Temporomandibular Joint: Disorders, Treatments, and Biomechanics S I and T

Transcription

Annals of Biomedical Engineering, Vol. 37, No. 5, May 2009 (Ó 2009) pp. 976–996
DOI: 10.1007/s10439-009-9659-4
Temporomandibular Joint: Disorders, Treatments, and Biomechanics
SHIRISH INGAWALE´1 and TARUN GOSWAMI1,2
1
Department of Biomedical, Industrial and Human Factors Engineering, Wright State University, 3640, Col. Glenn Hwy,
Dayton, OH 45435, USA; and 2Orthopaedic Surgery and Sports Medicine, Wright State University, Dayton, OH 45435, USA
(Received 5 May 2008; accepted 13 February 2009; published online 28 February 2009)
of the head and neck can cause TMD. The most common TMJ disorders are pain dysfunction syndrome,
internal derangement, arthritis, and traumas.18,21,22
With millions of people suffering in the United States
alone,21,32,87,115 TMD is a problem that should be
looked at more fully. Since a large fraction of TMD
causes are currently unexplained, the better
understanding of the etiology of TMDs will help prevent not only occurrence of TMDs but also failure of an
implanted joint in the same way as the joint it replaced.
The TMJ Bioengineering Conference, held in 2006,
underlined the importance of collective research efforts
from four major categories: tissue engineering, biomechanics, clinical community, and biology.22 At
Wright State University, Dayton, Ohio; our research
efforts focus on developing 3-D models of asymptotic
and diseased TMJs of men and women of different age
groups to enable better understanding of joint motion
and forces. The finite element analysis of these models
can provide useful information about the contact
stresses that possibly contribute to the dysfunction of
the joint. The similar approach can also be employed
for comparative evaluation of different TMJ implant
designs.
Abstract—Temporomandibular joint (TMJ) is a complex,
sensitive, and highly mobile joint. Millions of people suffer
from temporomandibular disorders (TMD) in USA alone.
The TMD treatment options need to be looked at more fully
to assess possible improvement of the available options and
introduction of novel techniques. As reconstruction with
either partial or total joint prosthesis is the potential
treatment option in certain TMD conditions, it is essential
to study outcomes of the FDA approved TMJ implants in a
controlled comparative manner. Evaluating the kinetics and
kinematics of the TMJ enables the understanding of structure
and function of normal and diseased TMJ to predict changes
due to alterations, and to propose more efficient methods of
treatment. Although many researchers have conducted biomechanical analysis of the TMJ, many of the methods have
certain limitations. Therefore, a more comprehensive analysis
is necessary for better understanding of different movements
and resulting forces and stresses in the joint components.
This article provides the results of a state-of-the-art investigation of the TMJ anatomy, TMD, treatment options, a
review of the FDA approved TMJ prosthetic devices, and the
TMJ biomechanics.
Keywords—Temporomandibular joint (TMJ), Temporomandibular disorder (TMD), TMJ implants, TMJ biomechanics.
BACKGROUND
Temporomandibular joint (TMJ) connects the
mandible or the lower jaw to the skull and regulates
the movement of the jaw (see Fig. 1). It is a bi-condylar
joint in which the condyles, located at the two ends of
the mandible, function at the same time. The TMJ is
one of the most complex as well as most used joint in a
human body.3,5,40 The important functions of the TMJ
are mastication and speech.
Temporomandibular disorder (TMD) is a generic
term used for any problem concerning the jaw joint.
Injury to the jaw, temporomandibular joint, or muscles
TEMPOROMANDIBULAR JOINT (TMJ)
TMJ Anatomy
TMJ, a joint that connects the mandible to the skull
and regulates mandibular movement, is a bi-condylar
joint in which the condyles, located at the two ends of
the mandible, function at the same time. The movable
round upper end of the lower jaw is called the condyle
and the socket is called the articular fossa (see Fig. 1).
Between the condyle and the fossa is a disc made of
fibrocartilage that acts as a cushion to absorb stress
and allows the condyle to move easily when the mouth
opens and closes.5,46
The features that differentiate and make the TMJ a
unique joint are its articular surfaces covered by
Address correspondence to Shirish Ingawale´, Department of
Biomedical, Industrial and Human Factors Engineering, Wright State
University, 3640, Col. Glenn Hwy, Dayton, OH 45435, USA. Electronic mail: [email protected] and [email protected]
976
0090-6964/09/0500-0976/0
Ó 2009 Biomedical Engineering Society
TMJ Disorders, Treatments, and Biomechanics
Ligament Disc
Articular fossa
977
separate axis of rotation. Rotation and anterior translation are the two primary movements. Posterior
translation and mediolateral translation are the other
two possible movements of TMJ.24
Muscle
TEMPOROMANDIBULAR DISORDER (TMD)
Condyle
The temporomandibular joint
FIGURE 1. Anatomical structure of the temporomandibular
joint (TMJ). Source: American Association of Oral and Maxillofacial Surgeons.5
fibrocartilage instead of hyaline cartilage. The bony
structure consists of the articular fossa; the articular
eminence, which is an anterior protuberance continuous with the fossa; and the condylar process of the
mandible that rests within the fossa. The articular
surfaces of the condyle and the fossa are covered with
cartilage.46 A dense fibrocartilaginous disc is located
between the bones in each TMJ. The disc divides the
joint cavity into two compartments (superior and
inferior).46,92 The two compartments of the joint are
filled with synovial fluid which provides lubrication
and nutrition to the joint structures.40,92 The disc distributes the joint stresses over broader area thereby
reducing the chances of concentration of the contact
stresses at one point in the joint. The presence of the
disc in the joint capsule prevents the bone-on-bone
contact and the possible higher wear of the condylar
head and the articular fossa.9,54,68,92 The bones are
held together with ligaments. These ligaments completely surround the TMJ forming the joint capsule.
Functioning of TMJ
The most important functions of the TMJ are mastication and speech. Strong muscles control the movement of the jaw and the TMJ. The temporalis muscle
which attaches to the temporal bone elevates the
mandible. The masseter muscle closes the mouth and is
the main muscle used in mastication.45 Movement is
guided by the shape of the bones, muscles, ligaments,
and occlusion of the teeth. The TMJ undergoes hinge
and gliding motion.3 The TMJ movements are very
complex as the joint has three degrees of freedom, with
each of the degrees of freedom associated with a
Temporomandibular disorder (TMD) is a generic
term used for any problem concerning the jaw joint.
Injury to the jaw, temporomandibular joint, or muscles
of the head and neck can cause TMD. Other possible
causes include grinding or clenching the teeth, which
puts a lot of pressure on the TMJ; dislocation of the
disc; presence of osteoarthritis or rheumatoid arthritis
in the TMJ; stress, which can cause a person to tighten
facial and jaw muscles or clench the teeth;
aging.7,15,31,38,48,92,97 The most common TMJ disorders
are pain dysfunction syndrome, internal derangement,
arthritis, and traumas.18,21,22
TMD is seen most commonly in people between the
ages of 20 and 40 years, and occurs more often in
women than in men.21,22,106,107 In 1996, the National
Institutes of Health estimated that 10 million Americans had painful TMJ dysfunction and more women
being affected by it than men. Some surveys have
reported that 20–25% of the population exhibit
symptoms of TMD while it is estimated that 30 million
Americans suffer from it, with approximately one
million new patients diagnosed yearly.21,32,87,115
Disc Displacement
Coordinated movement of condyle and disc is
essential to maintain the integrity of the disc. Disc displacement is the most common TMJ arthropathy and is
defined as an abnormal relationship between the articular disc and condyle.26,97 As the disc is forced out of the
correct position there is often bone on bone contact
which creates additional wear and tear on the joint, and
often causes the TMD to worsen.15,97 Disc displacement
generates a popping sound when the disc is first forced
out of alignment as the mouth opens up and then again
as the disc is forced back into place as the mouth is
closed. Clinically, this popping sound or clicking is
regarded as an initial symptom of the temporomandibular joint internal derangement (TMJ-ID).97
The anterior disc displacement has different degrees
of severity. Wilkes developed staging classifications for
the TMJ related internal derangement, or disc displacement (see Table 1).31,108,109 These stages were
defined based on clinical or radiological findings, or
based on the anatomic pathology of the jaw. The early
stage included slight displacement with clicking, and
no pain or dysfunction. The last stage included
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S. INGAWALE´
AND
T. GOSWAMI
TABLE 1. Wilkes’ staging classification of internal derangement of TMJ with respect to clinical, radiologic, and surgical findings.
