Micro-grooved Surface Topography Does Not Influence Fretting

Micro-grooved Surface Topography Does Not Influence Fretting Corrosion of
Tapers in THA: Classification and Retrieval Analysis
A Thesis
Submitted to the Faculty
of
Drexel University
By
Christina Marie Arnholt
In partial fulfillment of the requirements
For the degree
of
Masters of Science in Biomedical Engineering
June 2015
i
Copy Right
ii
Acknowledgements
First I would like to thank my mom, Cindy Arnholt for always being supportive and
believing that I could accomplish anything. This wouldn’t have been possible without you.
Additionally I would like to thank my boyfriend of 6 years, Chris Ousey for always being
supportive and proud of me.
To Dr. Steven Kurtz I would like to say thank you for taking a chance on me. When
we first met I had done nothing notable and had no proof of what I could do. Knowing
that someone with as much experience believed in me gave me the strength to push myself
far beyond what I dreamed possible. Your far sighted style of thinking has given me a better
approach to work and life. I admire and hope to be like you someday.
To Daniel Macdonald thank you for being a good friend and mentor. Thank you for
answering the phone in the middle of the night and talking me off the ledge. Thank you for
proof reading all of my papers and being a sounding board to bounce ideas off of. I don’t
think this would have been possible without you.
To Michael Beeman thank you for being the other neurotic person in the lab. Thank
you for booking all of my flights, for answering all of my silly questions and for handling all
of the paper work. I will miss you.
To Richard Underwood thank you for being a good friend and mentor. Thank you
for joking with me, fighting with me and pushing me to be better. I respect your opinions
even if I don’t always say it. I look forward to working with you in the future.
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To Drexel Implant research center family I love you guys. Thank you for dealing
with me when I’m half asleep and talking incessantly, for making fun of me for drinking too
much coffee and for always listening. I will always feel like I was a part of something great
because of you guys.
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Table of Contents
List of Figures ....................................................................................................................................viii
List of Tables ....................................................................................................................................... xi
Abstract ................................................................................................................................................ xii
1. Problem Statement .......................................................................................................................... 1
2. Specific Aims .................................................................................................................................... 3
1. Develop a method to characterize a broad range of machined smooth stems and
intentionally micro-grooved stems. ............................................................................................... 3
2. Using multivariate analysis of covariance to determine the associated factors with
fretting corrosion. ............................................................................................................................ 3
3. Introduction and Background ........................................................................................................ 5
3.1. Total Hip Arthroplasty ............................................................................................................ 5
3.2. Tapers ......................................................................................................................................... 6
3.2.1. Introduction of Tapers ..................................................................................................... 6
3.2.2. History of Tapers .............................................................................................................. 9
3.3. Clinical Implications ...............................................................................................................10
3.4. Corrosion .................................................................................................................................11
3.4.1. Background ......................................................................................................................11
v
3.4.2. Mechanically Assisted Crevice Corrosion ....................................................................12
3.4.3. Semi Quantitative Scoring Method ...............................................................................14
3.4.2. Taper Corrosion Implant Factors .................................................................................15
3.6. Surface Topography ...............................................................................................................18
3.6.1 Introduction of Surface Topography ............................................................................18
3.6.2. Current Analysis of Surface Topography in THA......................................................19
4. Aim 1: A Classification System for Surface Topography .........................................................22
4.1. Hypothesis ...............................................................................................................................22
4.3. Design Block Diagram ...........................................................................................................22
4.4. Materials and Methods ...........................................................................................................23
4.4.1. Implant Information .......................................................................................................23
4.4.2. Microscopy and Visual Inspection ................................................................................24
4.4.3. White Light Interferometry............................................................................................24
4.4.4. Stylus Profilometer ..........................................................................................................24
4.4.5. Final Design Validations ................................................................................................25
4.5. Results ......................................................................................................................................26
5. Aim 2: Patient and Implant Factors Associated with Fretting Corrosion .............................36
5.1. Hypothesis ...............................................................................................................................36
5.2.1. Clinical Information ........................................................................................................36
5.2.2. Design Variables ..............................................................................................................37
vi
5.2.3. Fretting and Corrosion Evaluation ...............................................................................38
5.2.4. Statistical Analysis............................................................................................................40
5.3. Results ......................................................................................................................................40
7. Discussion and Conclusion ..........................................................................................................45
References ...........................................................................................................................................50
vii
List of Figures
Figure 1. Image of total hip arthroplasty components used in a standard procedure ............... 6
Figure 2. Description of Morse Taper Design and terminology ................................................... 7
Figure 3. Diagram of THA head neck taper junction ..................................................................... 8
Figure 4. Description of crevice corrosion reaction with stagnant fluid, reactive species and
released metallic ions .........................................................................................................................12
Figure 5. Example of imprinting within femoral head of a micro-grooved stem surface .......18
Figure 6. Three different components photographed using Nikon D80 (Chiyoda Tokyo) and
3D digital microscope (Keyence VHX-5000 series Digital Microscope). Although figure C is
clearly micro-grooved, A and B are subjectively classified using this method ..........................28
Figure 7. Figure 4.5.2. Two different components analyzed using white light interferometry.
The components are clearly distinguishable between smooth and micro-grooved. Figure A
has a Wmax value of 1.73 um, while figure B has a value of 11.30. Although the filters
utilized during data collection made this method not appropriate .............................................29
Figure 8. Plot of PA values collected from 398 femoral stem components using a stylus
profilometer. It is clear that there is no separation describing two different surface
topographies. .......................................................................................................................................30
Figure 9. Plot of PA values for different geometries ....................................................................30
Figure 10. Typical surface profiles (Left) and micrographs (Right) of A) un-patterned
smooth, B) low amplitude and wavelength patterned smooth surface and C) micro-grooves
from femoral neck taper surfaces. ...................................................................................................32
viii
Figure 11. Amplitude values of known smooth and micro-grooved components. .................33
Figure 12. Wavelength values of known smooth and micro-grooved components ................34
Figure 13. ROC curve representing different cutoff values of amplitude (A) and wavelength
(W) ........................................................................................................................................................34
Figure 14. Reported area under the curve values and standard errors from ROC curve
analysis .................................................................................................................................................34
Figure 15. Known micro-grooved and smooth wavelength vs. amplitude plot with final
classification system marked with black line. .................................................................................35
Figure 16. Classification of 398 THA components.......................................................................35
Figure 17. Design variables measured from the femoral stem. ..................................................38
Figure 18. Exemplar components for each of the scores in the four point scoring method. 39
Figure 19. Figure 5.3.1. Stacked bar plot of the fretting corrosion damage score of femoral
heads with respect to surface topography classification. From the plots it is observed that the
smooth taper surface topography has a higher damage score potentially due to confounded
factors such as engagement length, implantation time and flexural rigidity ..............................41
Figure 20. An example of a micro-grooved surface topography with a fretting corrosion
damage score of 4 (A) and a femoral head that has an imprinted corrosion pattern (B). ......42
Figure 21. Distribution of implantation time by fretting corrosion damage scores of the
femoral head taper. A significant positive correlation (p < 0.0001) was observed between
implantation time and fretting corrosion score of the femoral head in multivariate analysis of
variance. ...............................................................................................................................................42
Figure 22. Distribution of apparent engagement length by fretting corrosion damage scores
of the femoral head taper. A significant negative correlation (p < 0.0001) was observed
ix
between the apparent engagement length and fretting corrosion score of the femoral head in
multivariate analysis of variance. ......................................................................................................43
Figure 23. Distribution of flexural rigidity of the stem trunnion by fretting corrosion damage
scores of the femoral head taper. A significant negative correlation (p = 0.008) was observed
between the flexural rigidity and fretting corrosion score of the femoral head in multivariate
analysis of variance. ............................................................................................................................44
x
List of Tables
Table 1. Scoring criteria for semi quantitative scoring method ...................................................15
Table 2. Design matrix quantifying compliance of different measurement methods with the
design specifications listed in section 4.2. Maximum compliance was described with a score
of 5 and minimum compliance was described with a score of 0 ................................................27
xi
Abstract
Micro-grooved Surface Topography Does Not Influence Fretting Corrosion of
Tapers in THA: Classification and Retrieval Analysis
Christina Marie Arnholt
Dr. Steven Kurtz, Ph.D.
