The Dual Motion of Opening and Closing the Mouth

Jaw opening begins with pure rotation of the mandibular condyle against the inferior surface of the articular disc within the lower joint compartment. This rotational phase accounts for the first approximately 20–25 millimeters of opening.

Further opening requires translation: the disc and condyle glide forward together onto the articular eminence within the upper joint compartment. Closing reverses the sequence, with translation occurring first followed by rotation as the teeth approach occlusion.

This combination of rotation and translation—sometimes called ginglymoarthrodial motion—allows a greater range of opening than a pure hinge joint could provide. Lateral and protrusive movements similarly depend on coordinated activity in both the upper and lower compartments of each joint.

The smooth sequencing of these motions is one of the most elegant mechanical achievements of the human musculoskeletal system.

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