Anterior Displacement of the TMJ

Disk: Repositioning of the Disk

Using a Mitek System. A 3D
Finite Element Study
In this paper the behaviors of the temporomandibular joint (TMJ) with an anteriorly
A. Pérez del Palomar displaced disk without reduction and with a surgically repositioned one were compared
with the response of a healthy disk during jaw opening. The movement of each joint was
M. Doblaré obtained imposing the same opening path between incisors and assuming that the move-
e-mail: mdoblare@unizar.es
ment of the condyle is determined by the passive action of the masticatory muscles and
the restrictions imposed by the articulating surfaces and the ligaments. A fiber-reinforced
Group of Structural Mechanics and
porohyperelastic model was used to simulate the behavior of the articular disk. The
Materials Modeling,
influence of the friction coefficient in the diseased joint was also analyzed, finding that
Aragón Institute of Engineering Research (I3A),
the final displacement of the complex condyle-disk was smaller as the friction coefficient
University of Zaragoza,
increased. On the other hand, its displacement in the repositioned joint was different than
Spain
in the healthy case because the artificial sutures used in the surgery do not fully stabilize
the disk posteriorly as the retrodiscal tissue does. The stress response of the disk changed
in both pathologic cases: in the displaced joint the highest stresses moved from the
intermediate zone (healthy case) to the posterior band, and in the reconstructed one the
most loaded zone moved posteriorly at total opening. Besides, local stress concentrations
appeared in the neighborhood of the artificial sutures and therefore damage of the disk
and releasing of the sutures might be possible postoperatively.
关DOI: 10.1115/1.2246238兴

Keywords: temporomandibular joint, disk displacement without reduction, Mitek anchor,

porohyperelastic model, finite element analysis

1 Introduction tion. In this case, the disk is permanently anteriorly displaced with
respect to the condyle. The structural changes associated with the
Temporomandibular joint 共TMJ兲 disorders are a common cause
functional limitation associated with this pathology have been
of persistent facial pain, headaches, jaw clicking, and jaw locking
well demonstrated 关14兴. However, patients still show a mouth
关1兴. The causes of the internal derangement of the temporoman-
opening of 30– 40 mm owing to some remaining sliding motion
dibular joint are not clear, although one of them is believed to be
关15兴. This limitation will be temporary, because continuous use of
macrotrauma due to impact or hyperextension, since 45% of TMJ
the mandible will gradually push the disk further forward 关16兴,
patients report previous trauma to head and neck 关2兴. Disk dis-
and the mandible range of motion can improve to a point where in
placement is the most common consequence of TMJ internal de-
most patients, surgical intervention to increase mouth opening is
rangement, affecting in its early stage about 40% of the population not required 关17兴. However, when the disk displacement is asso-
关3兴. Disk displacement can result in a reduction of the joint space, ciated with uncontrollable pain, surgical intervention is advised.
clicking, arthritis, condylar resorption, malocclusion, inflamma- Thus, about 5% of the patients need surgery 关18兴. The main cri-
tion, and compression of the bilaminar tissue. All of this can cause teria for choosing a specific surgical procedure are patient’s age,
varying degrees of pain and dysfunction 关4兴. Later stages of TMJ whether the patient is symptomatic or not, duration of the patho-
disk displacement are less common, being characterized by per- logical symptoms, the degree of functional limitation, whether
manent disk displacement, interference in jaw opening, pain and conservative treatment has failed, and the number of previous
degenerative changes in the joint 关5–7兴. Chronic disk displace- operations 关13兴. Although Annandale et al. 关19兴 first described
ment can lead to deformation of the disk, loss of flexibility, vas- surgical repositioning of the displaced temporomandibular articu-
cularization of the disk, and a breakdown of the fibrocartilaginous lar disk in 1887, it was not until Wilkes 关20兴 used arthrography to
covers of the condyles and fossa 关8兴. Perforation of the disk, or describe the anatomy, form, and function of the TMJ in 1978, that
more commonly of the bilaminar tissue posterior to the disk disk repositioning became an accepted surgical technique 关17,21兴.
关9,10兴, can occur as well as development of intracapsular adhe- The reported clinical results of surgical TMJ disk repositioning
sions 关11–13兴. These changes can lead to a progressive worsening procedures have been variable and often unpredictable, with fail-
of the jaw function, pain, and, finally, to complete locking of the ures related to a lack of long term stability 关21兴, indicating the
joint that makes it impossible for the affected subject to open the need for improved methods of disk stabilization.
mouth. There are several techniques to recover the position of the disk
One of the intra-articular disorders that have been identified to its physiological position. One type uses the remaining tissues
with the locking of the TMJ is disk displacement without reduc- of the joint to stabilize the disk, suturing the disk directly to the
retrodiscal tissue or to the condyle through an intraosseous hole
Contributed by the Bioengineering Division of ASME for publication in the JOUR-
关22兴. However, when articular disks are displaced, the posterior
NAL OFBIOMECHANICAL ENGINEERING. Manuscript received September 9, 2005; final band and lateral ligaments are usually stretched and thin, herni-
manuscript received April 19, 2006. Review conducted by Jeffrey A. Weiss. ated, ruptured, or degenerated, and they lack adequate integrity to