I. Early stage
Clinical: No significant mechanical symptoms other then opening reciprocal clicking; no pain or limitation of motion
Radiological: Slight forward displacement; good anatomic contour of the disc; negative tomograms
Anatomic pathology: Excellent anatomic form; slight anterior displacement; passive in-coordination demonstrable
II. Early intermediate stage
Clinical: One or more episodes of pain; beginning major mechanical problems consisting of mid-to-late opening loud clicking; transient
catching, and locking
Radiological: Slight forward displacement; beginning disc deformity of slight thickening of posterior edge; negative tomograms
Anatomic pathology: Anterior disc displacement; early anatomic disc deformity; good central articulating area
III. Intermediate stage
Clinical: Multiple episodes of pain; major mechanical symptoms consisting of locking (intermittent or fully closed); restriction of motion;
difficulty with function
Radiological: Anterior disc displacement with significant deformity or prolapse of disc (increase thickening of posterior edge); negative
tomograms
Anatomic pathology: Marked anatomic disc deformity with anterior displacement; no hard tissue changes
IV. Late intermediate stage
Clinical: Slight increase in severity over intermediate stage
Radiological: increase in severity over intermediate stage; positive tomograms showing early-to-moderate degenerative
changes—flattening of eminence, deformed condylar head, sclerosis
Anatomic pathology: Increase in severity over intermediate stage, hard tissue degenerative remodeling of both bearing surfaces
(osteophytosis); multiple adhesions in anterior and posterior recesses; no perforation of disc or attachments
V. Last stage
Clinical: Characterized by crepitus; variable and episodic pain chronic restriction of motion and difficulty with function
Radiological: Disc or attachment perforation; filling defects; gross anatomic deformity of disc and hard tissues; positive tomograms with
essentially degenerative arthritic changes
Anatomic pathology: Gross degenerative changes of disc and hard tissues; perforation of posterior attachment; multiple adhesions;
osteophytosis; flattening of condyle and eminence; subcortical cyst formation
Sources: Wilkes109; Gadd and Goswami31.
degenerative changes to the disc with possible perforation, flattening of bones, pain, and restricted
motion.31,109 In an early stage, there is a simple disc
displacement in the closed mouth position, usually
anteriorly, due to weakness of the discal ligaments.78,79
If the displaced disc returns to its normal position when
the mouth is opened, accompanied by a popping sound,
it is referred to as disc displacement with reduction
(see Fig. 2).72,78,86 If the displaced disc does not return
to the normal position and acts as an obstacle during
attempted mouth opening, the joint appears as locked.
This is referred to as disc displacement without reduction.72,78,86 Almost 70% of TMD patients have disc
displacement.21,26 According to Tanaka et al.97 stress
distributions in the TMJ with a normal disc position are
substantially different from those with anterior disc
displacement. It is suggested that the disc displacement
induces the change of stress distribution in the disc and
the increase of frictional coefficients between articular
surfaces, resulting in the secondary tissue damage.91,93
The internal derangement frequently precedes the onset
of TMJ osteoarthritis.92
Other Factors Causing TMD
Different types of functional malocclusion have
been shown to be partly responsible for signs and
symptoms of TMD. The functional unilateral posterior
crossbite (FUPXB) might be a contributing factor for
mandibular dysfunction.75 The habitual body posture
(HBP) during sleep is also speculated as being one of
the possible reasons for disc displacement.41 A study
conducted by Hibi and Ueda41 suggests that HBP
allows the ipsilateral condyle to displace posteriorly
and this posterior position causes anterior disc displacement. Juvenile chronic arthritis, a chronic arthritis
in childhood with an onset before the age of 16 years
and a duration of more than 3 months, is also reported
as a TMD risk factor.6
Animal studies indicate that the TMJ can adapt to
changing biomechanical stresses allowing affected tissues of the joint to maintain efficient function in the
presence of changing load demands. However, this
adaptability may be adversely affected by several factors including advanced age, tissue perturbations
caused by previous traumatic injury, enhanced sympathetic tone and hormonal influences.70 Milam and
Schmitz70 suggested that direct mechanical injury,
hypoxia-reperfusion injury, and neurogenic inflammation are the mechanisms involved in degenerative
processes affecting the TMJ. Mechanical stresses lead
to the accumulation of damaging free radicals in affected
articular tissues of susceptible individuals.69,71,74
Free radicals are molecules capable of independent
existence that have one or more unpaired electrons in
their outer orbits.37 If allowed to proceed unchecked,
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FIGURE 2. A schematic representation of the position of the TMJ disc in three different conditions: a healthy joint, anterior disc
displacement with reduction (ADDWR), and anterior disc displacement without reduction (ADDWOR). Source: Pe´rez-Palomar and
Doblare´78.