Surface topography of the femoral stem trunnion has been suggested as a factor in
fretting corrosion of the taper interface in total hip arthroplasty (THA). The purpose of this
study was to identify if femoral stem taper morphology was correlated with taper fretting
and corrosion. This was accomplished by first developing a method to characterize a broad
range of machined smooth stems and intentionally micro-grooved stems. Through
multivariate analysis of covariance, different factors were tested for correlation with fretting
corrosion. Finally a matched cohort was designed controlling all related factors to deduce
weather surface topography was related with fretting corrosion.
From a multi-institutional retrieval collection of over 3,000 THAs, 398 stems paired
with CoCr (ASTM-F75) alloy heads were collected as part of a multi-center, IRB-approved
retrieval program. Stems were fabricated from CoCr (ASTM-F75) or Titanium (Ti6V4Al)
alloys and were used in metal on polyethylene (M-PE) bearing total hip devices. Other stem,
femoral head alloys, and different bearing combinations were removed from this study. The
devices had a single location of modularity at the head neck junction.
In order to classify the surface topography into two groups visual inspection,
microscopy, white light interferometer (Zygo New View 5000) and a stylus profilometer
xii
(Talyrond 585, Taylor Hobson, UK) were used. The final method employed a stylus
profilometer with which linear profiles were measured using a diamond tip stylus capturing a
10 mm line trace. Commercial software (Ultra, Taylor Hobson, UK) was used to analyze a 1
mm representative as-manufactured region. Three parameters were calculated from the
profiles: average surface roughness, amplitude and wavelength of micro-grooves (if any).
Surface observations led to a classification system in which a surface had to contain a
periodic pattern, a wavelength > 140 μm and an amplitude of > 5 μm to be considered
micro-grooved. Fifty percent (200/398) of the femoral stem taper surfaces were classified as
smooth tapered stems. The remaining 50% (198/398) femoral stem taper surfaces were
classified as micro-grooved tapered stems.
A semi-quantitative scoring method was used to classify the stem-head pairs into
four groups based on quantity of damage on the female and male taper surfaces [1, 2]. For
this study the femoral head taper was characterized. With this scoring system, a score of 1 is
assigned when the damage is considered minimal and corresponds to fretting damage occurring
on less than 10% of the surface with no pronounced evidence of corrosion. A score of a 2
indicates mild damage where more than 10% of the surface has fretting damage or there is
corrosion attack confined to small areas. A score of a 3 reflects moderate damage where more
than 30% of the surface has fretting damage or localized corrosion attack. A score of 4
corresponds to severe damage over the majority of the taper (>50%) with abundant corrosion
debris. The femoral head taper was independently evaluated by three trained investigators
(C.A., G.B.H, and D.W.M.). Any differences between investigators damage scores were
resolved in a conference, resulting in a final damage score for both the femoral head and
stem.
xiii
Using multivariate analysis of covariance we found implantation time (p<0.0001),
apparent engagement length (p<0.0001), flexural rigidity (p=0.008) and head size (p=0.01)
were significant factors in fretting corrosion head damage scores. Surface topography (i.e.
smooth or micro-grooved, p=0.78), surface wavelength (p=0.60), and surface amplitude
(p=0.23) were not associated with femoral head fretting corrosion damage score.
Overall, the results of this study do not support trunnion surface morphology as a
contributing factor to fretting and corrosion damage at the modular head-neck interface.
This was deduced through an initial classification system, followed by statistical analysis,
which showed correlated factors and finally a controlled matched cohort that showed no
correlation between surface topography and fretting corrosion. This study was limited by
the use of a semi quantitative scoring method. Future work would include the use of
quantified volume of metal released from each cohort.
xiv
xv
1. Problem Statement
Total hip arthroplasty is a successful surgery that allows increased patient
functionality and loss of pain. Although it is a successful surgery, within the United States
the rates of both primary and revision surgeries have increased [3]. Because of this,
researchers have been trying to identify implant and patient factors associated with failure.
One failure mode involves mechanically assisted taper corrosion, which has previously been
linked to a multitude of factors including femoral head size, flexural rigidity and implantation
time [2].
Recently, surface topography has been suggested as a factor in fretting corrosion,
because of the recent observation of “micro-grooved” surface topography [4-7]. It is
believed that these surfaces were created to limit the stress concentration created from a
mismatch of cone geometry tolerances specifically for ceramic heads [8]. These surfaces
have been observed as large and smaller grooves with varying wavelengths and amplitudes
dependent on manufacturer and design [5, 7]. Femoral stems with these machined surfaces
can be paired with both ceramic and femoral heads [7]. It has been observed in several
studies that “imprinting” from the stem surface topography can occur on the inside of metal
femoral heads [8-12]. Imprinting is explained as in vivo corrosion damage [8, 11].
Currently, a standard method for classifying surface morphology does not exist.
Previous research has suggested using a classification system consisting of two groups:
threaded and non-threaded [13]. Additional studies have looked at the effects of surface
topography on fretting corrosion, however these studies are limited by observing the femoral
stem [14] which has been shown to not be the main cause of material loss. Other studies are
1
limited by number of manufacturers leading to confounded factors such as apparent
engagement length and surface topography [15, 16].
In order to accurately distinguish if there is a correlation between surface topography
and fretting corrosion, an adequately sized cohort classified for surface topography with
controlled implant, patient and clinical factors is needed.
2
2. Specific Aims
1. Develop a method to characterize a broad range of machined smooth stems and
intentionally micro-grooved stems.
To understand the potential impact of manufactured micro-grooves on the fretting
corrosion behavior of modular total hip arthroplasty (THA) components, a
classification system is needed to identify which femoral neck tapers have microgrooves. Additionally large variations in design dimensions exist between
manufactures, making it necessary to create a method of classification for all designs
and manufactures to deducing the “type” of trunnion surface. This methodology is
necessary for creating a standardization and comparability between studies of THA
trunnion topography. Developing a highly repeatable classification method was
crucial to the success of this study.
2. Using multivariate analysis of covariance to determine the associated factors with fretting
corrosion.
The use of step-forward multivariate linear modeling utilizing an ordinal logistic fit
allowed for the analysis of ordinal fretting corrosion scores. This was used to
evaluate the role of surface morphology in the context of other variables known to
influence corrosion damage. Through this statistical analysis, it will be possible to
3
deduce whether fretting corrosion is influenced by surface morphology and other
variables.
