Electromyography of masticatory muscles in three jaw registration positions

The purpose of this study was to investigate whether anteroposterior changes in mandibular position affect masticatory muscle activity. The electromyographic (EMG) activity of masticatory muscles during full and partial (10%) clenching in three mandibular bite registration positions--retruded contact position (RCP), intercuspal position (IP), and muscular position (MP)--was studied. Three groups of subjects with different ranges of anteroposterior positioning of the condyles were evaluated (normal occlusion, Class II, Division 2 malocclusion, and dual bite malocclusion). A posterior stabilizing splint for each registration position was made. EMG data were obtained from three bilateral muscles (masseter, anterior temporal, and posterior temporal). Clenching in the RCP elicited the lowest masseter muscle activity during full clenching, and the highest anterior temporal and posterior temporal muscle activity during partial clenching. If the relationships of the masticatory muscles are analyzed through a ratio that represents the interaction between biting and positioning muscles (masseter/posterior temporal muscle ratio), the RCP as compared with other positions had the lowest ratio. The RCP required more positioning muscle activity and permitted less biting muscle activity. There was no significant difference in the muscle activity between the IP and MP registrations. Small changes in jaw position (anterior to RCP) are not critical for the masticatory apparatus provided there is good intercuspation. The results of this investigation suggest that intercuspation in RCP is not the optimal position.