free radical-mediated reactions can be extremely
harmful by damaging extracellular and cellular molecules, and by excessive activation of cellular processes.69,71,122 Free radicals may accumulate in articular
tissues of the TMJ as a result of mechanical stresses
generated during functional or parafunctional movements of the jaw, or with clenching or bruxism. Accumulation of free radicals in the articular tissues of the
TMJ can cause significant tissue damage, microbleeding, and pain. In individuals predisposed to develop
excessive mechanical stresses in the TMJ because of
unique structural or functional characteristics of masticatory system, adaptive mechanisms of the TMJ may
be exceeded by free radical accumulation leading to a
dysfunctional state (i.e., disease state).71 Because
hemoglobin constitutes the largest iron store in the
body, it is speculated to be a potential source of redox
active iron which can catalyze the formation of free
radicals that might be damaging to the joint.120–122
Zardeneta et al.122 showed the presence of fibronectin
fragments, which may stimulate proinflammatory
responses, in samples obtained from symptomatic
human TMJs.42
The Gender Paradox
The majority of TMJ patients is female, aged
between 20 and 40 years.21,106,107 The female to male
patient prevalence is reported to be varying from 3:1 to
8:1.14,21,22,36,87 Because of the high predilection for the
TMJ symptoms in women compared with men, and
because these symptoms are more common during
childbearing years, some researchers suggest that the
female sex hormones may have a role in the pathogenesis of the TMJ disorders. Sex hormones are known
to influence the differentiation, growth and development, and metabolism of connective tissues. A study
conducted by Abubaker et al.1 suggests that sex hormones affect the extracellular matrix of the TMJ disc
of female and male rats.1 These effects on the biochemical composition of the disc can theoretically alter
the biomechanical properties of the connective tissue
such as those in the TMJ. Unfortunately, it is not yet
known whether the female sex hormones or the
estrogen receptors or some other factors are responsible for the TMD gender paradox.22
TREATMENT OPTIONS FOR TMD
Treatments for the various TMJ disorders range
from physical therapy and nonsurgical treatments to
various surgical procedures. Usually the treatment
begins with conservative, nonsurgical therapies first,
with surgery left as the last option. The majority of
TMD patients can be successfully treated by non-surgical
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S. INGAWALE´
therapies and surgical interventions may be required
for only a small part of TMD population. All nonsurgical treatment options must be exhausted before
undertaking the invasive methods for the management
of TMD. Many of the treatments listed below often
work best when used in combination. The correct
course of action may vary, for example: medication,
therapy, splints, arthrocentesis, discectomy, or prosthesis.15 The initial treatment does not always work and
therefore more intense treatments such as joint
replacement may be a future option. The success of
joint replacement surgeries significantly depends on the
number of prior surgeries with better outcomes for
patients with fewer previous TMJ surgeries.66,67,84,112,115
Self-Care
Physical therapy is often used by TMD patients to
keep the synovial joint lubricated, and to maintain full
range of the jaw motion. One such exercise for the jaw
is to open the mouth to a comfortable fully-open
position and then to apply slight additional pressure to
open the mouth fully. Another exercise includes
stretching the jaw muscles by making various facial
expressions.31 Avoiding extreme jaw movements, taking medications, applying moist heat or cold packs,
eating soft foods are other ways that may keep the
disorder from worsening.40
Splints
Splints are plastic mouthpieces that fit over the upper
and lower teeth (see Fig. 3). They prevent the upper and
lower teeth from coming together, lessening the effects
of clenching or grinding the teeth. The splints also
correct the bite by positioning the teeth in their most
correct and least traumatic position.18 Dental splints
AND
T. GOSWAMI
are often used as a short-term treatment during orthodontic management, before orthodontic therapy, or if
the TMJ disorders occur during dentofacial orthopedic
procedures.25 Bruxism is believed to cause the TMJ
dysfunction due to tooth attrition and subsequent
malocclusion; myofascial strain, fatigue or fibrosis of
masticatory muscles; and capsulitis and adhesions
within the TMJ joint space.47 Splints are used to help
control bruxism,17,19,33,34,47,81,85,95,117 a TMD risk factor in some cases. Splints are effective in reducing the
intensity of pain for patients with pain in jaw and
masticatory muscles by compensating for or correcting
perceived bite defects of the sufferer.17,19,33 The studies
on evidence-based medicine for splint therapy, however, have shown equivocal results.2,28,29,57,60,101 The
long-term effectiveness of this therapy has been widely
debated and remains controversial.17,19,85,117
Surgery
Surgery can play an important role in the management of TMDs. As different surgical approaches for
treating the same condition are often recommended in
the literature, it is essential to understand which
approach can be more beneficial when surgery is needed. Conditions that are always treated surgically
involve problems of overdevelopment or underdevelopment of the mandible resulting from alterations of
condylar growth, mandibular ankylosis, and benign and
malignant tumors of the TMJ.59 The surgical treatments
such as arthrocentesis, arthroscopy, discectomy, and
joint replacement are discussed below.
Arthrocentesis
Arthrocentesis is the simplest form of surgical
intervention into the TMJ performed under general
anesthesia for sudden-onset, closed lock cases (restricted
jaw opening) in patients with no significant prior history
of TMJ problems.4,18 Arthrocentesis is not only the least
invasive of all surgical procedures but also carries a very
low risk. It involves inserting needles inside the affected
joint and washing out the joint with sterile fluids (see
Fig. 4). Occasionally, the procedure may involve
inserting a blunt instrument inside the joint to dislodge a
stuck disc.4,18,61
Arthroscopy
FIGURE 3. A dental guard or splint. Source: Dental Care
Ottawa.20
Arthroscopy is a surgery performed to put the
articular disc back into place. During this surgery a
small incision is made in front of the patient’s ear to
insert a small, thin instrument that contains a lens and
light. This instrument is connected to a video screen,
allowing the surgeon to examine the TMJ and surrounding area. Depending on the cause of the TMD,
the surgeon may remove inflamed tissue or realign the
disc or condyle.18,35 However, if the ligament and retrodiscal tissue was previously stretched beyond its
elastic range, then just popping the disc back into place
is only a temporary fix as the joint still would not work
as well as usual. Therefore, an anchor—Mitek mini
anchor—and artificial ligaments have been used for
several years to stabilize the articular disc to the posterior aspect of the condyle (see Fig. 5).27,110,111
When disc repositioning and stabilization are indicated, the Mitek mini anchor system offers significant
advantages over other disc repositioning methods.27,62,114
The Mitek mini anchor has been analyzed in various
studies to assess its performance. A 2-year follow-up
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study showed a success rate of 90% in reference to
incisal opening, jaw and occlusal stability, and significant reduction in presurgical pain level.110 Fields and
Wolford27 have demonstrated osseointegration of the
Mitek anchor in human condyles. The Mitek anchors
are reported to remain intact and biocompatible for as
long as 59 months.27 Mehra and Wolford62 reported
that, in 105 patients (188 discs) treated with Mitek mini
anchors, the radiographic evaluation for the follow-up
over 14–84 months demonstrated no significant condylar resorption or positional changes of the anchors.
They also reported a statistically significant reduction
in TMJ pain, facial pain, headaches, the TMJ noises
and disability; and improvement in jaw function and
diet. The Mitek mini anchor also provides an effective
method for prevention of condylar dislocation while
permitting some controlled translation.114
Discectomy
FIGURE 4. Arthrocentesis. Source: Mayo Foundation for
Medical Education and Research.61
FIGURE 5. The Mitek mini anchor for repositioning and stabilization of TMJ disc. It is composed of a titanium alloy body,
5 mm in length and 1.8 mm in diameter. Two nickel titanium
wings provide the intra bony locking mechanism while an
eyelet in the body allows attachment of sutures which function as artificial ligaments. Source: Wolford.110
Discectomy is a surgical treatment, which is
often performed on individuals with severe TMD, to
remove the damaged and very often dislocating
articular disc without going to a more extreme treatment such as a joint prosthetic.18 However, removal of
the painful pathologic disc causes the TMJ reduced
absorbency and increased loading during articulation.39,90,92 Although materials such as tendon allografts are advocated for the use of disc replacement,
there are no ideal inter-positional materials that can
protect articular cartilage from degenerative changes
following discectomy.39
Joint Replacement
Joint replacement is a surgical procedure in which
the severely damaged part of the TMJ is removed and
replaced with a prosthetic device. While more conservative treatments are preferred when possible, in severe
cases or after multiple operations, the current end stage
treatment is joint replacement.92 If either a condyle or
a fossa component of the TMJ is replaced, the surgery
is called partial joint replacement. In total joint
replacement, condyle and fossa are both replaced (see
Fig. 6). Joint replacement is performed in certain circumstances such as bony ankylosis, recurrent fibrous
ankylosis, severe degenerative joint disease, aseptic
necrosis of the condyle, advanced rheumatoid arthritis,
two or more previous TMJ surgeries, absence of the
TMJ structure due to pathology, tumors involving the
condyle and mandibular ramus area, loss of the condyle from trauma or pathology.15,63,83,84,112,113 There
are now long-term studies available in the literature
that support the safety and efficacy of joint replacement under appropriate circumstances.65 However,
before a joint replacement option is ever considered for
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TABLE 2. Design requirements for a TMJ prosthesis.
No.
