Biomedical Engineering Reference
In-Depth Information
Fig. 4
Geometrical model of the TMJs under asymmetrical loading
two bony areas, and hence the location of the contact, changes with the displacement
and/or the mandible loading.
The model associated with the elementary joint is for the most part the result of
the local geometry of the articular surfaces. As this geometry of the temporal surface
evolves, the area is divided into four simple geometric sectors (Fig. 3 ) .
Under plane loading on the mandible (symmetrical tightening), the elementary
joint can thus be modeled by an isolated contact which, depending on the opening,
is located in sector 2 or sector 3 (Fig. 3 ) . The perpendicular to the tangent plane,
which is common to the two surfaces, is not contained in a horizontal or parasagittal
plane.
When loading is asymmetrical (unilateral tightening), the median plane of the
mandible is not combined with the sagittal plane. This displacement of the jaw
results in different configurations for the two elementary joints. For the “open” or
“working” profile, the mandibular condyle comes into contact occasionally with the
temporal slope mainly as a result of the action of the masseters (sectors 2 or 3).
The opposite condyle tends to move back simultaneously into the glenoid towards
its highest point (sector 1). The geometries of the local surfaces that are in contact
tend to be directed towards modeling a linear annular type of joint. The ligaments
(mainly the external lateral) restrict the amplitude of transversal displacement to 1
or 2 mm when movements are almost plane (excluding lateral excursion). In this
configuration, the hinge joint then takes on the same behavior as a kneecap.
The model (Fig. 4 ) corresponding to asymmetrical loading (with the sensor in
position 34 or 36) presents one point of contact for the left mandible condyle or
a “ball-and-plane pair” (1BPP) and a “spherical pair” (1SP, joint with two links
allowing spherical motion of one link relative to the other) for the right condyle.
This model is described further in Sect. 3 when defining the limiting conditions for
modeling the mandible.
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