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 defined by splines by means of a custom code, the temporoman-
dibular ligament was determined based on the observed disk po-
sition, the shape of bony components and anatomical knowledge
of the TMJ. The finite element meshes of disks and ligaments
were constructed using eight-noded brick elements. The retrodis-
cal tissue was modeled by a set of springs with a similar orienta-
tion. These were defined between the posterior part of the disk and
the temporal bone, and their stiffness was computed from the
stiffness of the retrodiscal tissue. The resulting finite element
model is shown in Fig. 2.
Fig. 1 A healthy joint with the disk located in its physiological
position „a…; anterior disk displacement without reduction „the
Three different cases were studied. First, a healthy joint with
disk is permanently displaced… „b…; repositioning of the disk the disk well positioned with respect to the condyle. This case was
with a Mitek anchor to its physiological position „c…. used as the control. Second, a severe displacement of the disk
without reduction was simulated. In this case, the disk is assumed
to be permanently displaced with respect to the condyle. The mesh
stabilize the disk and resist joint loading forces in the long term of the disk and diskal attachments was in this case modified in
关23兴. To overcome this problem, Wolford et al. 关13,24,25兴 devel- order to simulate the anterior displacement of the disk. The new
oped a surgical technique using a bone anchor 共Mitek anchor, location of the disk was obtained from a set of MR images of five
Mitek Products Inc., Westwood, MA, USA兲 to stabilize the TMJ different patients of ages ranged from 59 to 75 years with an an-
articular disk. This anchor is placed inside the condyle and ties the terior disk displacement without reduction. The diskal attachments
disk posteriorly using artificial ligaments. were modified to join the new position of the disk to the same
All the results that have been reported for this device are based points of the condyle in the healthy joint. Finally, the reposition-
on clinical evidence 关13,24,25兴, but no finite element study has ing of the disk was studied. The disk was relocated again on its
been developed to predict even qualitatively the most likely con- physiological position and the influence of introducing a reposi-
sequences of the introduction of this device on the behavior of the tioning system on the biomechanical behavior of the disk was
articular disk. As far as we know, all finite element simulations analyzed. Here, the collateral ligaments and the retrodiscal band
that have been performed for the temporomandibular joint only were eliminated. The three cases analyzed are shown in Fig. 2.
include the study of the joint in healthy and pathologic situations The model was considered symmetric, and thus the two temporo-
but without reproducing any surgical technique 关26–39兴. In this mandibular joints were treated in the same way.
paper, one case of anterior disk displacement without reduction A fiber-reinforced porohyperelastic model for the disk, which
共Fig. 1共b兲兲 is analyzed. Here, the consequences of a repositioning includes a transversely isotropic behavior for the solid phase was
operation 共Fig. 1共c兲兲 are also studied and compared with a healthy used. Collagen fibers were oriented in the mediolateral direction
joint without any surgical treatment using a computational model in the bands and in the anteroposterior direction in the intermedi-
of that surgical procedure. ate zone 关37,42,43兴. To take into account the fact that, as the tissue
is compacted, the porosity is smaller and consequently the perme-
2 Material and Methods ability decreases, a nonlinear permeability function that depends
The geometric data of the model developed here were obtained on the deformation was also included 关44兴. The disk was consid-
from previous works 关34,37,40兴. However, the methods are de- ered to be stress free in the initial configuration, i.e., in the closed
scribed briefly below. The contours of the cranium 共temporal mouth. The constitutive model for the disk was implemented as a
bone兲 and the mandible were obtained from CT images 共image user subroutine in the general-purpose finite element code
matrix of 1024⫻ 1024 pixels兲, and soft tissues contours were con- ABAQUS v.6.4 关45兴; this model is extensively detailed in 关41,46兴.
structed from MR data 共image matrix of 256⫻ 256 pixels兲 from a For the ligaments, a Neo-Hookean hyperelastic model with c1
healthy male patient aged 65. Bones were considered to be rigid = 6 MPa 关47兴 was used. With regard to the retrodiscal tissue, an
since they are much stiffer than the relevant soft tissues. For them, equivalent stiffness for the springs of 6.5 N / mm was introduced
a surface tesselation 共STL兲 was created and meshed automatically 关34,40兴, taking into account that the stiffness of this material has
in the commercial package I-deas v.9. The deformable parts of the been reported to be around 1.5 MPa 关48兴. The material properties
joint, the articular disk, and the ligaments were manually created of these tissues are summarized in Table 1.
and meshed. The definition of the disk was based on MR images There is little or no information regarding the displacement of
that were segmented semiautomatically with the different contours the condyles of the mandible in an anteriorly displaced disk with-

Fig. 2 Finite element model of the TMJ with details of the three cases analyzed: a
healthy TMJ, a displaced TMJ, and a repositioned disk. MED: medial; LAT: lateral. The
details shown in the middle and right parts are marked with a circle in the left figure.