Description
1
2
3
4
5
6
7
8
9
10
11
Imitation of condylar translation during mouth opening
Unrestricted mandibular movements
Correct fit to the skull
Correct fit to the mandible
Stable fixation to the bony structures
Expected lifetime of more than 20 years
Low wear rate
Wear particles tolerated by the body
Biocompatible materials
Sufficient mechanical strength
Simple and reliable implantation procedures
Source: van Loon et al.106
FIGURE 6. Temporomandibular joint replacement. Source:
Mayo Foundation for Medical Education and Research.61
a patient, all non-surgical, conservative treatment
options must be exhausted; and all conservative surgical methodologies should be employed.83,84
TMJ IMPLANT DEVICES
TMJ devices are used as endosseous implants for
articular disc replacements, condylar replacements,
fossa replacements, and total joint prostheses. The
important characteristics for a TMJ implant to be
successful are biocompatible materials; functionally
compatible materials; low wear, and fatigue; adaptability to anatomical structures; rigidly stabilized
components; and corrosion resistant and non-toxic
nature.115 van Loon et al.106 have stipulated the specific requirements for the TMJ prosthesis to be a successful treatment option for the TMD patients (see
Table 2). They highlight the life expectancy of TMJ
implant as one of the most critical requirements. In
order to reduce the frequency of painful revision surgery, a TMJ device should have an expected lifetime of
more than 20 years. As the maximum life of most TMJ
prosthetics is 10–15 years, Gadd and Goswami31 suggested that the locking screws and locking compression
plate for the condylar part of the prosthetic should be
researched to increase implant stability and to avoid
bone loss due to revision surgery.
A total TMJ prosthesis was not described until
1974.50 Till then, surgeons had concentrated on
implanting either a fossa or a condylar head, but not
both.88 Although alloplastic TMJ prostheses were in
use since early 1960s; those became popular in 1980s
with the introduction of the Vitek-Kent prosthesis.
Many other companies then introduced their own designs of alloplastic TMJ devices.113 However, many of
the alloplastic devices failed in delivering the intended
results due to their vulnerability to the repeated
mechanical stresses encountered in the TMJ with
functional movements of the jaw. The predicted in vivo
service life of such devices was one to three years.68
The United States Food and Drug Administration
(FDA), in 1993, halted the manufacture of the TMJ
implants—except for Christensen and Morgan implants which were on the market prior to the enactment
of the medical device law in 1976102—due to lack of
safety and efficacy information to support its indicated
use.113 In 1993, the Dental Products Advisory Panel
reclassified TMJ implants into Class III—the highest
risk category.102,104 This means that all manufacturers
of TMJ devices would be required to submit a Premarket Approval Application (PMA)—demonstrating
safety and effectiveness—when called for by the FDA.
On December 30, 1998, the FDA called for PMAs from
all manufacturers of the TMJ implants.102,104 Although
many individuals and research groups introduced different designs of the TMJ prosthetic devices, only four
TMJ implants (from three manufacturers) are approved by FDA since December 30, 1998: (1) Christensen/TMJ Implants, Inc., total joint implant, (2)
Christensen/TMJ Implants, Inc., partial joint implant,
(3) Techmedica/TMJ Concepts implant, and (4) Walter
Lorenz/Biomet implant.102,104,105,113
Christensen TMJ Implant
The Christensen TMJ implant system was introduced in early 1960s.32,88 Later, in 1995, it was
described as a total joint replacement system for the
TMJ.88 The Christensen prosthesis system includes
either a partial or total TMJ prosthesis available as a
stock device (see Fig. 7). The Christensen fossa eminence prosthesis (FEP) is fabricated entirely of Cobalt–
Chrome (Co–Cr) alloy and is approximately 20–35 mm
across and 0.5 mm thick with a polished articulating
surface.32,83 This device can support either unilateral or
bilateral partial joint reconstruction. The Christensen
983
FIGURE 7. The Christensen prostheses. (a) Fossa eminence prosthesis. (b) Total prosthesis. Source: TMJ Implants, Inc.100
condylar prosthesis has a Co–Cr alloy frame work with
a molded Polymethylmethacrylate (PMMA) head and
is available in three lengths of 45, 50, and 55 mm.
Co–Cr bone screws and drill bits sized to the screws are
used to fix the FEP to the base of the skull and condylar
device to the ramus.32,83,99
Christensen’s implant registry data, from 1993 to
1998, shows that 55% of the patients who received
either partial or total Christensen TMJ prostheses were
under the age of 40, and 83% were under the age of 50.
The number of women in the registry (3081, 87%)
compared with men (434, 12%) emphasizes the gender
paradox.32 A total of 58% of the patients received
partial joint prostheses while total TMJ prostheses
were placed in 42% patients.32
Chase et al.15 studied effectiveness of the Christensen
TMJ prosthesis system in treating patients with severe
TMD. The study dealt with patients who were
recalcitrant to nonsurgical treatments or had had prior
surgical procedures that did not alleviate their symptoms. The results of this study indicated that the
Christensen TMJ prosthesis system might offer a
treatment modality for severe TMJ dysfunction with a
high degree of success.15 Wolford et al.112 reported that
a metal condylar head against a metal fossa in the
Christensen TMJ prosthesis device can increase the
metal wear debris, create stress loading of the fossa
component, cause metalosis and corrosion, and increase
exposure of elements in hypersensitive patients.
Techmedica/TMJ Concepts TMJ Implant
Techmedica, Inc. developed the joint prosthesis in
1989 as a custom-made device (see Fig. 8). However, in
July 1993, FDA halted the manufacture of any TMJ
FIGURE 8. The Techmedica/TMJ
Source: Wolford et al.112
Concepts
prosthesis.
devices developed after 1976, due to lack of safety and
efficacy information to support its indicated use. In
1997, Wolford et al.116 presented a 5-year follow-up
study on 36 patients with 65 TMJs reconstructed with
the Techmedica (now known as TMJ Concepts) total
joint prosthesis. This study reported the overall success
rate of 90% for long-term occlusal and skeletal stability and pain reduction of 89% after reconstruction.
Based on outcomes of this five year study, in 1997, the
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Techmedica/TMJ Concepts implant was approved by
the FDA.110,112,113
The Techmedica/TMJ Concepts total joint prosthesis uses materials that are well proven in orthopedic
joint reconstruction for hip and knee replacements.112,113 The fossa component of this device is
made from commercially pure titanium mesh
(ASTM F67 & F1341) with an articular surface made
of ultra-high-molecular-weight polyethylene (UHMWPE
ASTM F648).83,84,99,110,112 The body of condylar
component is made from medical grade titanium alloy
(ASTM F136) with a condylar head of cobalt–chromium–molybdenum alloy (ASTM F1537).83,84,99,110,112
Both the fossa and condylar components are secured
with titanium alloy (Ti–6Al–4V) screws.83
Prior to joint replacement surgery, the patient
undergoes a computed tomography (CT) scan of jaws
according to a specific protocol.83,113,115 Using the CT
data, a 3-dimensional plastic model of the TMJ and
associated jaw structures is made using stereolithographic
technology, a rapid prototyping technology, to produce
an anatomically accurate plastic model.83,99,113,115 This
model allows selective repositioning of the mandible on
the model into a predetermined functional and esthetic
position.113,115 The condyle is removed and any necessary
bony recontouring of the fossa and mandibular ramus is
completed and marked on the plastic model, since all the
alterations on the model must be accurately duplicated on
the patient intraoperatively.99,113 A custom-made Techmedica/TMJ Concepts total joint prosthesis conforming
to the patient’s specific anatomical morphology and jaw
interrelationship is then fabricated on the plastic
model.113,115 The data generated in the computer is utilized to guide multi-axis milling systems in shaping the
implants to the anatomy found on the anatomical models.99 To ensure optimum fit, implant shapes are finalized
by hand contouring with careful attention to anatomical
details.99
Compared to the ‘‘off-the-shelf’’ implant devices, a
patient-fitted Techmedica/TMJ Concepts prosthesis
provides a better fit and stabilization of its components
to the host bone thereby mitigating any micro-movement leading to loosening of the components and
maximizing the opportunity for osseointegration of
components and fixation screws.65,67,112 Osseointegration can contribute to improved patient function and
decreased micro-movement, which limits overall prosthesis wear and stress.112 Based on material selection,
treatment philosophy, and clinical experience, this
implant is reported to have provided the service life of
up to 14 years without evidence of untoward wear or
failure.65,113,115
A prospective study by Wolford et al.115 evaluated
the five to eight year subjective and objective results of
42 consecutive patients who had TMJ reconstruction
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using the Techmedica/TMJ Concepts custom made
total joint prosthesis. This study demonstrated that the
Techmedica/TMJ Concepts total joint prosthesis is a
viable technique for TMJ reconstruction as a primary
procedure and for patients with previous multiple TMJ
surgeries and severely damaged joint.
