[The study of the relationships between the masticatory muscles activity and the craniofacial morphology in mandibular prognathism].

Facial asymmetry is usually manifested in mandibular prognathism. This is probably because of the intervention of masticatory muscles in some way or another. Functional factors of masticatory muscles are also believed to be responsible for the relapse of facial disharmony even after surgical orthodontic treatment. In the present study, cephalometric analysis was made on a total of 43 patients with mandibular prognathism. The functional activity of their masticatory muscles was measured quantitatively by using electromyography. The data thus obtained were analyzed with respect to their bearings on the morphological points of reference given on posteroanterior cephalogram. Our findings are as follows: 1) At the time of maximal clenching with a bite splint, differences in the activity of left and right masseter muscles had a significantly positive correlation with the deviations from the point Menton on the frontal image, and a significantly negative correlation with the angles of the mandibular ramus to the median line on the frontal view. However, no relation was established between the activity of the masseter muscles and that of the temporal muscles on either side. 2) At the time of maximal clenching in central occlusion, differences in the activities of the masseter muscles on both sides were significantly and positively correlated with the deviations from the point Menton on the frontal image, and significantly and negatively correlated with the angles of the mandibular ramus to the median line. Also a negative correlation was noted between the differences in the activity of temporal muscles on both sides and the deviations from the point Menton. 3) These results suggest that the larger the deflection of the mandible, the larger the activity of the masseter muscle on the deflective side.