Passive resistance increases differentially in various jaw displacement directions

In the present study, the passive resistance of the human jaw system was quantified in relation to the three-dimensional jaw displacement and the Posselt-envelope, using both in vivo measurements and computer simulation.

In eight subjects, the jaw was passively displaced with a step-wise increasing force in three orthogonal directions. Muscle relaxation was monitored using electromyography (EMG) with visual feedback. A biomechanical model of an average human system was used to examine the contributions of the jaw muscles.

The largest excursion was found for the vertical direction. Protrusive and lateral directions were more restricted. In protrusive and lateral directions, the jaw could generally move beyond the Posselt-envelope. The stiffness of the jaw increased with proceeding jaw displacement in all directions. The stiffness was larger in the protrusive direction than in the vertical and lateral directions. The model's predictions of stiffness were comparable to the in vivo measurements. However, in protrusive direction, the maximum jaw displacement was larger than in vivo. The estimated passive muscle forces showed that vertical displacement was mainly restricted by the complete group of closing muscles, while protrusive and lateral jaw displacement was restricted by selective individual muscles.

The human jaw system has larger motion range in the protrusive and lateral directions than can be exploited by active muscle use. Stiffness of jaw displacement is higher in the protrusive direction compared to the vertical and lateral directions.