In 2002, Mercuri et al.,67 in a 107 months (standard
deviation 15.5 months) follow-up study of 97 patients
treated with Techmedica/TMJ Concepts, reported a
76% reduction in mean pain score, a 68% increase in
mean mandibular function and diet consistency score,
and a 30% increase in mandibular range of motion
after 10 years. In 2007, Mercuri et al.,65 in a mean
follow-up of 11.4 years (standard deviation 3.0; range
0–14) study of the patient-fitted Techmedica/TMJ
Concepts total reconstruction system, reported a significant reduction in pain scores, an increase in mandibular function and diet consistency scores, and
improvement in mandibular range of motion after
14 years. Both studies found the long-term quality of
life improvement scores to be statistically related to the
number of previous TMJ operations the patient had
undergone.65,67 Comparison analysis demonstrated
significantly better outcomes for patients with fewer
previous TMJ surgeries and without exposure to alloplastic TMJ devices.65,67 In 2008, Mercuri et al.64
published the results of 20 TMJ reankylosis patients
(total of 33 joints) treated with Techmedica/TMJ
Concepts total TMJ prosthesis system with the
autogenous fat grafted around the articulating portion
of the prosthesis at implantation. The follow-up data
for 50.4 ± 28.8 months showed improvement in
reported pain; increased jaw function; diet consistency;
and a significant improvement in postreplacement
maximum interincisal opening and quality of life.
Dingworth et al.23 and Wolford et al.112 published
the results of first direct clinical comparison of preimplantation and postimplantation subjective and
objective data from two similar groups of patients who
underwent reconstruction with two different TMJ
reconstruction systems. These studies evaluated 23
patients treated with Christensen prostheses (followed
for a mean of 20.8 months) along with 22 patients
implanted with Techmedica/TMJ Concepts prostheses
(followed for a mean of 33 months). The investigators
reported statistically significant improved outcomes
relative to post-surgical incisal opening, pain, jaw
function, and diet for the Techmedica/TMJ Concepts
prosthesis group compared to the Christensen prosthesis group.23,112
W. Lorenz/Biomet TMJ Implant
The W. Lorenz TMJ implant is a ‘‘ball and socket’’
type prosthetic joint similar to a knee or hip implant
FIGURE 9. The W. Lorenz/Biomet TMJ prostheses. (a) Condylar prosthesis. (b) Total prosthesis. Source: Biomet Microfixation.11
(see Fig. 9). This device is made of common materials
with over 30 years of successful use in orthopedic joint
replacement.84,103 The condylar component is manufactured from Cobalt–Chromium–Molybdenum (Co–
Cr–Mo, ASTM F799) alloy with a roughened titanium
porous coating on the host bone side of the ramal
plate.83,84,103 The ramus of mandibular component is
available in lengths of 45 mm, 50 mm, and 55 mm. The
fossa component is manufactured from a specific grade
of ultra-high molecular weight polyethylene (UHMWPE)
called ArCom which has shown a 24% reduction in wear
compared to traditional UHMWPE.83,84,103 The swan
neck curvature on the medial surface of condylar neck
avoids the inherent fitting problems of the right angle
design found in most metallic condylar prosthesis.83,84
The fossa is available in three sizes with predrilled holes
for the screws. It also has an exaggerated circumferential
lipping to protect the condyle from possible heterotopic
bone formation and to avoid condylar dislocation.83,84
Both the condyle and fossa implants are attached to
bone using self-retaining, self-tapping bone screws made
of titanium alloy (Ti–6Al–4V).83,84,103
After three year follow-up of 50 patients (69 joints;
31 unilateral and 19 bilateral) reconstructed with Lorenz/Biomet prosthesis, Quinn83,84 reported significant
improvement in pain intensity, mouth opening, and
functional diet capability. This study also reported one
complication of staph scalp infection, necessitating the
removal of fossa prosthesis after 10 months of service.
According to FDA documentation, a total of 268 joints
(92 unilateral and 88 bilateral) were reconstructed with
W. Lorenz/Biomet total TMJ replacement system after
appropriate non-surgical treatment and/or previous
implant failure.103 The average patient follow-up for
19.6 months demonstrated improvement in patients’
condition through decrease in pain, increase of function, increase in maximal incisal opening, and satisfaction with the treatment outcome.103 Barbick et al.7
have demonstrated that the Lorenz/Biomet prosthesis
985
fits satisfactorily in the majority of patients undergoing
surgical Techmedica/TMJ Concepts custom joint
replacement with minimal anatomical reduction. The
Lorenz/Biomet total prosthesis has not been studied in
pregnant women or children, therefore, the safety and
effectiveness for these patients is not known.103 The
safety and effectiveness of revision surgery using a
second set of W. Lorenz/Biomet total TMJ replacement
system implants is not known.
The information about the FDA approved TMJ
implants is summarized in Table 3. Outcomes of some
selective studies of patients treated with FDA
approved total TMJ devices are summarized in
Table 4. Investigating the outcomes of FDA approved
implants in a controlled comparative manner, and
evaluating biological characteristics of failed implants
compared to controls is essential to determine the
mechanism of implant failure. More rigorous comparative evaluations of the available implants can be
possible with the advent of the TMJ Implant Registry
and Repository (TIRR).22
ALLOPLASTIC MATERIALS
As the use of an alloplastic material eliminates the
donor site morbidity and the need for tissue harvesting,
several different alloplastic materials have been used to
replace lost articular tissues of the TMJ.65,68 Alloplastic materials used as medical devices have traditionally been viewed as biologically inert substances
that can be designed to achieve desirable mechanical
properties.30,68,123 Silicone rubber and Proplast/Teflon
(PT) were widely used materials in alloplastic TMJ
implants from mid 1970s to late 1980s. Silicone rubber
was used as permanent or temporary interpositional
material in TMJ reconstructive surgery since animal
studies had revealed that silicone rubber implants
placed into the TMJ after discectomy were typically
encapsulated by a fibrous reactive tissue capable of
functioning as a ‘‘pseudo-disc’’.68 Implants composed
primarily of carbon fiber and polytetrafluoroethylene
(PTFE/Teflon or PT) were introduced in the mid 1970s
to reconstruct the TMJ after discectomy.68 Early
reported successes with the use of these materials
included greater implant stability, and soft-tissue
ingrowth into the more porous PT implants.68
Adverse Tissue Responses to Alloplastic Materials
In many TMJ patients, alloplast materials initially
provided pain relief and improved function of the joint.