4
3. Introduction and Background
3.1. Total Hip Arthroplasty
The use of modern total hip arthroplasty (THA) to combat patient ailments such as
osteoarthritis, osteonecrosis, dysplasia and inflammatory arthritis began between the 1960s
and 1970s when Sr. John Charnley introduced the Charnley hip [3, 17]. Surgery is commonly
indicated for chronic pain and osteoarthritis. This approach is considered a secondary
solution to less invasive techniques such as activity modification, weight loss or even intraarticular injections [3]. This surgery is clinically successful with reports of survivorship over
93% survivorship after eight years [18]. The surgery accounts for a 2.7 billion dollar annual
burden with 284,000 primary surgeries and 45,000 revision surgeries each year [19].
THA is a surgery in which an artificial hip is used to replace a damaged native hip
structure. The native hip is composed of a ball and socket joint. The femoral head is located
at the superior aspect of the femur and its articulating surface is covered with cartilage. This
fits into a concave cavity also covered with cartilage called the acetabulum. During surgery
the femoral head is removed and the femoral shaft is reamed creating a canal, in which the
femoral stem is implanted [17]. The acetabulum is commonly replaced with a metal cup
called the acetabular shell and an ultra-high molecular weight polyethylene (UHMWPE) liner
[17], Figure 1.
5
Figure 1. Image of total hip arthroplasty components used in a standard procedure
A single modular taper junction is incorporated in most THA components between
the femoral head and stem. Modularity allows for interchangeability of femoral heads[20],
and an easier insertion of the femoral stem[20, 21]. Modularity also allows the combination
of different materials and different sized femoral heads, which can be decided on a per
patient basis [21]. Additional modular junctions have been incorporated into modern THA
designs to assist with the femoral neck angle and limb length discrepancies [22], while these
allow for a more successful surgery in the short term, they may result in increased corrosion
in the long term [1].
3.2. Tapers
3.2.1. Introduction of Tapers
The Morse taper, invented in 1864 by Stephen A. Morse, was originally used to
connect rotating machinery components. [23]. Machinery handbooks define this style of
taper as “self-handling”, and indicate its common uses such as drilling and machine lathing
6
[24]. The term self-handling means that the taper can be impacted and maintain the juncture
without any additional design features such as locking pins. This strong association originates
from the friction created between mating components due to close contact resulting from
smaller taper angle [25]. In contrast, self-releasing styled tapers, which have a much larger
taper angle and decreased contact between mating surfaces require an additional locking
mechanism to secure the fit. Through the control of length and diameters with a constant
tapering slope, machinists are able to control the amount of contact a taper has with its
mating surface. Machining tapers are described by taper per foot, which fluctuates according
to the taper number [25]. For example a 1 foot taper with a 1/3 inch diameter on one end
and 2/3 inch diameter on the other end, would be described as being tapered 1/3 inch per
foot, Figure 2.
Figure 2. Description of Morse Taper Design and terminology
7
The use of taper junction became widely used in orthopedics with the introduction
of ceramic femoral heads [23]. The original method for attaching a ceramic femoral head to
a metallic stem was to glue and then screw the components together which caused ceramic
failure through fracture [23]. In 1974 Professor Mittelmeier used the Morse taper to facilitate
the connection, decreasing the rate of head fractures [23]. The term Morse taper is loosely
used to describe the cone shape and self-handling capability of orthopedic tapers. However,
general naming techniques such as 11/13 or 12/14 describe the small and large diameters of
THA tapers. These tapers are used today in orthopedics (total knee arthroplasty, total hip
arthroplasty, and shoulder arthroplasty), dentistry, and still in machining tools. A taper
consists of a protruding cone (male component) fitting into a tapered concave shape (female
component). The two components are assembled intraoperatively. This process is
commonly called impaction, in which a hammer is used to secure the femoral head onto the
femoral stem, Figure 3.
Figure 3. Diagram of THA head neck taper junction
8
3.2.2. History of Tapers
In the 1990s femoral tapers were used throughout THA in multiple bearing
combinations such as metal on polyethylene and metal on metal. Taper corrosion was noted
at the head neck junction and attributed to particle generation, implant fracture and mixed
alloys [20, 22, 26-29]. The use of taper connections was continued however, because it was
determined that the problems originated from poorly designed components and were
potentially not the cause of the failures in THA [30].
In the 2000s large head metal on metal devices were widely used in THA [31]. These
devices were used because they had lower rates of dislocation and the metallic bearing was
predicted to have lower wear. However, the clinical experience differed. These systems had
reported taper fretting corrosion resulting in metallic debris release and biological reactions
[31-35]. It was speculated that the larger head size allowed for increased torque at taper and
bearing surfaces leading to increased wear [16].
In the 2010s metal on metal devices continued to have debris release leading to
biological reactions [32, 36, 37]. Metal on polyethylene bearing couples exhibited blackened
taper trunnions and elicited reactions to metallic debris similar to metal on metal devices [38,
39]. These findings supported future research into other implant and patient factors that
may be associated with fretting corrosion at the head neck taper junction.
9
3.3. Clinical Implications
As previously stated, fretting corrosion at the head neck junction has the possibility
of releasing metallic debris and corrosion byproducts. It is important to note that the metal
release is not biologically inert. The size of metal particles has been associated with the type
of response elicited by metallic debris. Meyer et al. showed in vivo periprosthetic tissues have
stored metal debris, eliciting a foreign body reaction and necrosis [36]. Through staining for
CD68, a macrophage marker, and CD3, a lymphocyte marker there were sightings of both
reactions although the former was more abundant [36]. These findings were supported by
Howie et al. who observed large numbers of macrophages around cementless metal on metal
tissues while also noting minimal giant cell and lymphocyte activity [40]. Jacobs et al.
reported the association of larger particles with giant cells or necrotic regions, while smaller
particles were found within macrophages. Mathiesen et al. reported patients with
hypersensitivity experience a lymphocyte dominated reaction. Macrophages and giant cells
were also reported having phagocytized wear particles [29].
The reaction is similar to a biological reaction to bacteria in which macrophages and
other foreign body cells migrate to the metallic particles and phagocytize them [37, 41]. The
reaction differs because the metallic particles remain after the cells die, which then repeats
the process escalating it to an elevated inflammatory response [41]. Cells are hypothesized to
die due to toxic levels of cobalt after the removal of the passive layer within the lysosome of
the cell [37]. An inflammatory mass can develop in response to the cytokines further
continued by the repeating of the process [37].
10
Metal release can lead to adverse local tissue reactions [38, 42] which can manifest as
pseudo tumors or hypersensitivity [43, 44]. Although it was originally observed in metal-onmetal bearing surfaces [36, 37], these phenomena can also occur in metal-on-polyethylene
bearings [38, 39]. It is sometimes difficult to diagnose these reactions because patients often
complain about pain or recurrent instability [38, 42, 45]. Revision surgeries reveal black
deposits at the head/neck junction associated with discoloration of the capsule,
granulomatous mass or effusions [38, 42]. Before revision surgery this can be determined
using serum ion levels observing for elevated cobalt levels and metal artifact reduction
sequences (MARS) MRI observing for large masses [38, 42, 44, 46, 47]. Sectioning of the
periprosthetic tissue can also describe the reactions present around the device. Campbell et
al. proposed an aseptic lymphocytic vasculitis-associated lesions (ALVAL) score which
describes examination of synovial lining integrity, inflammatory cell infiltrates, and tissue
organization [48].