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 Table 1 Material properties of the soft tissues that compose the joint

out reduction 关38,49兴 and in a repositioned joint. However, it has for the prescribed motion. The action lines of the passive muscles
been reported that the final opening movement of the mandible considered are shown in Fig. 3. In addition, the temporal bone was
measured between the incisors is similar to that in a healthy joint fixed in all simulations.
关13,15,50兴. Therefore, it was assumed that the displacement be- In order to define the restrictions imposed by the articulating
tween the incisors was the same in the three analyzed cases and surfaces, nine contact candidate surfaces were defined in each
the movement of the different components of the joint 共condyles side. The top and bottom surfaces of the disk were assumed to
and disks兲 in the three scenarios was guided by the passive restric- touch the surface of the temporal bone and condyle respectively;
tions imposed by the ligaments, the articulating surfaces and the the temporomandibular ligament contacts with the jaw and the
muscles. Regarding the restrictions imposed by the muscles, the lateral part of the disk; and the collateral ligaments were assumed
masticatory muscles can be divided into jaw openers 共lateral to interact with the disk, the temporomandibular ligament and
pterygoid and digastric兲 and jaw closers 共masseter, temporal and with both poles of the condyle. In the healthy case, a friction
medial pterygoid兲. As mentioned above, the opening movement of coefficient of 0.0001 was considered for all these contact surfaces
the jaw in the model was introduced by prescribing the motion of because the joint was considered to be well lubricated 关39,54兴.
the incisors of the lower jaw. Thus, the masseter, temporal and However, for both pathologic cases, the influence of the friction
medial pterygoid were modeled as passive elements and the di- coefficient was analyzed since it has been reported that abnormali-
gastric and lateral pterygoid were not included as it was assumed ties in the lubrication system contribute to TMJ dysfunction 关55兴.
that they were primarily responsible for providing the active force Thus, it has been observed that the friction coefficient may in-

Fig. 3 Imposed displacement to the incisor of the lower jaw during the opening move-
ment †51‡ and action lines of the medial pterygoid, anterior, and posterior portions of the
temporal and deep and superficial portions of the masseter muscles †52,53‡

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 Fig. 4 Displacement of the central point of the right condyle „point C in Fig. 3… in the anteriorly displaced disk without
reduction „ADDWOR… „a… and in the repositioned joint „b… for different friction coefficients, compared to the displacement of the
condyle in the healthy joint

crease due to deterioration in the lubrication system 关56兴. Further- placed inside the condyle. Since the bone has been simulated as a
more, it has been suggested that in pathological joints the friction rigid body, a fixed joint between the artificial sutures and the bone
coefficient between articulating surfaces is higher than in healthy was introduced, neglecting the effect of the anchor itself. The
ones 关57兴. position of the anchor may vary slightly from case to case, but is
As mentioned above, the passive forces exerted by the closing generally positioned 8 – 10 mm below the superior aspect of the
muscles were included. The muscles introduced were the medial condyle and just lateral to the midsagittal plane; therefore, this
pterygoid, anterior and posterior portions of the temporal muscle point was located on the rigid surface of the condyle 共see Fig. 2兲.
and the deep and superficial portions of the masseter muscle. Two Ethibond sutures 1-0 共Ethicom Inc., Somerville, NJ, USA兲
These elements were defined as connector elements between the were attached to the disk. These sutures join the disk to the
insertion points 关53,58,59兴 with a stiffness that depends on its condyle with two bows, separated 5 – 8 mm from each other, one
length. Thus, passive muscle behavior was modeled by a nonlin- in the medial and the other in the lateral part of the disk 关13兴. The
ear stress-strain relation 关60兴, in which the stress is related to the Ethibond sutures, made of polyester coated with polybutilate,
strain 共defined as the elongation relative to the optimum length of were simulated as truss elements with a section of 0.0869 mm2
the muscle兲, and the passive muscle stiffness. When the length of and a Young‘s modulus of 4561 MPa 关63兴.
the muscle is at or below its optimum length, its passive resistance The movement depicted in Fig. 3 obtained by Travers et al. 关51兴
is negligible, but it increases if it is stretched beyond this length. was introduced for the three cases to the middle point of the lower
Knowing the passive muscle stress for any length of muscle, the jaw just between the incisors. In order to check whether this im-
force due to the passive components can be computed by means posed movement, the restrictions of the articulating surfaces, the
of the physiological cross-sectional area 共PCSA兲 of the muscle ligaments, and the passive forces exerted by the muscles lead to a
关53兴. This behavior is defined in the following way: physiological movement of the remaining components of the
k·⑀ joint, the trajectory of the center of the condyle was analyzed for
F= · PCSA all cases 共point C in Fig. 3兲.
1 − ⑀/a It can be observed that although the final opening movement of
where the mouth was considered to be the same, the movement of the
condyle in each case was different. The displacement of the
l − lfree condyle in the healthy joint can be related to the experimental
⑀=
lfree results obtained by Travers et al. 关64兴. They measured a condylar
and movement of 11.9 mm in a straight line during the opening move-
ment, while our results yielded 11 mm for the same point. For the
Sfree case of the anterior displacement without reduction 共ADDWOR兲,
lfree = lrest · several calculations were made, varying the friction coefficient
Srest
between the articulating surfaces. In Fig. 4共a兲 it can be observed
where l is the final length of the muscle, lref is its optimum length, that the final displacement of the condyle decreases as the friction
Sref is the optimum length of the sarcomere 共2.73 ␮m 关61兴兲, and coefficient increases. This can be related to the findings of Tanaka
lfree and Sfree are the lengths of the muscle and of the sarcomere in et al. 关38兴, who found that a more accurate condylar translation in
a free state 关60兴. The values of PCSA, lfree and Sfree which depend an ADDWOR was related with a friction coefficient of around
on the type of muscle, were obtained from van Eijden et al. 关62兴. 0.01. Besides, it can be observed that although in the pathologic
The remaining factors were considered constants, where k is the case the opening movement of the mouth was supposed to be
estimated force length stiffness 共k = 3.34 N / cm2兲 and a is the pas- complete, the translation of the condyle is limited as it has been
sive force length asymptote 共a = 0.7兲 关60兴. Thus, passive forces found widely in the clinical experience 关50,65兴; therefore, our
were defined to be dependent on their length 关61兴 taking into results and those of previous findings 关49,51,66,67兴 suggest that
account their estimated fiber and sarcomere lengths and cross- for the detection of anterior disk displacement, one should mea-
sectional area as a measure of their force capabilities. sure the condylar motion rather than the lower incisor motion.
Finally, the surgical procedure for the repositioning of the disk Finally, the displacement of the condyle in the repositioned joint
was simulated using the correct position of the disk with respect was also analyzed for different friction coefficients of the surfaces
to the condyle and stabilizing it with an artificial device. The 共Fig. 4共b兲兲. This displacement could not be compared with any
Mitek mini anchor is a cylindrical device made of titanium that is other work in the literature because there is no information about