However, in most patients, these implant materials
(silicone rubber and PT) were found to gradually break
down as they could not sufficiently withstand the
986
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TABLE 3. Summarized information about the FDA approved TMJ implants.
Prosthesis
TMJ implant
Christensen/TMJ
Implants, Inc.
Property
Condylar
Features
Co–Cr alloy
20 mm x 35 mm (across)
0.5 mm (thick)
44
Co–Cr bone screws, drill bits
Co–Cr alloy. PMMA for head.
45, 50, 55 mm
Materials
Pure titanium (ASTM F67 &
F1341). UHMWPE (ASTM
F648) for articular surface
Dimensions
Available sizes
Accessories
Materials
Patient-specific
Patient-specific
Ti–6Al–4V alloy screws
A specific grade UHMWPE
(ArCom)
Dimensions
Available sizes
Accessories
–
3
Medical grade titanium
alloy (ASTM F136).
Co–Cr–Mo alloy (ASTM
F1537) for head
Patient-specific
Patient-specific
Co–Cr–Mo alloy (ASTM
F799) +
Roughened titanium porous
coating.
45, 50, 55 mm
3
Mainly a stock device
Serves as partial as well as
total prosthesis
The company has its own
implant registry
Some researchers believe
that the metal head
against the metal fossa
can cause more metal
wear debris
A custom-made device
Service life is reported up
to 14 years without
evidence of untoward
wear or failure
Materials
Dimensions
Available sizes
Accessories
Techmedica/TMJ
Concepts
W. Lorenz/Biomet
Fossa eminence
contact stresses generated during functional movements
of the jaw.30,83,110 The structural failure of the implants
resulted in formation of microparticulate implant debris
which elicited a foreign-body response characterized by
the presence of multinucleated giant cells.68 The
breakdown particles provoked the foreign body giant
cell reactions resulting in severe pain, headaches,
inflammation, fibrosis, malocclusion, progressive bone
and soft-tissue destruction, and severely limited joint
function often requiring further surgery.30,83,110,113
Zardeneta et al.123 suggested that the severity of the
biologic response to implant debris may be dependent
largely on the size of the debris particles. Implants
reduced to small particles elicit a more intense
inflammatory response than implants degraded to
larger particles.68,110,123 The inflammatory response to
PT or silicone rubber debris continues despite removal
of the failed implants because these materials are not
substantially degraded in vivo.68,110 Patients with previous exposure to failed materials and bony destruction resulting from foreign body inflammatory reaction
are likely to experience high pain and poor long-term
outcomes with alloplastic reconstruction.89 In such
revised reconstruction cases, implants showed the
potential to fragment in situ resulting in nonbiodegradable particles that stimulate a giant cell reaction
3
Co–Cr bone screws, drill bits
The ‘swan-neck’ curvature
of condylar component
offers better fitting of the
device
Service life is reported up
to 3 years without
evidence of untoward
wear or failure.
Long-term follow-up is
not available
and lead to degeneration of local structures, pain, and
limitation of mandibular opening.89
The next generation of joint replacements will
incorporate live tissues in an effort to reconstruct the
joint to its normal state. The TMJ tissue engineering
strategies, in the long-term, may need to combine the
disc and mandibular condyle along with other tissues
such as retrodiscal tissue in a single implant.22
Understanding the development and breakdown mechanisms of the TMJ lubrication may enable
us to develop a ‘‘good as new’’ treatment remedy for
TMDs.92
LOADING AND KINEMATICS OF TMJ
Mandibular motions result in static and dynamic
loading in the TMJ. During natural loading of the joint,
combinations of compressive, tensile, and shear loading
occur on the articulating surfaces.92 The analysis of
mandibular biomechanics helps us understand the
interaction of form and function, and mechanism of
TMDs necessary to develop methods to prevent, diagnose, and cure joint disorders.10,38,48,52,56,58 It also aids
in the improvement of the design and the behavior of
prosthetic devices, thus increasing their treatment
21
90
17
45
36
38
58
61
Quinn83
Gerard and Hudson32
Speculand et al.88
Wolford et al.112
Wolford et al.115
Mercuri et al.66
Mercuri et al.65
Christensen TMJ, Inc.
Techmedica TMJ Concepts, Inc.
Patients
Chase et al.15
Source
Device
102
97
69
65
60
26
109
34
Joints
Population
4.9 (range: 0–28)
4.2 (range: 0–12)
2.9 (range: 0–16)
?
?
?
?
2.9
Number
of prior TMJ
surgeries
0–14 years
(Mean = 11.4 years)
60–120 months
(Mean = 107.4 months)
60–96 months
5 years
1–120 months
(Median = 14.5 months)
12–84 months
(Mean = 50 months)
36–108 months
1–10 yr
(Mean = 2.4 yr)
Follow-up
TABLE 4. Results of the total TMJ reconstruction with FDA approved devices.
95% had pain improvement
86% had increased ability to eat
91% had improved incisor opening
No failures or complication reported
9 fractures of the condylar prostheses, at
an average of 5.8 yr after placement,
were reported. These factures were
attributed to uncorrected parafunctional habits and device fixation
82% had a pain improvement
88% had improved function
82% had improved interincisal opening
One condylar and one TJR prosthesis
required replacement
97% had overall functional improvement
77% could eat all types of food
76% reported satisfaction with the
treatment
No prosthesis required replacement
90% overall success rate for long-term
occlusal and skeletal stability
89% had pain reduction
Statistically significant improvement in
incisal opening, jaw function, and pain
level; and long-term stable occulsion in
all cases
Statistically significant decrease in lateral
excursion movements
Complications occurred in six patients
76% had reduction in mean pain scores
68% had increase in mandibular function
and diet consistency scores
30% improvement in mandibular range of
motion after 10 years
After 5 years of implantation, one case
required replacement of a ramus
component due to screw loosening
Pain levels, mandibular function, diet
consistency, and maximum interincisal
opening increased significantly over
time
85% had improved quality of life
Results
987
3 years
5.7 (range: 0–13)
69
Follow-up
Joints
Number
of prior TMJ
surgeries
Patients
50
Source
Quinn84
Device
Lorenz–Biomet
Population
TABLE 4. continued.
22 of the patients have had the joints
in function for longer than 3 years
Significant improvement in pain intensity,
mouth opening, & functional diet
capability.
One complication required removal of
fossa component
S. INGAWALE´
Results
988
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efficiency.56 As the TMJ components are difficult to
reach and as the applications of experimental devices
inside the TMJ cause damage to its tissue, the direct
methods are not used often.