3.4. Corrosion
3.4.1. Background
Corrosion is described as an environmental affect causing the loss of material
properties or mass over time [49]. It can affect multiple industries and has raised concern
from a safety and economic perspective [49]. Corrosion impacts multiple materials, one
group being metals. Metallic components are subject to nine basic forms of corrosion:
uniform corrosion, intergranular corrosion, galvanic corrosion, crevice corrosion, pitting,
erosion corrosion, stress corrosion cracking, biological corrosion and selective leaching [49].
11
3.4.2. Mechanically Assisted Crevice Corrosion
Although older theories have included galvanic corrosion [28, 50, 51], it is currently
well accepted that mechanically assisted crevice corrosion is the mechanism of corrosion at
the head neck junction. The theory states: “Mechanical loading of prosthesis can result in
fretting of the modular taper interface, fracture of the passive oxide films, repassivation, and
crevice corrosion” [52].
Figure 4. Description of crevice corrosion reaction with stagnant fluid, reactive species and released metallic ions
The theory of mechanically assisted crevice corrosion is comprised of two
components: fretting and corrosion. Fretting is described as the small oscillatory movement
between two solid surfaces in contact [53]. In orthopedics fretting occurs during the human
walking cycle and is known to abrade the protective oxide film allowing for increased
exposure of virgin metal [2, 52]. Crevice corrosion is a geometry dependent (cannot exceed
3mm) type of corrosion that relies on stagnant fluids and reactive species (oxygen,
12
hydroxide, chloride) [49]. The local oxygen present in the fluid reacts with metal ions in
solution creating a concentrated cell that can react through the bulk material [49]. Released
metallic ions react with chlorine and hydroxide to make metal hydroxides and hydrochloric
acid, Figure 4 [49]. The increased acidity decreases the metals passivity creating an
environment supporting further corrosion [49].
The materials traditionally used in THA are known to form passive films making
them highly corrosion resistant [50]. Titanium (Ti-6Al-4V) is commonly used for femoral
hip stems and has excellent biocompatibility. The material has a low elastic modulus (110
GPa) and high fatigue resistance due to an almost perfect stoichiometric oxide allowing less
defects [50]. Cobalt chrome (Co-Cr-Mo), is also used in THA generally for the femoral head
or stem. It has high wear resistance and a higher elastic modulus compared with titanium
(200 GPa).
Within total hip arthroplasty the head neck junction has an initial tolerance creating a
narrow crevice with fluid penetration that can be exacerbated by irregularities within the
device [50]. The onset of walking introduces shear stresses into the junction that are large
enough to fracture the oxide film followed by spontaneous re-passivation [50]. The synovial
fluid contains oxygen and chloride species that can penetrate the crevice through the
minimal allotted fluid flow [28, 50]. As previously described this results in heightened
hydroxide and hydrochloric acid concentration [49, 52, 54]. The result of the chemical
species leads to a lower fluid pH in the crevice creating an active metal within the taper
junction. A reaction occurs within the inside and outside of the crevice allowing the
corrosion mechanism to continue [50]. As the corrosion continues the minimal amount of
13
fluid flow allows debris to escape the crevice and replenishes the reactive chloride and
oxygen species. This process repeats allowing for metal corrosion and wear.
3.4.3. Semi Quantitative Scoring Method
A historical study produced in 2002 by Goldberg et al. quantified fretting corrosion
damage of modular connections in 231 THA [2]. This study produced a methodology that
would score damage based on percent of area affected inside the femoral head and on the
surface of the femoral stem, Table 1 [2]. It was an easily repeatable method that has been
used in multiple research studies [1, 7]. This method was modified to include different
implant types and surfaces (Total knee arthroplasty [55], and acetabular shell and femoral
neck surfaces in metal on metal THA [1]). It was also modified to create a better description
of the wear specifically in regards to the phenomena of imprinting [1]. The accuracy of this
method has been proven through comparison of measured material loss and component
scores [56].
14
Table 1. Scoring criteria for semi quantitative scoring method
Damage
Score
Criteria
Minimal
1
Fretting on < 10% surface and no corrosion
damage
Mild
2
Fretting on > 10% surface and/ or corrosion attack
confined to one or more small areas
Moderate
3
Fretting >30% and/ or aggressive local corrosion
attack with corrosion debris
Severe
4
Damage over major (>50%) of mating surface with
sever corrosion attack and abundant corrosion
debris
3.4.2. Taper Corrosion Implant Factors
The distribution of forces throughout the taper are affected by different design
parameters and can affect the likelihood of fretting corrosion. Tapers have an angle
describing the slope of the line connecting two different diameter circles. This angle can
affect the wear and fit of the taper [57]. Bishop et al. observed wear patterns within the
femoral head which they described as asymmetric (non-homogenous) and axisymmetric
(homogenous) [8]. The angle mismatch between the male and female taper allows non-
15
continuous contact of the taper could create an opened and closed end of the taper. This
potentially allows a toggling motion with greater motion at the open end that has been
explained through the turning moments resulting from joint force vectors [8, 16]. This
results in non-homogenous wear, which has also been confirmed through finite element
(FE) analysis that explains double the amount of wear on one side through off axis loading
[57]. This type of wear was correlated to decreased head offset and angle differences [8].
These findings were supported by stochastic FE simulation where angular mismatch, center
offset and patient mass were correlated with change in contact mechanics [58].
However Kocagoz et al. observed taper angle clearance, described as the difference
between femoral head taper angle and femoral stem taper angle, and reported no correlation
with observed fretting corrosion score [9]. This study observed a matched cohort of ceramic
and CoCr components highlighting both distal and proximal engagement, featuring
previously described wear patterns. However the distribution of taper angle clearance was
similar between all scored cohorts.
It has been speculated that the femoral head size or diameter is linked with fretting
corrosion. This has been explained through increased moment arm and torque, specifically
occurring in metal on metal hips [16, 59, 60]. Axisymmetric wear was hypothesized to be
related with axial rotation due to friction, supported by increased revision rates of metal on
metal bearing systems with larger diameter heads [8]. The moments were speculated to be
large enough to rotate the head around the stem taper in combination with toggling effects
of axial loading, allowing the pattern to encompass the entire stem surface [8]. The
hypothesis of frictional torque being associated with wear has been disputed due to the lack
of correlation with surface wear [16]. Additionally, it was hypothesized that frictional torque
16
could initially destabilize the femoral head and screw the femoral head down onto the
femoral stem creating the observed wear pattern without damaging the bearing surface [16].
It was hypothesized that the magnitude of impaction used to assemble the femoral
head and stems could affect the success of components. Nassif et al. utilized an analytical
model to demonstrate that radial stresses at the junction were uniform after a large
impaction applied through the center of the trunnion [59]. If the initial impact force was too
low or offset it could create asymmetrical radial stresses affecting the wear of the internal
taper [59]. Although it is difficult to regulate, studies show a linear relationship between
impaction force and disassembly strength [21]. Multiple impactions aligned with the taper
axis elicited the most secure fit [21], essentially capitalizing on the self-handling quality of the
taper. Additionally it was shown that assembly under dry conditions allowed for a minimal
fretting corrosion [21].