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 Fig. 5 Position of the right disk and condyle at maximum opening in the three analyzed
scenarios. In gray the initial configuration and in black the final configuration

the trajectory of the condyle in postsurgical joints, but the dis- joint is rather different. In the latter case, the disk is more rigidly
placement of the condyle in the repositioned case with a friction joined to the condyle and therefore it does not move posteriorly
coefficient of 0.01, resulted in 8% lower than in the healthy case. when the movement progresses.
The stress response of the disk in the healthy joint is shown in
Fig. 6. As mentioned above, this case will be used as the control
3 Results and the rest will be compared to it. It can be seen that the most
In the following, only the results in the disk of the right joint loaded part of the disk is the intermediate zone. Both the maxi-
will be presented, since the three cases have been considered sym- mum and minimum principal stresses were located in this part of
metric. In Fig. 5 the final configurations of the disk and condyle in the disk with the intermediate zone being responsible of absorbing
the three scenarios are shown. As mentioned before, although the most of the load. The posterior band was mainly working in ten-
imposed displacement between incisors was the same, the final sion and therefore the maximum principal stresses were higher
displacement of the condyle and subsequently of the disk were than the minimum principal in that zone. This effect comes from
different in each case. Here, the results shown for the displaced the fact that the retrodiscal tissue pulls from the disk posteriorly,
and repositioned disk correspond to a friction coefficient of 0.01. in order to achieve a correct opening movement of the mouth.
The influence of this coefficient in the stress response of the disk Moreover, taking into account that shear stresses seem to be re-
will be discussed later. It can be observed that, in the displaced lated to damage of the soft tissues 关68兴, maximum tangential
case, the disk is permanently displaced with respect to the stresses were also monitored, obtaining their maximum at the lat-
condyle. However, one of the most significant results is that the eral and medial poles of the disk.
position of the disk in the healthy case and in the repositioned The results in the anteriorly displaced disk are presented in Fig.

Fig. 6 Healthy joint. Minimum principal „SMIN…, maximum principal „SMAX…, and tangen-
tial stresses „STANG… „MPa… in the right disk. PB: Posterior Band; AB: Anterior band; IZ:
Intermediate Zone; L: Lateral; M: Medial.

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 Fig. 7 Anteriorly displaced disk. Minimum principal „SMIN…, maximum principal „SMAX…,
and tangential stresses „STANG… „MPa… in the right disk. PB: Posterior band; AB: anterior
band; L: lateral; M: medial.

