorations in stress
bearing areas and as a reference material for evaluating wear resistance of a novel
restorative material.
The SEM observation of the enamel surface opposing the gold specimen revealed
minimally damaged wear scar, characterised by small patches of shining gold
smear adhered to the enamel surface. High magnification SEM images of the wear
scar showed fine crack lines perpendicular to the sliding direction and minute
95
areas of surface loss (Fig. 3-6). Transfer of material from the gold specimen to the
antagonist has been demonstrated by previous in vitro wear studies (Hacker et al.
1996; Ramp et al. 1997) and this indicates that adhesive wear is the predominant
wear mechanism. The adhesive wear mechanism of this wear pair can be
explained in relation to the atomic structure of the gold material, which possesses
high ductility due to a large number of slip systems (Ramp et al. 1997). The
enamel is comparatively harder than the gold specimen. As the gold specimen
slides against the enamel surface, the frictional force results in plastic deformation
of the ductile gold specimen and at the same time, adhesive forces develop at the
asperity junctions between the enamel and gold specimen in contact. The adhesive
forces at the asperity junctions may become stronger than the inter-atomic
metallic bonds of the gold material and that fragments of gold adhere to the
enamel surface during subsequent sliding movement (Hutchings 1992). Plastic
deformation of the gold specimen also leads to a more conformal contact between
the enamel and gold, reducing the frictional surface and wear rate subsequently.
In addition, a thin layer of gold particles adhered onto the enamel surface may
contribute to lower friction energy by serving as a solid lubricant or protective
coating (Ramp et al. 1997). The thin gold layer on the enamel surface prevents
direct contact between the enamel surface and the gold specimen, minimising
wear damage on the enamel surface. However, this phenomenon may not be
commonly evident in clinical evaluations, as the transferred material would be
susceptible to removal by food particles and mechanical brushing.
Low damage on the enamel surface opposing type ІІІ gold could be attributed to
superior surface finish that can be achieved with gold specimens. Al-Hiyasat et al.
(1998) have measured the surface roughness of gold and different dental ceramic
materials (aluminous porcelain, bonded porcelain, low fusing hydrothermal
ceramic and feldspathic machinable ceramic) and demonstrated that the gold
samples had a significantly smoother surface than the ceramic groups. However, a
96
positive correlation between surface smoothness and the antagonist wear was not
evident among the different dental ceramic materials and the authors suggested
that microstructural differences between the different ceramic materials may
account for different amount of wear on the opposing enamel (Al-Hiyasat et al.
1998).
4.4
Influence of wear condition on the friction and wear
behaviours of enamel
Two typical wear tests, two- and three-body wear were conducted to investigate
the effect of food particles on the friction and wear behaviour of enamel opposing
IPS e.max Press or type ІІІ gold. The results of the two-way ANOVA revealed
that the wear condition did not significantly affect the steady-state  value (p =
0.55785) during wear testing as shown in Table 3-3. However, SEM observations
showed some differences in morphology of wear scar between the two- (Fig. 3-5
and Fig 3-6) and three-body wear testing (Fig. 3-8 and Fig. 3-9).
For the enamel and IPS e.max Press wear pair, lower steady-state and smaller
area of wear scar were observed under the three-body wear condition, compared
with those under the two-body wear condition. The SEM images in Fig. 3-5 and
Fig. 3-8 show significant differences in the morphology of the wear scars between
the two wear conditions. The worn enamel under the two-body wear condition is
characterised by deep ploughing, numerous cracks and bulk delamination, while
the three-body wear scar is characterised by shallow cracks and scratches
accompanied by small pits. The bulk delamination under the two-body wear
condition indicates that as the IPS e.max Press specimen slides across the enamel,
the enamel is plastically deformed and microcracks develop within the subsurface
(Zheng and Zhou 2007). These subsurface microcracks propagate and emerge to
the surface to eventually form a particle which becomes displaced. However,
under the three-body wear condition, the PMMA beads in the food slurry hinder
the direct contact between the IPS e.max Press and the enamel specimens and the
97
normal load is distributed by the PMMA beads entrapped between the wear pair.
Therefore, the maximum stress on the enamel surface is reduced compared to that
under the two-body wear condition. The reduced contact stress correlates with the
lower frictional force and less severe wear damage observed on the enamel wear
scar. Surface pitting of the worn enamel under the three-body wear condition
suggests that the wear on the enamel is caused by the abrasion of the moving
PMMA beads. These results agree with the in vitro study by Zheng and Zhou
(2007) who reported lower wear rate and less wear damage on the enamel with the
introduction of food slurry in the lubricating medium (Zheng and Zhou 2007).
The study by Zheng and Zhou (2007) however, used a titanium ball opposing the
enamel and the food slurry consisted of ground rice and miller seed shells in
phosphate buffer solution.