In Vivo Testing
Some of the earlier studies suggest that the TMJ can,
in certain specific combination of muscle forces, be a
force-free joint.13 These studies were contrasted by
observations of Brehnan et al.12 and Hylander45 who
showed through the direct measurements that considerable forces were exerted on the TMJ during occlusion
as well as mastication.13 In face of these contrary
reports, Breul et al.13 performed stress investigation
using MRI scans of the TMJ in five different positions of
occlusion. For each position of the condyle, the
momentary center of rotation in the head of the mandible and the tangent attached to the temporal surface
were determined.13 The line connecting these two points
indicated the direction of the resulting compressive
force. By means of the force and the estimated extension
of the area available to the force transmission, the stress
distribution was calculated independently from the
position.13 This analysis showed that the TMJ was
subjected to pressure forces during occlusion as well as
during mastication and it was slightly eccentrically
loaded in all positions of occlusion.13
Korioth and Hannam55 indicated that the differential static loading of the human mandibular condyle
during tooth clenching was task dependent and both
the medial and lateral condylar thirds were heavily
loaded. Huddleston Slater et al.44 suggested that when
the condylar movement traces coincide during chewing, there is compression in the TMJ during the closing
stroke. However, when the traces do not coincide, the
TMJ is not or only slightly compressed during chewing.44 Naeije and Hofman73 used these observations to
study the loading of the TMJ during chewing and
chopping tasks. Mandibular movements of ten healthy
subjects were recorded using a jaw movement recording system during chewing and chopping of a latexpacked food bolus on the left or right side of the
mouth. Distances traveled by the condylar kinematic
centers were normalized with respect to the distances
traveled during maximum opening.73 The coincidence
of the opening and closing condylar movement traces
were judged without knowing their origin. The analysis
showed that the distances traveled by the condylar
kinematic centers were shorter on the ipsilateral side
than on the contralateral; and the kinematic centers of
all contralateral joints showed a coincident movement
pattern during chewing and chopping.73 The indication
that the ipsilateral joint is less heavily loaded during
chewing than the contralateral joint may explain why
patients with joint pain occasionally report less pain
while chewing on the painful side.
Hansdottir and Bakke38 evaluated the effect of TMJ
arthralgia on mandibular mobility, chewing, and bite
force in TMD patients (categorized as disc derangements, osteoarthritis, and inflammatory disorders)
compared to healthy control subjects. The pressure
pain threshold (PPT) was measured with an electronic
algometer during slight jaw opening. The PPT value
was determined as the amount of pressure applied at
which the sensation of pressure changed to pain.38
Maximum unassisted jaw opening was measured with
a ruler at the central incisors.38 Unilateral bite force
was recorded with a strain-gauge transducer placed on
the mandibular first molar. The transducer was covered with polyvinyl chloride tubes for protection, and
the force was measured during maximum clenches (2-s
duration) as the stored peak values on the digital display.38 The PPT, maximum jaw opening, and bite force
were significantly lower in the patients as compared to
that in controls. The patients were also found to have
longer duration of chewing cycles. The bite force and
jaw opening in patients were significantly correlated
with PPT.38 The most severe TMJ tenderness (i.e.,
lowest PPT) and the most impeded jaw function with
respect to jaw opening and bite force were found to be
more severe in the patients with inflammatory disorders than the patients with disc derangement or osteroarthritis.38
In Vitro Testing
Indirect techniques such as humanoid robotic
approach, physical modeling using photo-elastic systems, moire fringe technique, and laser holographic
interferometry were tried by researchers to evaluate
mandibular biomechanics.8,10,82,90,96 However, these
methods had limited success due to their ability to
evaluate only the surface stress of the model but not its
mechanical properties.
Mechanical Testing
Osteoarthritis of the TMJ is associated with articular cartilage degradation and eventual joint destruction due to collagen damage caused by excessive shear
strain.96,119 As shear strain can result in fatigue,
damage, and deformation; data on shear behavior
might help a better understanding of tissue damage in
the articular cartilage.94,96 Previously it was reported
that the shear stress is very sensitive not only to the
frequency and direction of the loading but also to the
amount of shear and compressive strain.94 To characterize the dynamic shear properties, Tanaka et al.96
tested the shear response of cartilage of 10 porcine
989
mandibular condyles using an automatic dynamic
viscoelastometer. The results showed that the shear
behavior of mandibular condylar cartilage is dependent on the frequency and amplitude of the applied
shear strain suggesting a significant role of shear strain
on the interstitial fluid flow within the cartilage.
As TMJs are mostly used dynamically during
habitual tasks, dynamic analyses seem to be the most
appropriate. Beek et al.8 performed sinusoidal indentation experiments and reported that the mechanical
behavior of disc was nonlinear and time-dependent.
Beek et al.10 simulated these experiments using axisymmetric finite element model and showed that a
poroelastic material model can describe the dynamic
behavior of the TMJ disc. Tanaka et al.90 carried out a
series of measurements of frictional coefficients on 10
porcine TMJs using a pendulum-type friction tester.
The results showed that the presence of the disc reduces
the friction in the TMJ by reducing the incongruity
between the articular surfaces and by increasing synovial fluid lubrication. This study highlighted importance of alternatives to discectomy to treat internal
derangement and osteoarthritis of the TMJ.
Finite Element Modeling
The finite element modeling has been used widely in
biomechanical studies due to its ability to simulate the
geometry, forces, stresses and mechanical behavior of
the TMJ components and implants during simulated
function.16,52,53,56,76–80,82,91,94,98 Experimental or clinical validation of theoretical predictions should be the
goal in any simulation endeavor.56 Chen et al.16 performed stress analysis of human TMJ using a twodimensional (2D) finite element model developed from
magnetic resonance imaging (MRI). Although there
are limitations for using a 2D finite element model to
estimate stress/strain for a three-dimensional (3D)
joint, it is possible to estimate the relative changes of
the stresses corresponding to a 2D motion of the TMJ.
However, the 3D models are more realistic.16
Figure 10 shows the meshes of the TMJ model. The
maximum von Mises stress, seen at the posterior portion of the disc, was about 8.0 MPa. The compressive
stress (about 8.0 MPa) was much higher than the
tensile stress (3.7 MPa). Due to convex nature of the
condyle, the compressive stresses were dominant in the
condylar region whereas the tensile stresses were
dominant in the fossa-eminence complex owing to its
concave nature.16
Although TMJ is a bicondylar joint, very few finite
element simulations have analyzed the different
responses of two sides of the joint. Beek et al.9 developed a 3D linear finite element model and analyzed the
biomechanical reactions in the mandible and in the
990
S. INGAWALE´
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T. GOSWAMI
FIGURE 10. Meshes of the TMJ model consisting of the disc, condyle and the fossa-eminence complex. Source: Chen et al.16
TMJ during clenching under various restraint conditions. Tanaka et al.91,98 developed a 3D model to
investigate the stress distribution in the TMJ during
jaw opening, analyzing the differences in the stress
distribution of the disc between subjects with and
without internal derangement. In 2008, Tanaka
et al.,93 from the results of finite element model of the
TMJ based on magnetic resonance images, suggested
that increase of the frictional coefficient between
articular surfaces may be a major cause for the onset of
disc displacement.
All of the above mentioned simulations considered
symmetrical movements of mandible, and the models
developed only considered one side of the joint. Pe´rezPalomar and Doblare´76 used the combination of the
finite element models of the TMJ comprising the two
joints and models for soft components to study
clenching of mandible. However, these movements
were considered to be symmetric. In 2005, Koolstra
and van Eijden52 developed a combination of rigidbody model with a finite element model of both discs
and the articulating cartilaginous surfaces to simulate
the opening movement of the jaw. Using the same
model, Koolstra and van Eijden53 performed finite
element analysis to study the load-bearing and maintenance capacity of the TMJ. The results indicated that
the construction of the TMJ permits its cartilaginous
structures to regulate their mechanical properties
effectively by imbibitions, exudation and redistribution
of fluid; and refreshment of this fluid can be performed
during normal function. However, these studies did
not dynamically simulate the TMJ as a two-sided joint
incorporating both discs and the most relevant ligaments and considering a nonsymmetrical movement of
the jaw.