7. It can be observed that, although the imposed displacement of medial part of the disk. Furthermore, the zones with higher
the jaw is the same, the movement is totally different with respect stresses moved posteriorly with respect to the healthy case.
to the healthy one, and consequently the stress pattern obtained. Besides, the artificial sutures modified the stresses in the poste-
The disk is permanently displaced with respect the condyle, there- rior band of the disk. The sutures are tied to the disk at two points
fore, the posterior band is supposed to absorb the compression located medially and laterally. This local application of the pulling
exerted by the condyle when it moves. Thus, in this case the most forces provokes stress concentrations as depicted in Fig. 9. There,
loaded part is located on the posterior part of the disk. Further- a detail of the posterior band where the sutures are located is
more, the friction coefficient in this simulation was increased with shown. In the neighborhood of those points the maximum princi-
respect to the healthy joint, due to the assumption of the degen- pal stresses tripled the mean value obtained in the posterior band.
eration of the surfaces, thus, the tangential stresses were distrib- These local concentrations could lead to perforations of the pos-
uted both on the top and bottom contacting surfaces with the terior band, as detected clinically 关69兴.
temporal and condyle surfaces, respectively. A comparison between the mean values obtained in the locked
Finally, the opening movement in an ideal repositioned joint is and in the repositioned disks with respect to the healthy one was
shown. As before, the movements of the disk and condyle were also made. To obtain more significative conclusions, these values
different from those in the healthy case, due to the presence of the were computed distinguishing different parts. The disk was di-
sutures that anchor the disk posteriorly and to the increase of the vided into the anterior and posterior bands, intermediate zone, and
friction coefficient. In this repositioned joint, the bilaminar tissue medial and lateral zones, and the mean value of the stresses was
is elongated and maybe even perforated, therefore, this tissue was computed in each zone. In Fig. 10, these comparative plots are
removed and the disk was only attached by means of the artificial depicted.
sutures. These prevented the disk from moving forward to an an- In the central part of the disk 共lateral, intermediate and medial
teriorly displaced position. However, in this simulation it was ob- zones兲 the mean stresses were similar in the healthy and in the
served that the sutures, in the absence of retrodiscal tissue, not repositioned joint; nevertheless, it can be observed that, while in
only prevent the disk from moving forward but backward, too. the healthy case the highest stresses were located in the lateral
The stress distribution in the disk after surgery is shown in Fig. part of the disk, in the repositioned joint, these were located in the
8. It can be observed that the stress pattern with respect to the medial part. In addition, the maximum shear stresses were mainly
healthy one is rather different as it could be inferred from the located in this latter zone in the repositioned joint, with stresses
movement of the joint that was obtained in this case. Here, the that were 30% higher than in the healthy one. On the other hand,
upper surface of the intermediate zone was mainly absorbing the behavior of the disk in the displaced joint was totally different,
compression stresses, but the bottom surface underwent both the the central region of the disk underwent lower stresses than in the
maximum and minimum principal stresses. In this case, higher other two analyzed cases, and the distribution of the stresses
maximum principal stresses appeared in the posterior band of the through the lateral, intermediate, and medial zones was much
disk because of the pulling exerted by the sutures. Finally, in this more uniform. The behavior of the bands can also be analyzed.
case, the shear stresses were higher and more localized in the The anterior band was nearly unloaded in the three scenarios,

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 Fig. 8 Repositioning surgery. Minimum principal „SMIN…, Maximum principal „SMAX…, and
Tangential Stresses „STANG… „MPa… in the right disk. PB: Posterior Band; AB: Anterior
band; L: Lateral; M: Medial.

while the posterior band was highly compressed in the displaced such as the anterior displacement of the articular disk, and even
joint. Finally, and taking into account that shear stresses are a less about the influence of a repositioning technique on the overall
good indicator to predict damage in soft tissues, higher stresses behavior of the joint. There are many factors that may affect the
were obtained in the medial region of the repositioned disk and in response of a joint, being clear that the mechanical environment
the posterior band of the displaced joint. In this latter zone, the of the joint will alter its response. Here, we have tried to obtain
shear stresses were 100% higher than in the healthy case. some additional understanding of the joint behavior in pathologic
Finally, the direction of the maximum principal stresses are situations. As far as we know, this is the first computational analy-
shown for the three cases 共Fig. 11兲. It can be observed that in the sis that dynamically analyzes the alteration of the biomechanical
healthy disk 共Fig. 11共a兲兲, these directions are clearly oriented in response of the articular disk of the TMJ in an anteriorly displaced
the direction of the collagen fibers 共in the anteroposterior direction disk without reduction and in a joint that has been repositioned
in the intermediate zone and in the lateromedial direction in the surgically.
anterior and porterior bands兲. In the displaced joint, the higher First, the role of lubrication in the response of the TMJ was
stresses were concentrated in the posterior band; there, the direc- analyzed. Many investigators have discussed the mechanism of
tions of the maximum stresses are a bit different but they are lubrication and stress distribution in this joint, and have recog-
oriented in a mainly lateromedial orientation. However, in the nized that the increase in friction in synovial joints may be caused
repositioned joint, it can be seen 共Fig. 11共c兲兲 that the directions of by a change in the lubrication system 关56,57兴. Moreover, it has
the stresses in the posterior band are more heterogeneous, appear- been reported that normal TMJ movements depend primarily on
ing the maximum stresses in the anteroposterior direction. the free sliding of the disk, understanding that aberrations in the
lubrication system contribute to TMJ internal derangement. In our
4 Discussion simulations, both for the displaced joint and for the repositioned
The function of the temporomandibular joint is still uncertain. one, we found that the increase of the friction coefficient prevents
Not much is known about the causes that lead to derangements the disk and condyle from moving in the same way as they do in
a healthy joint. Because no information about in vivo friction in
the TMJ is available, it is impossible to compare the friction co-
efficients used in this study with those measured in vivo. How-
ever, Tanaka et al. 关57兴 showed that the frictional coefficients
predicted in their simulations were higher in the symptomatic pa-
tients 共suffering from anterior disk displacement兲 than in the as-
ymptomatic ones. The results presented in this paper for both
pathologic scenarios have been selected for a friction coefficient
of 0.01. This assumption was made because there is no informa-
tion about this parameter in damaged joints, and a mean value of
Fig. 9 Repositioning surgery. Stress concentrations around friction coefficient within the range of 0.001–0.1 关70兴 was con-
the artificial sutures in the posterior band of the repositioned templated. The increase of friction leads to higher stresses in the
disk. surfaces of the disk, especially shear stresses in the inferior sur-

