Muscle activity during function and its correlation with craniofacial morphology in a sample of subjects with Class II, Division 1 malocclusions.

To evaluate the relationship between craniofacial morphology and jaw muscle activity in a sample of skeletal Class II, Division 1 malocclusions, electromyographic and cephalometric analyses were undertaken in a sample of twenty adolescent children. For each subject, electromyographic activity was recorded from the left anterior temporal, masseter, and orbicularis oris muscles at rest, in maximum intercuspation, and during clenching. In addition, mandibular displacement was measured simultaneously with a kinesiograph, and twenty-seven anatomic points were digitized from a lateral head film. A computer-based system was used to sample the data and to determine correlation coefficients between selected parameters. Five significant correlations were found (at the 99 percent confidence level) for the rest position data. High anterior temporal amplitude was correlated with flat palatal planes and large ramus heights; high masseter amplitude correlated with upright maxillary incisors. Anterior positions of the mandible at rest correlated with flat occlusal planes and small pogonions. In contrast, significant correlation coefficients could not be identified for the maximum intercuspation recordings. However, low orbicularis oris muscle amplitude during clenching was correlated with high lower face height, overerupted mandibular molars, and elongated maxillary incisors. This interdependence of jaw muscle activity and craniofacial morphology in Class II, Division 1 subjects suggests a contribution from the musculature to the development of the dentition.