Morphological differences in individuals with lip competence and incompetence based on electromyographic diagnosis.

The study group consisted of 19 subjects with positive overjet and overbite, and 17 subjects with skeletal open bite. Two bipolar surface electrodes were attached to the skin of the upper and lower lips. The mean integrated amplitude of the electromyographic (EMG) activity was obtained at the mandibular rest position with the lips in contact and with the lips apart. Subjects were divided into two groups based on positive or negative values of the difference in integrated EMG activity of the mentalis muscle between the two lip positions. Subjects displaying a negative value were classified as having competent lips and those displaying a positive value were classified as having incompetent lips. The EMG activity of the mentalis muscle was found to be more indicative of lip sealing as compared with the EMG activity of the depressor of the lower and the upper lips. The activities of the mentalis muscle at the mandibular rest position with the lips in contact and with the lips apart appear to offer an objective criterion for the evaluation of lip incompetence. In addition, the vertical dimension of the face, as well as the proclination of the incisors, appear to affect lower lip function.