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 Fig. 10 Mean stress values in the different zones of the disk in the three
analyzed scenarios. Lat: Lateral Zone; IZ: Intermediate Zone; MED: Medial
Zone; AB: Anterior Band; PB: Posterior Band.

face of the disk. Thus, from these pathologic scenarios, it seems disk displacement without reduction兲 and a repositioned disk 共by
that if the lubrication system is compromised and the friction co- means of a Mitek anchor兲 were analyzed and compared with that
efficient between articulating surfaces grows, the stresses in the of a healthy disk. Many simplifications had to be made, but some
disk will also increase, and therefore the degenerative process in global trends can be inferred from our simulations. In the first
the joint may develop faster. place, the movement of the complex condyle-disk was very dif-
The stress distributions in the disk of a locked joint 共anterior ferent in the three cases. While in the healthy one the disk and the

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 Fig. 11 Directions of the maximum principal stresses at the top surface of
the disk in the three cases. „a… Healthy, „b… Displaced, „c… Repositioned.

condyle move together through the articular eminence and at possible that if the tissue tears, these sutures could be released and
maximum opening the retrodiscal band pulls the disk posteriorly, then the disk redisplaced anteriorly, as found clinically 关18兴.
in the anteriorly displaced joint the disk is permanently displaced In this paper, we have presented the differences in the behavior
with respect to the condyle and it cannot be reduced posteriorly to of the disk under different conditions. However, many simplifica-
its physiological position. Obviously, the displaced position of the tions were made for the development of the finite element models.
disk will determine the stress patterns observed. For the same We must emphasize the qualitative and comparative goal of this
friction coefficient, the displacement of the condyle in the dis- study due to the several assumptions made for the geometry of the
placed case was a 10% lower than in the healthy one, and for a pathologic joint, the material properties of the human soft tissues
friction coefficient in the diseased joint of 0.01, this difference and the loading conditions. First, the displaced disk in the AD-
was 16.4%. On the other hand, the movement of the repositioned DWOR was introduced by modifying the position and geometry
joint was also different because the sutures anchor the disk rigidly of the healthy disk. Here, the position of the disk was calculated
to the condyle, preventing it from moving posteriorly taking into with the mean position observed in five different patients, so,
account also that there is no retrodiscal tissue for pulling it pos- taking into account the wide variability among subjects, the re-
teriorly. Thus the relative movement between the disk and condyle sults obtained can be only considered as qualitative. In the case of
was also different that in the healthy case. The consequences of the Mitek anchor, the disk should be usually reshaped because its
this different movement are not clear. It is known that posterior morphology and appearance is altered and a total recovery is not
connective tissues play an important role for filling the posterior possible 关8兴. In this case, a total recovery of the position of the
joint spaces and controlling the disk position during jaw opening disk was supposed. In the clinical evidence, it is not possible to
关71兴. Our results showed that while the disk was pulled posteriorly obtain this total recovery but the aim of this work was to compare
in the healthy case, it remains almost fixed to the condyle in the the results in the same joint 共healthy and repositioned兲 where the
repositioned one, thus, although the movement of the condyle was discal attachments and retrodiscal tissue have been replaced by
similar in both cases, the motion of the disk was different. There- the sutures of the Mitek anchor device. It is clear that the presence
fore, it can be suggested that if the retrodiscal tissue is not too of a disorder in the joint may affect the articulating surfaces of the
damaged, the movement of the disk would be more physiological temporal bone and condyle. Therefore, the introduction of a more
if the disk is sewn to the retrodiscal tissue instead of to the realistic geometry of the components may probably modify the