Additional factors associated with fretting corrosion include flexural rigidity,
trunnion length and trunnion thickness. Decreased flexural rigidity has been correlated with
decreased fretting corrosion [2, 45]. This term involves the elastic modulus and the moment
of inertia through the trunnion, which includes the neck diameter [2]. Trunnion length and
thickness have also been linked in the literature with fretting corrosion. Brown et al. showed
through laboratory studies that shorter neck length and increased trunnion thickness were
correlated with fretting corrosion [30]. This was supported in the literature by laboratory
and retrieval studies [16, 30, 59]. Additionally it was shown that alloy combination was not
associated with fretting corrosion [30]. These findings were contradicted by FE models in
the literature [58].
17
3.6. Surface Topography
3.6.1 Introduction of Surface Topography
Continuous sightings of imprinting,[1, 7-9, 11, 16, 59] or transfer of topography
from the femoral stem to the femoral head have lead researchers to question the impact of
femoral stem surface topography on fretting and corrosion,
Figure 5. The introduction of micro-grooved surfaces occurred to limit the local stress
concentrations due to a mismatch of cone geometry tolerances specially for ceramic heads
[8]. This was supported by Kocagoz et al. who reported through a matched cohort that
ceramic femoral heads exclusively had a proximal contact within the femoral head [9]. This
was done to prevent increased hoop stresses in the femoral head which allow crack
propagation [9, 61]. Femoral stems with these machined surfaces can be paired with both
ceramic and cobalt chromium (CoCr ASTM-F75 or ASTM-F90, here after referred to as
CoCr) alloy femoral heads.
18
Figure 5. Example of imprinting within femoral head of a micro-grooved stem surface
3.6.2. Current Analysis of Surface Topography in THA
Researchers have hypothesized that stem taper surface finish may influence taper
corrosion due to the introduction of crevices through the structure of topography. Kop et al.
conducted a study of THA with multiple points of modularity containing micro-grooved
surfaces [4]. The findings reported that devices composed of CoCr with micro-grooved
surfaces contained more corrosion. The opposite trend was observed with Ti alloyed
components [4]. An in vitro study conducted by Panagiotidou et al. noticed contradicting
trends where the passive film of micro-grooved stems was breached, leading to increased
corrosion [5]. These results are contradictory and require further analysis.
Currently, a standard method for classifying taper surface morphology is not available.
Previous research has suggested using a classification system consisting of two groups:
threaded and non-threaded [13]. A 3D optical profilometer used in conjunction with an
19
interference microscope was employed to measure Sa (defined as the arithmetic deviation of
the surface) and Sz (defined as the difference in the height between 5 peak heights and 5
valleys) parameters of 11 never implanted stems from 5 different manufacturers[13].
Additionally the thread height, the pitch, skewness (Ssk), and kurtosis (Sku) parameters were
also measured. [13]. The measurements were taken at the proximal, mid-point and distal
segment [13]. The authors reported features of the designs including variable micro-groove
heights on a case-by-case basis for all of the designs included within the study. An implied
classification was also suggested on a case-by-case basis. Seven stem designs were classified
as micro-grooved designs based on the thread heights and presented 3D surface topography
images: Profemur, Synergy, Summit, Silent, Secur-fit Max, Corail and Tri-Lock [13]. The
ABGII, Taper-lock, Accolade, and SROM were classified as non-micro-grooved[13].
Currently, it is unknown what affect (if any) the groove amplitude and wavelength would
have on fretting corrosion in vivo. Additionally, it is not clear if the classification system can
be adapted to retrieved implants with severe corrosion.
Additional studies have involved a single manufacture when observing metal on metal
hips. DePuy Synthes (Warsaw, Indiana) Corail and SROM designs were observed in each
study [15, 16]. The Corail stem contained a micro-grooved surface topography and the
SROM contained a smooth surface. A classification was not utilized in the study by Langton
et al. however the study speculated a relationship between surface topography and fretting
corrosion. They claimed a potential increase of wear due to contact stresses resulting from
plastic deformation of the taper surface[16]. A related study used Contour GT-K 3D optical
profilometer (Bruker, UK) to measure three stems for height and pitch of the machined
grooves [15]. An objective lens of 53 was used to scan the surface with a back scan of 80
20
mm and length of 50 mm [15]. The methodology used was not expanded to other stem
designs or manufactures. The study could not conclude any information regarding the
relationship between topography and fretting corrosion due to the confounded factors.
Another study conducted by Ha et al. created a system of characterizing machining
height and spacing [14]. This was done by femoral stem surface measurements and femoral
head bore measurements. The former was done using white light interferometry (NewView
6k, Zygo, Middlefield, CT) and the latter was accomplished using profilometry (Surftest
SJ201P, Mitutoyo, Aurora, IL) [14]. The system observed fretting corrosion scores from the
semi-quantitative scoring method [2]. The study concluded that machining height and
implantation time predicted the fretting corrosion scores observed [14]. A limitation of this
study was that the material loss from the femoral stem was studied and used as a predictor.
Our previous research has shown that the majority of the material loss occurs in the femoral
head with negligible loss form the femoral stem.
The existence of multiple techniques with no clearly defined repeatable method of
determining surface topography makes it difficult to compare results from different studies
and truly discern the impact of surface topography on fretting corrosion.
21
4. Aim 1: A Classification System for Surface Topography
4.1. Hypothesis
In this study, we asked: can a classification system be developed to differentiate
between intentionally micro-grooved and smooth femoral neck taper topographies? It was
hypothesized that upon visual inspection a distinct difference would be observed between
surface topographies rendering additional measurements unnecessary.
4.2. Design Specifications
Historically femoral stem dimensions have been shown to differ between
manufactures and designs. Because of this, it is crucial that the proposed classification
system is design and manufacture independent. Additionally the method should be accurate
and repeatable. It should not employ any manipulation on the measurements and show a
clear difference between smooth and micro-grooved topographies. The machine utilized in
the method should be easily accessible by different research groups across the country and
would ideally not take an excessive amount of time to complete measurements as the
number of components being classified may exceed 100 components.
4.3. Design Block Diagram
22
Femoral Stem Tapers:
398 Components
5 Major Manufactures
79 Femoral Stem Designs
Visual Inspection
3D Digital Microscopy
White Light Interferometry
Stylus Profilometer
Manufacture and design
independent binary
classification system
depicting surface
topography
4.4. Materials and Methods
4.4.1. Implant Information
Approximately 3,100 THA systems were collected between 2000 and 2014 during
routine revision surgeries as part of an IRB-approved, multi-institutional, orthopedic implant
retrieval program. Head/stem pairs were selected based on the following inclusion criteria: a
metal-on-polyethylene bearing system, a CoCr alloy femoral head paired either with a
titanium-6aluminum-4vanadium (Ti6V4Al, here after referred to as Ti alloy) alloy or a CoCr
alloy stem, both the head and femoral stem were implanted/explanted at the same time, and
both retrieved components were available for analysis. We excluded devices with any type of
modular stem or with a femoral head incorporating a taper adapter sleeve. We also excluded
femoral stems that were not fabricated from CoCr or Ti alloy (e.g., femoral stems made from
TMZF were excluded).
