Associations between jaw-opening muscle activity and craniofacial morphology

The purpose of this study was to investigate the relationship between jaw-opening muscle activity and craniofacial morphology in a sample of 64 adults with skeletal Class I malocclusions. EMG recordings from the inferior head of the right lateral pterygoid and the anterior belly of the right digastric muscle were analyzed together with mandibular displacement data sampled simultaneously with a kinesiograph. Incisor separation at the rest position and the threshold mandibular rotation position which corresponded to the onset of muscle activity in response to a graduated mandibular rotation were identified. Incisor separation was converted to a mandibular angle on lateral cephalograms. Anatomic points were digitized and sixteen traditional cephalometric variables were determined. The rest position angle did not correlate with the threshold angle for the anterior belly of the digastric muscle, but a significant correlation was identified with the inferior head of the lateral pterygoid muscle. One canonical correlation (r = 0.741; p less than 0.01) was identified between the mandibular rotation and dentoskeletal variables. Negative loadings with lateral pterygoid muscle threshold and rest position angles were obtained. The corresponding dentoskeletal variables had a positive loading with mandibular plane angle, total and lower anterior face height and gonial angle and a negative loading with lower posterior face height. The results suggest a possible relationship between lateral pterygoid muscle activity at two specific mandibular positions and the resultant form and posture of the mandible.