In comparison, the enamel and type ІІІ gold wear pair exhibited higher steadystate  under the three-body wear condition than that under the two-body wear
condition. The average steady-state after 500 cycles was 60% higher under the
three-body wear condition (0.40 ± 0.058) than under the two-body wear condition
(0.25 ± 0.052). The SEM images of the wear scars under the two different
conditions (Fig. 3-6 and Fig. 3-9) have shown similar surface characteristics, but
the degree of fracture lines and cracks perpendicular to the sliding direction was
more severe under the three-body condition than under the two-body condition.
The worn enamel surface under the different wear conditions displayed small
patches of gold smear on the enamel surface, confirming the adhesive wear as the
predominant mechanism for this wear pair. The rationale for the higher steadystate  for the enamel and gold wear pair under the three-body wear condition
cannot be fully understood. However, it is assumed that abrasion of the moving
PMMA beads may remove the thin gold layer adhered to the enamel surface. This
causes continuous deformation and abrasion on the tip of the gold specimen to
form new protective gold coating. As a result, the contact surface between the
98
gold specimen and enamel surface increases, generating greater frictional force
and wear damage on the enamel surface as observed by SEM (Fig. 3-9).
Generally, material-enamel wear pairs showed greater variability in the
evolutionary pattern of under the three-body wear condition than those under
the two-body wear condition. It has been suggested that more parameters such as
contact force, different surface speed and the abrasive medium influence the wear
results under the three-body wear (Pelka et al. 1996). The abrasive particles in the
food slurry may become displaced out of the contact during some stages of sliding
movement. When the PMMA beads are forced out of the contact, direct contact
between the restorative specimen and enamel occurs. The in vitro study by
Pallave et al. (1993) found that minor alterations of the food-film thickness at the
contact area resulted in considerable changes in wear rates and wear-rate rankings
of composite materials. When the average slurry-film thickness decreased from
10m to 3m, wear increased significantly by a factor of two to three and was
exclusively of erosive wear (Pallav et al. 1993). However, at a slurry-film
thickness of 1 m, direct contacts between the antagonist and composite filler
particles started to occur, reducing the erosive wear. Ultimately, direct contact
phenomena predominated, diminishing the wear rate of the various materials to
different degrees. This may partly explain greater inconsistencies observed among
the material-enamel wear pairs under the three-body wear condition.
99
4.5
Conclusions
Within the limitations of this in vitro study, the following conclusions can be
drawn:
1. Type ІІІ gold possesses a lower  and causes less wear damage on
opposing enamel than IPS e.max Press.
2. Abrasive wear was the main wear mechanism for the enamel and IPS
e.max Press wear pair, while adhesive wear occurred predominantly for
the enamel and type ІІІ gold wear pair.
3. The effect of food particles on the friction and wear behaviours of enamel
is dependent on the antagonistic material. PMMA slurry reduced the
steady-state  value for the enamel and IPS e.max Press wear pair, while
it increased the  value for the enamel and gold wear pair. However, the
effect of wear condition on the steady-state  was not statistical
significant to draw a reliable conclusion. Further investigation is needed
to clarify the effect of wear condition on the wear behaviour of enamel
with a larger sample size.
Although the in vitro wear model cannot simulate the oral environment
completely, it provided an invaluable tool to study the mechanisms of wear and
factors involved in the wear process of enamel. The in vitro wear data can
therefore provide some reference to the clinical material selection. However,
clinicians should be aware of various intrinsic and extrinsic factors that contribute
to wear of dentition.
100
4.6
Future research
As a result of this study, a number of areas were identified that would benefit
from further research.
1. Currently, various dental ceramic materials are available for fabrication of
all ceramic restorations. It is known that the wear behaviour of different
dental ceramics is influenced by their microstructural and mechanical
properties as well as the environmental conditions. There is a growing
interest in the use of high strength ceramic substructure such as zirconia
without veneering to optimise the mechanical properties of the
restorations. Therefore, it would be of great interest to investigate the
friction and wear behaviour of human enamel opposing a range of dental
ceramics with different microstructures, surface treatment and fabrication
methods to identify the underlying wear mechanisms and controlling
factors involved in the wear process.
2. Long-term in vivo wear data is required to verify the correlation between
the in vitro and in vivo wear data.
101
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APPENDIX
Abstracts accepted for presentation
The Academy of Australian and New Zealand Prosthodontists
11-14th July 2012
Background
Wear of teeth and restorative materials is a complex and multifactorial
phenomenon with the interplay of biological, mechanical, and chemical factors.
The wear behaviour of various dental biomaterials has been extensively studied to
improve our understanding of the underlying mechanisms and for the
development of restorative materials with excellent wear resistance. Due to the
lack of standardised experimental methods, however, the wear behaviour and
mechanisms of different restorative materials on opposing enamel are still not
clear.
Aim
The aim of this research was to investigate the wear behaviour of human tooth
enamel opposing two different indirect restorative materials; heat pressable
lithium disilicate glass ceramic (IPS e.max Press) and type ІІІ gold.