The final study cohort included the head-stem pairs from 398 total hip devices,
which were produced by 5 major manufacturers (in alphabetic order): Biomet (Warsaw,
23
Indiana; n=30 of 398); DePuy Synthes (Warsaw, Indiana; n=45 of 398); Smith and Nephew
(Memphis, Tennessee; n= 14 of 398); Stryker (Mahwah, New Jersey; n= 84 of 398); Zimmer
(Warsaw, Indiana; n=219 of 398), as well as other manufacturers (n= 6 of 398). There were
different stem designs within manufacturer cohorts and the stems were manufactured from
CoCr alloy (57%; n=227 of 398) and Ti alloy (43%; n=171 of 398). The femoral heads
ranged in size from 22 to 44mm.
4.4.2. Microscopy and Visual Inspection
Components were analyzed using a 3D microscope (Keyence VHX-5000 series
Digital Microscope) and visual inspection. Features such as notable patterned ridges, macro
appearance of grooves and overall texture were observed.
4.4.3. White Light Interferometry
A pilot study of 68 retrieved femoral neck tapers surfaces were measured with a
white light interferometer (Zygo New View 5000). Each component was scanned using a
prefabricated application controlling the settings of the machine. The Rmax (maximum
peak-to-valley roughness), Wmax (maximum height of the waviness) [62] and number of
points collected were reported.
4.4.4. Stylus Profilometer
24
The horizontal straightness function of a Talyrond 585 (Taylor Hobson, UK)
roundness machine1 or stylus profilometer was used to measure 10mm long profiles on an
unworn region of the femoral stem tapers. A conispherical diamond tip stylus with 5μm
radius was used [63].
Unworn regions were selected by visual inspection of the surface. Regions were
avoided that appeared to have been engaged with the femoral head, had areas featuring
discoloration, and regions with clear evidence of damage generated by the surgeon during
removal or impaction.
Surface profiles were analyzed using the Ultra commercial surface analysis software
(Taylor Hobson, UK). From each profile, a representative as-manufactured region of 1mm
in length was selected to standardize results. The as-manufactured region was selected based
on uniformity and trace amplitude. Regions of plastic deformation or noticeable impaction
were excluded. The amplitude and wavelength of the micro-grooves was calculated from the
representative 1mm portion of the profiles. Filters and corrections were purposefully not
utilized to avoid data manipulation or distortion of the long wavelength surface topography,
such as the micro-grooves. The amplitude was calculated as the vertical distance between the
peak and valley of a representative groove. The wavelength was calculated by taking the
average distance between 5 peaks.
4.4.5. Final Design Validations
In order to validate the final chosen method of analysis, a true positive and negative
group was constructed through literature search and manufacture information. Munir et al.
1
The Talyrond 585 has a radial straightness unit with ceramic datum, which allows the radial arm
to take surface roughness profiles.
25
published that known smooth (Taperloc (n=2)) and micro-grooved (Tri-Lock (n=12), Corail
(n=5), Synergy (n=3) and Summit (n=1))[13]. Additionally Stryker orthopedics (n=24) does
not make any micro-grooved components, which is partially confirmed by the lack of
ceramic femoral heads. These components were blindly analyzed and the data was evaluated
with a binary logistic analysis and an ROC curve. The binary logistic analysis is testing that
the observations will fall into one of two categories based on one or more the tested
variables. The assumptions of this analysis include: dichotomous dependent variable,
categorical nominal independent variable, and mutually exclusive exhaustive categories. The
ROC curve is the illustration of the performance of a binary classifier system with varying
thresholds. The plot represents all positives against false positives. The area under the curve
shows the accuracy of the classification method.
4.5. Results
After attempting to measure each component using five different methods of
measurement a design matrix was completed evaluating the compliance with the necessary
specifications, Table 2.
26
Table 2. Design matrix quantifying compliance of different measurement methods with the design specifications listed in section 4.2.
Maximum compliance was described with a score of 5 and minimum compliance was described with a score of 0
Visual
3D Digital
White Light
Stylus
Stylus Profilometer
Inspection
Microscope
Interferometry
Profilometer
Wavelength and
PA values
Amplitude
Accuracy
1
1
3
5
5
Repeatability
1
1
4
5
5
Design
5
5
3
5
5
5
5
3
5
5
5
5
1
5
5
1
1
3
2
5
Independent
Manufacture
Independent
Unaltered
measurement
(i.e. no use of
filters)
Clear
Difference
between
Topographies
Visual inspection of the components was given one of the lowest compliance ratings
with a total score of 18 out of 30. This method was completely subjective making it not
accurate or repeatable. A machine was not employed making allowing for unaltered data,
however it did not produce consistent results. The use of a 3D digital microscope had the
same compliance rating of 18. It shared many of the features of the visual inspection
27
method, lack of accuracy and repeatability, Figure 6. However the machine itself is not
readily available to research labs and the technique of taking pictures adds an element of
time to the measurement.
Figure 6. Three different components photographed using Nikon D80 (Chiyoda Tokyo) and 3D digital microscope (Keyence
VHX-5000 series Digital Microscope). Although figure C is clearly micro-grooved, A and B are subjectively classified using this
method
The white light interferometer had the lowest compliance rating of 17. This was
because it utilized filters to increase the number of data points collected which also lead to a
depleted accuracy and repeatability score, Figure 7. Additionally the machine had trouble
measuring different geometries and alloy compositions of a specific manufacture making it
not design or manufacture independent. It was able to present a difference between
topographies that was not fully trusted due to the previously described pitfalls.
28
Figure 7. Figure 4.5.2. Two different components analyzed using white light interferometry. The components are clearly
distinguishable between smooth and micro-grooved. Figure A has a Wmax value of 1.73 um, while figure B has a value of 11.30.
Although the filters utilized during data collection made this method not appropriate
The last two methods involve the profiles measured using stylus profilometer, PA
values and wavelength and amplitude dimensions. The PA values shared the benefit
accuracy, repeatability, design and manufacture independence and unaltered data and had a
score of 27. Where these two methods diverge is in the distinctive difference between
surface topographies. Figure 8 shows a plot of PA values collected from 398 femoral
components with no clear differentiation indicating two different groups. The PA value is
29
1
𝐿
described as 𝑃𝐴 = 𝐿 ∫0 |𝑦(𝑥)| 𝑑𝑥 which indicates that it is the average of roughness values.
Because of this feature multiple geometries can have the same reported measurement, Figure
9.
Figure 8. Plot of PA values collected from 398 femoral stem components using a stylus profilometer. It is clear that there is no
separation describing two different surface topographies.
Figure 9. Plot of PA values for different geometries
30
The use of x and z descriptive parameters to define the topography was then
explored. In order to employ this method additional structuring was necessary. Initial review
of femoral stem surface profiles revealed that there were profiles with distinct periodic
structures and others without. The non-periodic profiles had random surface topography
and were deemed smooth tapered stems (Figure 10 A). The periodic structured surfaces were
separated into two groups based on trends observed from the amplitude and wavelengths
measured from femoral stem surface profiles. In order to determine the cut off values for
amplitude and wavelength values a validation was performed using known true positive and
negative values. Binary logistic analysis was performed testing wavelength values (ranging
from 80µm to 160µm varying by 20µm) and amplitude values (ranging from 3µm to 6µm
varying by 1µm). These values were selected by observing differences between known
smooth and micro-grooved amplitude, Figure 11 and wavelength values, Figure 12. The
results of the ROC curve showed that the methods chosen were accurate, Figure 13. The
area under the curve reported ten perfect combinations of wavelength and amplitude, Figure
14. However, after further analysis of the data, Figure 15, there was an optimal predictor of
wavelength and amplitude. The validation study and initial observations, revealed that in
order to be considered micro-grooved, the surface topography of the femoral stem taper
must have a periodic pattern, a wavelength >140μm, and an amplitude >5μm. If surface
topography did not meet any of the requirements, they were considered smooth, Figure 16.
