Biomedical Engineering Reference
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22.2.1.3 loading and boundary Conditions
The magnitude of masticatory muscle forces reaches its maximum under centric occlusion.
Therefore, the force vectors of the bilateral masticatory muscles (i.e., the superficial and deep
masseter, anterior and posterior temporalis, medial pterygoid, and superior and inferior lateral
pterygoid) corresponding to centric occlusion were applied to the three models. The magnitude of
each muscle force was assigned according to its physiological cross section and the scaling fac-
tor (Koolstra et al. 1988). In addition, the origin and direction of each muscle force were defined
from anatomical measurements (Faulkner et al. 1987). Boundary conditions had the models fixed
at the occlusal surface and the external regions of the temporal bone (Tanaka et al. 2000, 2001b;
Hu et al. 2003).
22.2.2 a
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As expected, the movement of the condyles and the discs was observed to be different among the
three models. The interaction between the discs and the cartilages and the tensile forces in the disc
attachments was related to the movement of the discs. The condyles moved backwards in the three
models, and the maximum displacements were 0.07 mm in the bond model and 0.3 mm in the con-
tact and gap models. In the bond model, the discs and the articular cartilages were bonded together,
so the movement of the condyles was restricted by the fixed temporal bones and the discs couldn't
move between the articular surfaces. The anterior and posterior attachments of the discs were in
tension, but the bonded discs and the articular cartilages did not produce sufficient tensile forces in
the discal attachments.
In the gap model, the movements of the condyles could be simulated similar to the contact
model. The discs slightly rotated around the condyles with the anterior band rotating upwards and
the posterior band rotating downwards. The rotation of the discs resulted in an abnormal condyle-
disc-fossa position. The anterior and posterior attachments of the discs were also in tension during
rotation around the condyles. However, the maximum tensile force occurred in the temporal poste-
rior attachments, which should be slack (Chen et al. 1998). Meanwhile, the maximum compressive
stresses of the gap elements were located between the center of the condyle and the posterior band
of the disc and between the intermediate zone of the disc and the anterior of the temporal bone,
resp e ct ively.
In the contact model, the discs were observed to move along with the condyles without rotation,
consistent with previous biomechanical research (Chen and Xu 1994; Chen et al. 1998; Devocht et al.
1996; Beek et al. 2000, 2001; del Palomar and Doblare 2006b, c, 2007). Moreover the backward
and upward slide of the discs occurred between the articular surfaces of the condyles and the fossa-
eminences. The posterior bands of the discs were finally located between the crests of the condyles
and the articular fossas, in agreement with the normal disc position (Incesu et al. 2004). Thus, the
contact elements between the disc and the articular cartilages could provide more accurate charac-
terization of movement of the condyle and the disc. The anterior attachments of the discs were in
tension, with the bilaminar zones slack, produced by the backward and upward slide of the discs.
The maximum tensile force in the temporal anterior attachment was 0.41 N, similar to Chen et al.
(1998). Moreover, the maximum contact stresses occurred between the intermediate zone of the disc
and the anterior of the condyle and the temporal bone. The contact area and the maximum contact
stress on the lower interface were higher than those on the upper interface, consistent with previous
results (Beek et al. 2001).
The stress distributions in the discs induced by the interaction between the discs and the carti-
lages varied among the three models. In the bond model, the average von Mises stresses were 0.15
MPa in the anterior bands, 0.23 MPa in the intermediate zone, and 0.42 MPa in the posterior band
(Figure 22.2). Likewise, the maximum tensile and compressive stresses of the discs fell over the
posterior band (Table 22.1), in combination with a poor ability to resist tensile loading (Kang et al.
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