condyle. quantitative results but not the qualitative trend of the simulations.
With regard to the stress behavior of the disk, we obtained that Some assumptions were also made regarding the material prop-
in the case of the displaced joint, the disk blocks the translation of erties of the tissues involved. The ligaments were treated as iso-
the mandible, therefore the highest stresses appeared in the poste- tropic hyperelastic, however they should have been considered as
rior band of the disk. The intermediate zone and anterior band prestressed transversely isotropic materials since they are com-
remained almost unloaded, and the highest shear stresses also ap- posed of fibers oriented along their longitudinal direction. As far
peared in the posterior band of the disk. These stresses were over as we know there are no experimental measurements of the strain
50% higher than in the other two joints analyzed. As mentioned distributions in these elements. Moreover, the mechanical proper-
previously, these stresses have been related to damage in soft tis- ties of these ligaments are not available. Therefore, average values
sues by several authors 关68,72兴. Besides, it has been shown clini- for their stiffness were considered and a simplification of their
cally that in displaced disks, the posterior band becomes thicker behavior was used 关37兴. There is little information about the me-
and can act as a pseudodisk 关73兴, however, the retrodiscal tissue chanical properties of the articular disk in pathologic temporo-
may be totally disrupted. In this simulation, the retrodiscal band mandibular joints. Only Tanaka et al. 关77兴 performed experiments
was simplified as an equivalent set of springs, and therefore, we to human temporomandibular joint disks in patients with severe
are not able to analyze its stresses, however, it could be argued internal derangement. They found that pathologic disks were more
from our results that if the highest stresses were located in the rigid than normal disks, but the differences were not too much
posterior band of the disk, the retrodiscal tissue could also be significant; therefore, in this work the same properties for the
overloaded, and then disrupted 关74兴. articular disk were introduced for all the cases. In addition, it is
In the repositioned disk, the results showed that the biome- known that damaged cartilage behaves in a different way, and it is
chanical response of the disk after this surgery was similar to the also known that the permeability increases as the stiffness of the
healthy one. The main difference has been discussed above and is solid matrix decreases. The influence of these parameters were not
related to the position between the disk and condyle. This differ- taken into account and it will be analyzed in further develop-
ence leads to changes in the most loaded zones of the disk, that ments. Regarding the initial configuration of the TMJ disk in our
move posteriorly with respect to the healthy case, with nearly analysis, it must be noted that the disk is located inside an articu-
40% higher stresses in the medial part of the disk. Moreover, it lar capsule filled with synovial fluid, and that it has been reported
has been reported that articular disk perforations are more fre- that the intra-articular pressure varies during the different move-
quently located in the lateral part of the joint 关75,76兴. This result ments. Likewise, it seems reasonable to think that, like other tis-
coincides with the healthy disk but in the repositioned one, the sues, the disk might be subject to residual initial stresses. How-
highest stresses were located in the medial zone. Besides, some ever, to the best of our knowledge, there are no data in the
local stress concentrations appeared in the surrounding zone of the bibliography that describe, even qualitatively, the initial stress or
sutures leading to stress values that tripled the average ones. The deformation of the disk in the resting position. For this reason, we
role of the sutures is to anchor the disk to the condyle, and there- preferred to consider the disk as stress-free in the closed position
fore, these artificial ligaments pull the disk posteriorly introducing than to impose a fictitious deformation which, due to the nonlin-
local concentrations of stresses in those zones of the disk. As a earity of the solid matrix behavior, may impact the value of the
result, the sutures may damage the surrounding tissue making stresses obtained. To analyze the influence of the Mitek anchoring

