2007 Volume 61 Issue 1 Pages 23-38
In this study, we examined the relationships among dentofacial deformity, lip-sealing function, and mastication in patients with jaw deformities in the vertical dimension. For the cross-sectional study, 17 volunteers with normal occlusion, 17 patients with jaw deformities and 13 patients who had received orthognathic surgical treatment were used. For the longitudinal study, 5 patients who had received orthognathic surgical treatment and who had corresponding records were selected. The purpose and methods of this study were explained to the subjects so they were properly informed and we had their consent. Surface electrodes were placed on the skin over the lower lip and the bilateral masseter muscles. The electrical signals were rectified and averaged using holster-type electromyography (ME3000P, Mega Electronics). Based on the EMG activities of the lower lip at rest, we divided the subjects into incompetent lip and competent lip groups. In addition, we obtained the averaged amplitude of the lower lip and the duration of masseter muscle activity while chewing gum with the lips in contact and apart. Unpaired t-tests, paired t-tests and Mann-Whitney tests were used for this study. The following results were obtained. 1)In the cross-sectional and longitudinal subjects, the greater mandibular clockwise rotation and anterior facial height significantly (p<0.05) decreased after orthognathic surgery in the jaw deformity group. However, the degree of mandibular clockwise rotation was still greater in the jaw deformity group than in the control group. 2)All subjects in the pre-surgery group showed lip incompetence. The higher EMG activities of the lower lip at rest decreased after surgery, but half the subjects of the post-surgery group showed lip incompetence. 3)The lower lips of the pre-surgery group patients showed higher EMG activities while chewing gum with the lips in contact as compared with the patients in the control group. After orthognathic surgical treatment, the EMG activities of the lower lip significantly decreased, and showed no significant difference when compared to results for the control group. 4)The duration of 15 chewing strokes was significantly longer for the pre-surgery group than for the control group. Nevertheless, no significant decrease was observed after surgery. These results indicate that significant changes in the vertical position of the mandible may affect the recovery of masticatory muscle movement associated with the lip sealing function. The choice of surgical method for the facial height is important for patients suffering from lip incompetence.