31
Figure 10. Typical surface profiles (Left) and micrographs (Right) of A) un-patterned smooth, B) low amplitude and wavelength
patterned smooth surface and C) micro-grooves from femoral neck taper surfaces.
32
Figure 11. Amplitude values of known smooth and micro-grooved components.
33
Figure 12. Wavelength values of known smooth and micro-grooved components
Figure 13. ROC curve representing different cutoff values of amplitude (A) and wavelength (W)
Figure 14. Reported area under the curve values and standard errors from ROC curve analysis
34
Figure 15. Known micro-grooved and smooth wavelength vs. amplitude plot with final classification system marked with black line.
Figure 16. Classification of 398 THA components
35
5. Aim 2: Patient and Implant Factors Associated with Fretting Corrosion
5.1. Hypothesis
In this study we asked: can statistical analysis of a large cohort of femoral stem
components be used to predict factors associated with fretting corrosion and determine
whether surface topography is an associated factor. It was hypothesized that implantation
time, flexural rigidity and stem alloy would be associated factors. Surface topography was
hypothesized to not be a correlated factor.
5.2. Materials and Methods
5.2.1. Clinical Information
Clinical information and operative reports were collected from each patient including
age, gender, revision reason, and implantation time. The average patient age at the time of
implantation was 60 ± 14 years. Forty-eight percent (191 of 398) of the patients were female.
The components were implanted on average (±SD) for 6 ± 7 years (range 0 to 32y). Sixtyfive percent (257 of 398) of all components were implanted during primary surgeries and the
36
head and the stem were implanted during the same surgery. The components were
predominantly revised for loosening (n=154), infection (n=130), peri-prosthetic fracture (29)
and polyethylene wear (n=26). We studied the medical records for evidence of metal-related
pathology. In only one patient was there a notation of evidence of metallosis at the time of
revision surgery.
5.2.2. Design Variables
Because taper corrosion damage is known to be multifactorial, we identified design
variables for our analysis to complement our understanding of the potential role of stem
taper morphology. Thus, in addition to surface topography, the femoral stem taper length
and diameter were measured, and the flexural rigidity was calculated. It has been reported in
the literature that increased flexural rigidity is correlated with lower head and femoral neck
taper fretting corrosion damage scores after THA [2]. The stem femoral neck taper diameter
was measured at the distal point of engagement with the femoral head when apparent, or the
distal end of the femoral stem taper for designs where the entire taper is engaged, Figure 17.
The flexural rigidity was calculated by multiplying the 2nd moment of area by the elastic
modulus (𝐸 ∗ (𝜋 ∗
(𝑁𝐷)4
64
)) [2]. The alloy composition was determined using X-ray
florescence (Niton XL3t GOLDD+; Thermo Scientific, Waltham, Massachusetts). The
apparent length of engagement was measured from the proximal edge of the femoral stem
taper to the point of femoral head engagement and the femoral stem taper length was
measured as the distance from the proximal to the distal edges of the femoral stem taper.
37
Figure 17. Design variables measured from the femoral stem.
5.2.3. Fretting and Corrosion Evaluation
The femoral head and stem components were received in both assembled and
disassembled states. Assembled components were disassembled using a femoral head
extractor. The components were photographed in their as-received condition and if
applicable after disassembly. Devices were cleaned in two consecutive 20-minute soaks in a
1:10 ratio of detergent (Discide®; AliMed, Dedham, Massachusetts) to water. After
decontamination they were placed in an ultra sonication bath twice for 30 seconds in water.
After cleaning, the junctions were visually inspected for evidence of fretting corrosion
damage
38
Fretting corrosion damage at the head taper of the head/neck junction was
characterized using a modified semi-quantitative score adapted from the Goldberg method
[1, 2]. For this study the femoral head female taper was characterized, Figure 18. In this
scoring system, a score of 1 is assigned when the damage is considered minimal and
corresponds to fretting damage occurring on less than 10% of the surface with no pronounced
evidence of corrosion. A score of a 2 indicates mild damage where either more than 10% of the
surface has fretting damage or there is corrosion attack confined to small areas. A score of a 3
reflects moderate damage where more than 30% of the surface has fretting damage or localized
corrosion attack. A score of 4 corresponds to severe damage over the majority of the taper
(>50%) with abundant corrosion debris. Irregular, acute artifacts on the surface were
considered iatrogenic damage and excluded from the taper damage assessment. The femoral
head taper was independently evaluated by three trained investigators (C.A., G.B.H, and
D.W.M.). Any differences between investigators damage scores were resolved in a
conference, resulting in a final damage score for both the femoral head and stem.
Figure 18. Exemplar components for each of the scores in the four point scoring method.
39
5.2.4. Statistical Analysis
The fretting corrosion scores in this study are ordinal in nature. Therefore, we used
step-forward multivariate linear modeling utilizing ordinal logistic fitting to evaluate the role
of surface morphology in the context of other variables known to influence corrosion
damage. Specifically, we included the following factors in the step-forward analysis: stem
surface finish (i.e., smooth vs. micro-grooved), as well as metal alloy coupling, flexural
rigidity, trunnion length, engagement length, head size, head offset, femoral stem taper
length, and implantation time. For all tests, the alpha level was set to 0.05. Statistical analyses
were performed using commercial statistical software (JMP 11.0, Cary , North Carolina).
5.3. Results
Mild to severe damage (fretting corrosion head damage score ≥ 2) was observed in
294 (74%, Figure 19) of the analyzed femoral head taper. Mild to severe damage (fretting
corrosion head damage score ≥2) was observed in 70% of femoral heads that were used with
micro-grooved stem tapers (n = 140 of 198, Figure 20), and in 77% of femoral heads that
were used with smooth stem tapers (n = 154 of 200). Severe damage (fretting corrosion head
damage score = 4) was observed in 15% of the analyzed femoral head tapers (n = 60 of
398). Severe damage was observed in 9% of femoral heads that were used with micro-
40
grooved stem tapers (17/198) and in 22% of femoral heads that were used with smooth
stem tapers (43/200).
Using multivariate analysis of covariance ordinal logistic fitting, implantation time
(p<0.0001; Figure 21), apparent engagement length (p<0.0001; Figure 22), flexural rigidity (p
= 0.008; Figure 23), and head size (p=0.01) were significantly associated with increased
fretting corrosion damage of the femoral head. However, these factors were only able to
explain approximately 19% of the variation in fretting corrosion damage scores. Surface
topography (i.e. smooth or micro-grooved, p=0.78), surface finish wavelength (p = 0.60),
surface finish amplitude (p = 0.23), femoral stem taper length (p=0.66), and alloy coupling
(p=0.95 were not associated with fretting corrosion damage scores.