In 2006, Pe´rez-Palomar and Doblare´77 developed a
3D finite element model that included not only the two
discs but also the most important ligaments and the
three body contact between all elements of the joints,
and analyzed biomechanical behavior of the soft
components during a nonsymmetrical lateral excursion
of the mandible to investigate possible consequences of
bruxism. The right lateral movement of the mandible
was performed in which case the right joint becomes
the ipsilateral TMJ (or working side) and the left joint
becomes the contralateral one (or nonworking side)
(see Fig. 11).77 The study reported maximum principal
stresses in the posterior band of the ipsilateral disc (up
to 2.5 MPa) and anterior band of the contralateral
disc; higher compressive stresses (up to 3.2 MPa) in the
posterior band and lateral part of the ipsilateral disc
(as it was compressed posteriorly against the temporal
bone); higher shear stresses (3.2 MPa) in the contralateral disc in the lateral part of the posterior band;
FIGURE 11. Schematic diagram of a lateral movement of the
mandible: (I) ipsilateral condyle, (C) contralateral condyle.
Source: Pe´rez-Palomar and Doblare´.77
and higher tensile stress in the contralateral ligaments
than that in the ipsilateral ligaments.77 This study
suggested that a continuous lateral movement of the
jaw may lead to perforations in the lateral part of both
discs, conforming with the indications by Tanaka
et al.91,94,98 Later, in 2007, Pe´rez-Palomar and
Doblare´79 suggested that unilateral internal derangement is a predisposing factor for alterations in the
unaffected TMJ side. However, it would be necessary
to perform an exhaustive analysis of bruxism with the
inclusion of contact forces between upper and lower
teeth during grinding.
Nearly 40% of the rear-end impacts during vehicle
accidents produce whiplash injuries.43 Whiplash injury
is considered as a significant TMD risk factor and has
been proposed to produce internal derangements of the
TMJ.49,80 However, this topic is still subject to
debate.22 In 2008, Pe´rez-Palomar and Doblare´,80
published the results of finite element simulations of
the dynamic response of TMJ in rear-end and frontal
impacts to predict the internal forces and deformations
of the joint tissues. The results, similar to suggested by
Kasch et al.,49 indicated that neither a rear-end impact
at low-velocity nor a frontal impact would produce
damage to the soft tissues of the joint suggesting that
whiplash actions are not directly related with TMDs.80
However; since this study has its own limitations such
as analysis of only one model, for low-velocity impacts,
without any restrictions like contact with some component of the vehicle; there is a need for more reliable
finite element simulations to obtain more accurate
numerical results.
Effects of TMJ Surgery
To assess the surgical replacement of TMJ, pre- and
post-surgical in vivo kinetics and kinematics of jaw had
been reported in the literature.51,66,118 TMJ reconstruction using the partial or total TMJ prosthetics, in
991
most cases, improves range of motion and mouth
opening in the TMJ patients. However, loss of translational movements of the mandible on the operated
side has been often observed, especially in anterior
direction, owing to various factors like loss of pterygoid muscle function, scarring of the joint region and
the muscles of mastication.118 Komistek et al.51
assessed in vivo kinematics and kinetics of the normal,
partially replaced, and totally replaced TMJs. Under
fluoroscopic surveillance, the subjects were asked to
open and close their jaw on a force transducer placed
between their molars nearest the joint. A data acquisition system recorded the bite force. The kinematic
data derived from fluoroscopy and the data output
from the force transducer were input into a mathematical model of the human jaw to determine the
kinetics of the TMJ.51 Less translation was reported in
the implanted fossa and total TMJ joints than in the
normal joints. The study suggests that total TMJ
implants only rotate and do not translate; and the
muscles do not apply similar forces at the joint when
the subject has a total TMJ implant, compared to a
subject who has a normal, healthy TMJ.
In the post-TMJ replacement follow-up studies,
Mercuri et al.67 obtained the measures of mandibular
interincisal opening and lateral excursions from direct
measurements using the measuring scale provided in
the survey, mailed to patients with instructions as to its
use. The assessment showed a 24% and a 30%
improvement in mouth opening after 2 years and
10 years, respectively. On the other hand, at 2 years
post-implantation there was a 14% decrease in left
lateral excursion and a 25% decrease in right lateral
excursion from the pre-implantation data.65,67 As the
loss of lateral jaw movement is a great disadvantage to
total TMJ prosthesis replacement, a future prosthesis
must allow some lateral translation as well as the
anterior movement of mandible on the operated side
when the mouth is opened.105
Most studies have collected the data by subjective
surveys or mandibular incisor motion rather than
condylar motion. Yoon et al.118 followed a kinematic
method that tracks the condylar as well as incisors
path of the TMJ motion. An electromagnetic tracking
device and accompanying software were used to record
the kinematics of the mandible relative to temporal
bone during opening–closing, protrusive, and lateral
movements.118 This was achieved using an electromagnetic sensor attached each to the upper and lower
plastic dental brackets, a magnetic source, and a digitizing probe used to locate anatomic points for defining
anatomic coordinate systems and landmarks of interest.118 Mean linear distance (LD) of incisors during
maximal mouth opening for the surgical patient group
was 18% less than the normal subjects. Mean LD for
992
S. INGAWALE´
FIGURE 12. Condyle kinematics healthy and diseased TMJs
during opening–closing and protrusive movements. Source:
Yoon et al.118
mandibular right and left condyles was symmetrical in
the normal group; however, in the surgical patient
group, measurements for operated condyle and unoperated condyle were asymmetric and reduced as compared with normal subjects by 57 and 36%,
respectively (see Fig. 12).118 In protrusive movements,
operated and unoperated condyles of surgical patients
traveled less and significantly differently as compared
with condyles of normal subjects, which moved almost
identically. For the surgical patient group, the mean
incisor LD away from the operated side and toward
the operated side as compared with the normal group
incisors were reduced by 67 and 32%, respectively.118
Various research articles on the TMJ underline the
importance of biomechanical analysis of the natural
joint to better understand the structural and functional
aspects; and of the reconstructed joint to assess the
implant function and performance. Numerous works
have focused primarily on calculating absolute magnitude of TMJ loading with finite element models.
Most of the methods reported in the literature have
certain limitations due to the complex nature of the
joint and also due to certain limitations of the techniques and software packages used for modeling and
analysis. The reported magnitudes of TMJ loading
differ significantly from one another because of difference in simulation conditions. The direct measurements also indicate a large discrepancies.92 Due to
these reasons; there is currently no universally agreedupon value of TMJ loading.92 Therefore, a more
comprehensive biomechanical analysis of the TMJ is
essential for better understanding of the movements,
applied forces, and resultant stresses in the natural
and/or artificial joint components.
CONCLUSION
The temporomandibular disorder (TMD) symptoms are exhibited by a large portion—nearly 20 to
AND
T. GOSWAMI
25%—of the population and, hence, this problem
should be looked at more fully. Though majority of the
TMD conditions can be successfully managed by various non-surgical and less-invasive treatments, joint
replacement becomes the only potential remedy for
certain TMD conditions. Assessing the outcomes of
the FDA approved TMJ implants in a controlled
comparative manner, and evaluating biological characteristics of failed implants compared to controls are
essential to determine mechanism of implant failure.
The biomechanical analysis is a useful tool to
understand the normal function, predict changes due
to alterations, and propose methods of artificial
intervention for the treatment of diseased or damaged
TMJ. The patient-specific computer models can be
used to estimate non-measurable TMJ loads through
finite element analysis to understand the underlying
mechanisms of TMD, necessary for developing and
improving the methods to prevent, diagnose and cure
joint disorders.
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Temporomandibular Joint: Disorders, Treatments, and Biomechanics S I and T

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