Methodology
Wear tests on human enamel, opposing IPS e.max Press and type ІІІ gold, were
conducted in a ball-on-flat configuration using a reciprocating horizontal wear
testing apparatus. Two typical wear conditions, two-body (with water as lubricant)
and three-body (with food slurry containing PMMA beads) wear, were carried out.
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Each wear pair was subjected to 1,100 cycles on the wear machine with a static
load of 9.8N and reciprocating amplitude of ~200μm. The frictional force of each
cycle was recorded and the friction coefficient (μ) for the different wear pairs was
calculated. Following completion of the wear testing, the wear scars on the
enamel specimens were observed under light microscopy and scanning electron
microscopy (SEM).
Results
The results showed that type ІІІ gold had a lower μ and caused less wear damage
on opposing enamel than IPS e.max Press in both two- and three-body wear
conditions. The enamel surface opposing the IPS e.max Press specimen exhibited
significant cracks, plough furrows, and surface loss, indicating abrasive wear as
the predominant wear mechanism. In comparison, the enamel surface opposing
type ІІІ gold showed little wear scar with small patches of gold smear adhering to
the surface, indicating adhesive wear.
Conclusion
Within the limits of the in vitro research, the following conclusions were drawn:
1. Type ІІІ gold possesses a lower μ and causes less wear damage on the
opposing enamel than IPS e.max Press.
2. Abrasive wear was the main wear mechanism for the IPS e.max Press and
enamel wear pair, while adhesive wear occurred predominantly for the
type ІІІ gold and enamel wear pair.
Further in vitro research is required to characterise and predict the wear behaviour
of human enamel against a range of dental restorative materials.
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University of Otago Faculty of Dentistry Research Day
2nd August 2012
Title:
In vitro investigation of wear behaviour of human enamel opposing lithium
disilicate glass ceramic and type ІІІ gold
Abstract:
This in vitro study investigated the effects of heat pressable lithium disilicate glass
ceramic (IPS e.max Press) and type ІІІ gold on the friction and wear behaviour of
human enamel. The friction-wear tests were conducted under two typical wear
conditions; two-body (tap-water lubrication) and three-body (food slurry
composed of PMMA beads) wear with the selected parameters of 9.8N load, and
~200mm stroke length. The results of this study indicate that the type ІІІ gold has
a lower friction coefficient and causes less wear damage on human enamel than
does IPS e.max Press. Abrasive wear was the main wear mechanism for the IPS
e.max Press and enamel wear pair, while adhesive wear occurred predominantly
for the type ІІІ gold and enamel wear pair.
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INFORMATION FOR PARTICIPANTS
May 2010
To whom it may concern
I am conducting research on wear properties of different dental restorative
materials on opposing enamel. The research is being conducted as part of studies
towards gaining a Doctor of Clinical Dentistry specialising in Prosthodontics, at
the Faculty of Dentistry, University of Otago.
The investigation requires sourcing extracted teeth with no defects and caries
present, so that the extracted teeth can be subjected to wear simulation, opposing
different dental restorative materials. The teeth will be sterilised following
extraction. Non-biting surfaces of extracted teeth will be sectioned and ground to
produce a flat enamel surface in preparation for the wear test.
It is hoped that the research may establish wear characteristics of the different
dental restorative materials and help clinicians in selection of an appropriate
restorative material in different clinical situations.
Patients who provide their extracted teeth for use in this research project will not
be identified and total confidentiality and anonymity is assured. The only means
of identifying extracted teeth used by the researchers will be by way of randomly
allocated numbers. Patient records or details will be kept confidential.
Upon completion of research, the teeth will be stored for a period of 5 years (in
case of value for further research or if need to review findings) before being
disposed off following contemporary guidelines for handling and elimination of
medical wastes.
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When you have read this information, your treating dentist will discuss it with you
further and answer any questions you may have. If you would like to know more
at any stage, please feel free to contact Professor Michael Swain, the chief
supervisor of this research.
Prof. Michael Swain
Biomaterials, Department of Oral Rehabilitation
School of Dentistry, University of Otago,
310 Great King Street, PO Box 647 Dunedin 905
E-mail: [email protected]
Your participation and cooperation by providing your extracted tooth for use in
this research would be greatly appreciated.
Yours Sincerely
Amy (Ahreum) Lee BDS
Postgraduate Student (Prosthodontics)
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PARTICIPANT CONSENT FORM
I have read the Information Sheet concerning this project and understand what it
is about. All my questions have been answered to my satisfaction. I understand
that I am free to request further information at any stage.
I, ................................................................................................................................
.......[name]of..............................................................................................................
......................[address] have read and understood the Information for Participants
of the above named research study and have discussed the study
with........................................................................[dentist].
I have been made aware of the procedures involved in the study, including any
known or expected inconvenience, risk, discomfort or potential side effects and of
their implications as far as they are currently known by the researchers.
I freely choose to participate in this study and understand that I can withdraw at
any time.
I also understand that the research study is strictly confidential.
I hereby agree to participate in this research study.
Signature
Patient name
Date
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