The positional changes of the mandible in comparison with the maxilla can be observed and measured at three places (Fig 7-1): between the condyle and the articular fossa, between the jawbones, and between the occlusal surfaces of the teeth.
Only the mandibular movements with tooth contact, ie, occlusal displacements, are of interest to the dental technician. The changes of position between condyle and condylar path can be measured, and thus the inclinations of the condylar path can be determined. The positional changes between the jaws are recorded when the jaw relation is being determined. The complex mandibular movements are attributed to two types of movement of the joint: (1) rotary or hinge movement and (2) translatory or sliding movement.
The temporomandibular joint (TMJ) is both a pivot (rotary) and a sliding joint, which permits the following three basic mandibular movements: opening and closing movement, forward and backward movement, and lateral movement.
Opening and closing movement (depression and elevation or abduction and adduction) can be interpreted as pure hinge movement. This kind of hinge action is possible up to an opening movement of approximately 10 mm when the mandible is pushed backward at the same time. For wider opening, the condyle slides forward and downward (ventral-caudal direction) on the condylar path.
Forward and backward movement (protrusion and retrusion) happens as a simultaneous sliding movement of both condyles on the particular condylar paths downward and forward.
Lateral movement (laterotrusion and medio-trusion) can happen when one condyle slides downward and forward on its condylar path while the opposing condyle rotates around a vertical axis in its articular fossa.
The translating condyle (mediotrusive, nonworking-side, balancing, idling, or orbiting condyle) denotes the condyle sliding downward and forward during lateral movement; the side of the jaw of the translating condyle is correspondingly known as the translation or mediotrusive side or the nonworking or balancing side because the teeth are taken out of contact on that side.
The rotating condyle (laterotrusive, working-side condyle) is the condyle that turns. It is found on the side toward which the mandible is being moved; this side is known as the active or working side (laterotrusive side) because here the teeth remain in full contact and perform masticatory activity.
The mandibular movements are performed in accordance with the function of the masticatory system during speaking, chewing, or swallowing and are therefore known as functional movements. A distinction is made between free movements without tooth contact and movements with tooth contact, the so-called articulatory movements. During speaking, the mandibular movements happen both with and without tooth contact; when swallowing and grinding up food in the final phase, occlusal contacts are required.
The masticatory process goes through specific, habitual movement cycles that can be divided into phases. The sequence of movements depends on the consistency of the food and varies considerably among individuals, but it does follow a set principle. This means that food is crushed not by a "chopping" action (simply opening and closing the rows of teeth) but by occlusal surfaces grinding as they push against each other.To do this, the mandible must be brought into a slightly lateral position from where the teeth slide back into terminal occlusion, with a grinding effect.
Movement phases of a masticatory cycle run smoothly into each other (Fig 7-2):
- Opening phase: Starts with lowering of the mandible to take in the food.
- Lateral displacement (or protrusive movement): The mandible slides to the chewing side to grasp the food.
- Closing phase:The mandible is raised until it approaches cusp-to-cusp contact so that the food is crushed but not ground up.
- Occlusion phase: The mandible slides out of the lateral position and into terminal occlusion, during which the food is made smaller by a constant increase in the force applied until maximal intercuspation of the teeth is achieved. Now the mandible is guided not merely by joints and muscles but by the occlusal pattern of the teeth.