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 device only the artificial sutures were introduced and not the an- 155–166.
关11兴 Barkin, S., and Weinberg, S., 2000, “Internal Derangement of the Temporo-
chor itself. It might be hypothesized that the technique that has mandibular Joint: The Role of Arthroscopic Surgery and Arthrocentesis,” J.
been simulated here could be the repositioning of the disk fixing it Can. Dent. Assoc., 66, pp. 199–203.
to the condyle instead of the proper Mitek anchoring. However, 关12兴 Emshoff, R., Brandlmaier, I., Bertram, S., and Rudisch, A., 2002, “Comparing
the sutures were placed in the same way as in the Mitek anchoring Methods for Diagnosing Temporomandibular Joint Disc Displacement Without
system, therefore, although the anchor itself was not introduced, Reduction,” J. Am. Dent. Assoc., 133, pp. 442–451.
关13兴 Mehra, P., and Wolford, L. M., 2001, “The Mitek Mini Anchor for TMJ Disc
the response of the disk in this repositioning technique could be Repositioning: Surgical Technique and Results,” J. Oral Maxillofac Surg., 30,
evaluated. Finally, notwithstanding the fact that one of the worst pp. 497–503.
effects of these systems is bone resorption, in this paper, bone 关14兴 Wilkes, C. H., 1989, “Internal Derangements of the Temporomandibular Joint.
surfaces were modelled as rigid, and thus this effect could not be Pathological Variations,” Arch. Otolaryngol. Head Neck Surg., 115共4兲, pp.
469–477.
analyzed. 关15兴 Nitzan, D. W., and Dolwick, F. M., 1991, “An Alternative Explanation for the
With regard to the imposed displacement to the mandible, some Genesis of Closed Lock Symptoms in the Internal Derangement Process,” J.
comments have to be made. The movement was simulated impos- Oral Maxillofac Surg., 49共15兲, pp. 810–815.
ing a displacement in the low incisor of the jaw and assuming that 关16兴 Nitzan, D. W., and Marmary, Y., 1997, “The ‘Anchored Disc Phenomenon’: A
Proposed Etiology for Sudden-Onset, Severe, and Persistent Closed Lock of
the movement of the joint will be a consequence of the passive the Temporomandibular Joint,” J. Oral Maxillofac Surg., 55, pp. 797–802.
resistance of the masticatory muscles, and the restrictions imposed 关17兴 Dolwick, M. F., and Dimitroulis, G., 1994, “Is There a Role for Temporoman-
by the ligaments and the articulating surfaces. The advantage of dibular Joint Surgery,” Br. J. Oral Maxillofac Surg., 32共5兲, pp. 307–313.
this method is that the same border motion is prescribed in the 关18兴 Dolwick, M. F., and Nitzan, D. W., 1994, “The Role of Disc-Repositioning
Surgery for Internal Derangements of the Temporomandibular Joint,” J. Oral
three scenarios while the movement of the joint will be deter- Maxillofac Surg., 6, pp. 271–275.
mined by the passive restrictions of its components. Thus, assum- 关19兴 Annandale, T., 1887, “On Displacement of Intraarticular Cartilage of the
ing that the muscles have the same stiffness in the healthy and Lower Jaw and its Treatment by Operation,” Lancet, 1, pp. 411–412.
pathologic situations, the influence of the biomechanical environ- 关20兴 Wilkes, C. H., 1978, “Arthrography of the Temporomandibular Joint in Pa-
tients With the TMJ Pain-Dysfunction Syndrome,” Minn Med., 61共11兲, pp.
ment in the disk can be better analyzed. 645–652.
Finally, the two joints of the mandible were considered in the 关21兴 McCarty, W. L., and Farral, W. B., 1979, “Surgery for Internal Derangements
same way, both displaced or both repositioned. In practice, how- of the Temporomandibular Joint,” J. Prosthet. Dent., 42, pp. 191–196.
ever, the behavior of both joints is not the same, so, different 关22兴 Fields, R. T., Cardenas, L. E., and Wolford, L. M., 1993, “The Pullout Force
for Mitek Mini and Micro suture Anchor Systems in Human Mandibular
treatment is applied to each joint and then one possible effect such Condyles,” J. Oral Maxillofac Surg., 55, pp. 483–487.
as the lateral deflection of the jaw during mouth opening could not 关23兴 Sommer, O. J., Aigner, F., Rudisch, A., Gruber, H., Fritsch, H., Milleri, W.,
be analyzed here. This will be studied in the future. and Stiskal, M., 2003, “Cross Sectional and Functional Imaging of the Tem-
Therefore, it is clear that a more accurate description of the poromandibular Joint: Radiology, Pathology and Basic Biomechanics of the
Jaw,” Radiographics, 23共6兲, p. 14.
elements of the joint would probably modify the magnitude of the 关24兴 Wolford, L. M., 1997, “Temporomandibular Joint Devices: Treatment Factors
stresses obtained, but we consider that the present work is a good and Outcomes,” Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod., 83,
qualitative tool to predict the response of the soft tissues in the pp. 143–149.
joint under different loading conditions, and to establish the influ- 关25兴 Wolford, L. M., Cottrell, D. A., and Henry, C. H., 1994, “Temporomandibular
Joint Reconstruction of the Complex Patient With the Tech-Medica Custom-
ence of a specific pathology or surgical procedure on the biome- Made Total Joint Prosthesis,” J. Oral Maxillofac Surg., 52共1兲, pp. 2–12.
chanical response of the disk. 关26兴 Chen, J., and Xu, L., 1994, “A Finite Element Analysis of the Human Tem-
poromandibular Joint,” ASME J. Biomech. Eng., 116, pp. 401–407.
关27兴 DeVocht, J. W., Goel, V. K., Zeitler, D. L., and Lew, D. A., 1996, “A Study of
Acknowledgment the Control of Disc Movement Within the Temporomandibular Joint Using the
The authors gratefully acknowledge the support of the Spanish Finite Element Technique,” J. Oral Maxillofac Surg., 54共12兲, p. 1431–1437.
关28兴 Chen, J., Akyuz, U., Xu, L., and Pidaparti, R. M. V., 1998, “Stress Analysis of
Ministry of Science and Technology through the research Project the Human Temporomandibular Joint,” Med. Eng. Phys., 20, pp. 565–572.
No. DPI2003-09110-C02-01 and the Spanish Ministry of Health 关29兴 Nagahara, K., Murata, S., Nakamura, S., and Tsuchiya, T., 1999, “Displace-
through the National Network IM3 共Molecular and Multimodal ment and Stress Distribution in the Temporomandibular Joint During Clench-
Medical Imaging, Associated Partner, 300⫹⫹, 2003–2005兲. ing,” Angle Orthod., 69共4兲, pp. 372–379.
关30兴 Beek, M., Koolstra, J. H., van Ruijven, L. J., and van Eijden, T. M. G. J., 2000,
“Three-Dimensional Finite Element Analysis of the Human Temporomandibu-
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