Muscle activity in normal and postnormal occlusion

Abstract

The EMG activity in the rest position of the mandible, during chewing, and during swallowing was compared in fifteen boys with normal occlusion and fifteen boys with Angle Class II, Division 1 malocclusion. The children's ages varied between 9 and 13 years, with a mean of 10 years 6 months in the normal-occlusion group and 10 years 11 months in the group with postnormal occlusion. The EMG activity was recorded unilaterally from the anterior temporal muscle, the posterior temporal muscle, and the masseter muscle and superiorly from the orbicularis oris muscle. The EMG activity during rest was recorded according to a five-grade scale, while the maximum mean voltage and duration of the EMG activity were recorded during chewing and swallowing. The EMG activity was related to the morphology of the face and dentition analyzed with profile roentgenograms. No difference was found between type of occlusion in the EMG activity in the rest position, which was greatest for the posterior temporal muscle for both types of occlusion. The EMG activity at rest in the anterior temporal muscle was greater in children with small gonial angles than in those with large angles. The boys with postnormal occlusion had a tendency to develop less EMG activity during chewing than boys with normal occlusion. In both types of occlusion the chewing activity was greater in the anterior than in the posterior part of the temporal muscle. In the orbicularis oris muscle the activity, irrespective of type of occlusion, was greater during the opening phase than during the closing phase of the chewing cycle. The children with postnormal occlusion showed less EMG activity during swallowing in the anterior temporal muscle and the masseter muscle than those with a normal occlusion. In children with postnormal occlusion the duration of the activity during swallowing was longer in the posterior than in the anterior part of the temporal muscle, while the inverse relationship was found for children with normal occlusion. In boys with small gonial angles, the amplitude of the activity in the masseter muscle during swallowing was smaller and the duration in the posterior temporal muscle was longer than in boys with large gonial angles.