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 w ere attached to the skin of the upper and lower lips. The mean integrated a mplitude of the electromyographic (EMG) activity was obtained at the mandib ular rest position with the lips in contact and with the lips apart. Subjec ts were divided into two groups based on positive or negative values of the difference in integrated EMG activity of the mentalis muscle between the t wo lip positions. Subjects displaying a negative value were classified as h aving competent lips and those displaying a positive value were classified as having incompetent lips. The EMG activity of the mentalis muscle was fou nd to be more indicative of lip sealing as compared with the EMG activity o f the depressor of the lower and the upper lips. The activities of the ment alis muscle at the mandibular rest position with the lips in contact and wi th the lips apart appear to offer an objective criterion for the evaluation of lip incompetence. In addition, the vertical dimension of the face, as w ell as the proclination of the incisors, appear to affect lower lip functio n.