Perioral muscle action and the incisor relationship: a new method of study.

Abstract

The purpose of this study was to investigate a method of quantifying the degree of lower lip cover of the maxillary central incisor at rest and during function. Subjects seated while watching an entertainment videotape were recorded in profile and full face using a colour video camera. Later, the tape was scanned for spontaneous perioral muscle activity and by using the frame advance mechanism of a videorecorder, appropriate stills from the action sequence were analysed. By using a fixed point facial landmark it was possible to relate the site of action of the lower lip to the upper central incisor crown. Using this technique, lower lip cover of the maxillary central incisors was measured during spontaneous activity in subjects with Class I and Class II division 1 incisor relationships. There was a trend towards greater lower lip cover of the maxillary central incisors in Class I than in Class II division 1 incisor relationships, both at rest and during spontaneous swallowing activity. Because of the large standard deviation, this difference was not significant in a small sample. A method has been developed for measuring lip cover of maxillary incisors during spontaneous activity to an accuracy of 1.1 mm. Intra-subject variability suggests the need for development of an integrating method.