Reference Planes

Sagittal Plane (Fig. 4-7). In the sagittal plane, the mandible is capable of a purely rotational movement as well as translation. Rotation occurs around the terminal hinge axis, an imaginary horizontal line through the rotational centers of the left and right condylar processes. The rotational movement is limited to about 12 mm of incisor separation before the temporomandibular ligaments and

Fig. 4-7. A, Rotation of the mandible in a sagittal plane can be made around the terminal hinge axis. B, After about 12 mm of incisal opening, the mandible is forced to translate. C, Maximum opening; the condyles have translated forward.

structures anterior to the mastoid process force the mandible to translate. The initial rotation or hinging motion is between the condyle and the articular disk. During translation, the lateral pterygoid muscle contracts and moves the condyle-disk assembly forward along the posterior incline of the tubercle. Condylar movement is similar during protrusive mandibular movement.

Horizontal Plane. In the horizontal plane, the mandible is capable of rotation around several vertical axes. For example, lateral movement consists of rotation around an axis situated in the working (laterotrusive) condylar process (Fig. 4-8) with relatively little concurrent translation. A slight lateral translation-known as Bennett movementt4 mandibular sideshift, or laterotrusion (Fig. 4-9)-is frequently present. This may be slightly forward or slightly backward (lateroprotrusion or lateroretru-sion). The orbiting (nonworking) condyle travels forward and medially as limited by the medial aspect of the mandibular fossa and the temporomandibular ligament. Finally, the mandible can make a straight protrusive movement (Fig. 4-10).

Frontal Plane. When observing a lateral movement in the frontal plane, the mediotrusive (or non-working) condyle moves down and medially while the laterotrusive (or working) condyle rotates around the sagittal axis perpendicular to this plane (Fig. 4-11). Again, as determined by the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed: as determined by the anatomy of the mandibular fossa on the laterotrusive side, this may be lateral and upward or lateral and downward (laterosurtrusion and laterodetrusion). A straight protrusive movement observed in the frontal plane, with both condylar processes moving downward as they

Fig. 4-8. Rotation in the horizontal plane occurs during lateral movement of the mandible. (The vertical axis is situated in the condylar process.) Normally there is relatively little translation (sideshift).

slide along the tubercular eminences, is shown in Figure 4-12.

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