Smoothness of chewing jaw movements in adults with mandibular prognathism

Indices such as smoothness, movement time, peak velocity, and symmetry of the velocity profile have been shown to be effective in explaining the degree of skilfulness of human saccadic eye, limb, and jaw motions. We investigated whether adult subjects with mandibular prognathism show impaired smoothness of the masticatory jaw movements. Forty-nine adults with skeletal Class III malocclusions and 52 healthy adults with acceptably good occlusions were selected respectively as Test and Control subjects. Subjects of the Test Group were subdivided into two groups: Class III(closed) showed full occlusal contact between the upper and lower teeth at the habitual intercuspal position, whereas Class III(open) showed inability of occlusal contact between the upper and lower anterior teeth. Each subject was asked to chew a piece of chewing gum. The normalised jerk-cost (NJC), movement duration, and tangential velocity profile during jaw-closing movements were compared between groups. Test Groups showed greater NJC (P < 0·01) with longer movement duration (P < 0·01) and lower peak velocity (P < 0·01) than the Control Group did. Class III(closed) showed greater NJC (P < 0·01) with longer movement duration (P < 0·01) and lower peak velocity (P < 0·01) than Class III(open) did. Results show that the mandibular movements made by the Test Groups exhibit lower skilfulness than those made by the Control Group. The jaw movement skilfulness of the prognathic patients decreases most drastically with existence of malocclusal contact between upper and lower anterior teeth.