Mandibular rest position in subjects with high and low mandibular plane angles

Studies have explored the relationship between different facial morphologies and mandibular posture; however, few conclusions can be drawn. The purpose of this study was to explore the effects of contrasting facial morphologies on both clinical and electromyographic rest positions of the mandible. Electromyographic and clinical rest positions were recorded in 10 subjects whose Frankfort horizontal-to-mandibular plane angles were less than 20 degrees (low-angle group) and in 10 whose angles were greater than 30 degrees (high-angle group). The vertical dimension of clinical rest position was recorded from soft tissue markings on the nose and chin after subjects wet their lips, swallowed, and pronounced Mississippi. Electromyographic rest was defined as the interincisal vertical dimension of minimal muscle activity as determined with an Enting Model 220 EMG integrator and the Myotronics K-5 kinesiograph. Head posture was held constant to the subject's visual axis. The vertical dimension of clinical rest position was significantly greater in the low-angle group (X = 4.6 mm., S.D. = 1.4) than in the high angle group (X = 3.1 mm., S.D. = 1.1); however, no differences were observed in the EMG rest positions, which were 9.9 and 9.7 mm., respectively, for the low- and high-angle groups. These results do not support a simple mechanical explanation of jaw posture; however, these measurements need to be repeated with subjects selecting their preferred head inclination.