Figure 19. Figure 5.3.1. Stacked bar plot of the fretting corrosion damage score of femoral heads with respect to surface topography
classification. From the plots it is observed that the smooth taper surface topography has a higher damage score potentially due to
confounded factors such as engagement length, implantation time and flexural rigidity
41
Figure 20. An example of a micro-grooved surface topography with a fretting corrosion damage score of 4 (A) and a femoral head
that has an imprinted corrosion pattern (B).
Figure 21. Distribution of implantation time by fretting corrosion damage scores of the femoral head taper. A significant positive
correlation (p < 0.0001) was observed between implantation time and fretting corrosion score of the femoral head in multivariate
analysis of variance.
42
Figure 22. Distribution of apparent engagement length by fretting corrosion damage scores of the femoral head taper. A significant
negative correlation (p < 0.0001) was observed between the apparent engagement length and fretting corrosion score of the femoral
head in multivariate analysis of variance.
43
Figure 23. Distribution of flexural rigidity of the stem trunnion by fretting corrosion damage scores of the femoral head taper. A
significant negative correlation (p = 0.008) was observed between the flexural rigidity and fretting corrosion score of the femoral
head in multivariate analysis of variance.
44
7. Discussion and Conclusion
This retrieval study classified and assessed the surface topography of 398 explanted
femoral stem tapers of 79 different designs from 5 major manufacturers. The final
classification method was validated using binary logistics and ROC curve. Statistical analysis
was used to identify predictive factors for fretting corrosion and determine if surface
topography was a predictive factor. The results of this study do not support the hypothesis
that trunnion surface morphology is a contributing factor to fretting and corrosion damage
at the modular head-neck interface.
The strengths of this study lie in the control of known factors and the quantity of
components, designs and manufactures assessed. Initially 398 components were assessed
using a multitude of methods to definitively characterize the surface as micro-grooved or
smooth. Initial methods such as macro inspection and digital 3D microscopy were not
complex enough to bear the intricate differences between topography. They were subject to
analysis and not repeatable. White light interferometry presented possible data manipulation
and a lack of ability to process certain geometries and alloys. Additionally this method gave
a limited field of view (0.719 × 0.719 mm). Our previous research found that the surface of
the stems is highly anisotropic with axial symmetry, and therefore the surface topography is
best characterized using a 2D surface profile measured along the whole length of the stem
taper. In the current study, the use of a diamond tipped stylus profilometer allowed reliable
measurements from the surface of the explanted stem tapers and overcame the problems of
analyst subjectivity, repeatability, data dropout and limited field of view associated with other
methods.
45
Previous studies looking at the surface topography of taper junctions have proposed
using a plethora of surface roughness parameters [13], metrologists have warned against the
“parameter rash” of sophisticated surface roughness parameters whose functional and
tribological significance is not understood [64]. We therefore chose to characterize the
micro-grooves using amplitude and wavelength rather than the less tangible parameters
available with software packages, whose significance is currently unclear.
Analysis has been conducted on the effect of stem taper surface topography and in
vivo fretting and corrosion damage [4-7]. In the laboratory, researchers have cited increased
fretting corrosion in micro-grooved stem tapers for cobalt chrome alloy stems, but found
the opposite effect for titanium alloy stems [4]. Additional studies assessing surface
topography have looked at an implant collection with limited manufactures or at the material
loss of the wrong component. This presents limitations by confounding associated factors
such as apparent length of engagement and surface topography leading them to conclude
that topography is a predictor of fretting corrosion damage [15]. Other studies have chosen
to observe the femoral stem as a predictor of fretting corrosion [14], while previous research
has shown that the femoral head bears the bulk of material loss. This could also lead to
incorrect conclusions about the impact of surface topography on fretting corrosion.
It is the opinion of this researcher that different patient and implant factors have a
larger impact on implant survivorship. Surface topography may not be correlated with
increased fretting corrosion because of the amount of contact area present in smooth and
micro-grooved stems. Either the micro-grooves may have a summation of forces along
multiple contacting grooves that is equivalent to the contact area of a smooth stem or a local
deformation may occur in micro-grooved stems where the contact patch is equivalent to a
smooth stem contact patch.
46
Additional factors may be associated with fretting corrosion. One of these factors is
the seating of the femoral head on the femoral stem [8, 16]. The presence of different wear
patterns observed in metallic femoral components [9] identifies a lack of correlation with
impaction site (proximal or distal) but rather firm seating. Head size speculated to create a
larger moment arm around the taper in combination with toggling has an affect about the
entire taper creating uniform wear in some areas of the taper supported by the axisymmetric
wear observed in this study [16, 59, 60]. The trunnion length and flexural rigidity also affect
the connection between the taper and the flexibility of the taper itself. Increased contact in a
loose seating with a flexible surface can affect the passive film of the material and the
fretting allowing further corrosion of the components.
If attempting to design around the problem of corrosion different aspects of the
component design have to be considered. To stop crevice corrosion the main method is to
prevent crevices which cannot be done due to the crevice at the interior of the femoral head
[49, 54]. However, another description of crevice corrosion is dissolution of the anode
through electron exchange within the material itself creating a gradient. Separation of the
metallic components with a polymer of some type would prevent the passage of electrons.
It can also prevent destructive fretting which can fracture the passive film, initiating
corrosion. Additionally increased contact surface area would decrease the amount of fretting
within the femoral head and lead to less fretting corrosion. This could be done by increasing
the trunnion diameter, the compaction force or decreasing the taper angle.
There were several limitations in this study. First, the method used to assess fretting
corrosion was semi-quantitative and does not provide a measure of material loss from the
taper junction. Previous studies have established that there is a correlation between visual
scoring methods and the volume of metal debris released at these junctions, but for head
47
tapers with a score of 3 or 4, there is a wide range of volume of material loss [9, 56].
However, use of this semi-quantitative method allowed for comparisons with prior research
in the literature that also utilized this method [1, 7]. Another limitation is that this study was
limited to explanted components; inspection of never-used implants could provide
additional insight into the as-manufactured surface topography [13]. However, we were able
to analyze regions that did not have gross evidence of damage, corrosion or deformation
that appeared to be representative of the as-manufactured surface. In addition, this study was
confined to monoblock stem designs incorporating a metal-on-polyethylene (M-PE) bearing
and, thus, the findings of the study should not be extrapolated to hip systems incorporating
multiple modularity or metal-on-metal bearing systems. However, the devices included in
this study are representative of M-PE total hip replacement devices currently used in the
United States.
In summary, this study developed and provided evidence to support a contact stylus
measurement technique for distinguishing surface topography. Observations of a periodic
pattern with amplitudes greater than 5μm and wavelengths longer than 140μm represented a
micro-grooved stem taper. Statistical analysis was used to determine predictive factors within
a 398 component cohort and predict the significance of surface topography on fretting
corrosion at the head neck junction. The current retrieval analysis does not support the
hypothesis that surface morphology is associated with fretting corrosion in metal on
polyethylene bearing systems, when studied in the context of other design and clinical
factors.
The findings of this study support the interchangeable use of micro-grooved stems
with ceramic and CoCr femoral heads. Clinically these findings are important as the
48
modularity of femoral stem was introduced to allow for interchangeable femoral head
materials.
49
References
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