Electromyographic relaxed mandibular position in long-faced subjects Joseph E. Van Sickels, D.D.S.,* John D. Rugh, Ph.D.,** Gary W. Chu,*** and Robert R. Lemke***+ University of Texas Health Science Center, Dental School, San Antonio, Tex. C linical and electromyographic (EMG) mandibular rest position in patients with different dentofacial mor- phologies has been the subject of investigations.‘,* These and other articles have shown that there are two separate positions of the mandible: a clinical rest position (inter- occlusal distance, 1 to 3 mm) and an EMG relaxed position (interocclusal distance, 5 to 12 mm).lw4 Reliable individual differences have been reported in both of these measurements.1e4 Clinical rest position is influenced by a number of factors including head position, exercise, emotional tension, pain, and age.le5 In addition, when treating partially dentulous and edentulous patients, it is important to understand that the clinical rest position varies with the vertical facial morphology.‘) ‘34 Peterson et al.’ studied clinical and EMG rest position in subjects with contrasting facial morphologies. The vertical dimension of clinical rest was found to be significantly greater in a low-angle group than in a high-angle group. No difference between the two groups was seen in the vertical measurement of EMG rest position. The investigators suggested that EMG resting position of the mandible, and perhaps the resting muscle length, may be determined more by functional demands than by facial morphology. On the other hand, Wessberg et al.* evaluated an electrically stimulated jaw position and clinical rest position in three morphologic groups subdivided on the basis of cephalometric standards for vertical maxillary excess (VME), vertical maxillary normal (VMN), and vertical maxillary deficiency (VMD). Clinical rest position in subjects classified as VMD was found to be greater than that seen in VME patients. Likewise, a position they termed “physiologic rest position” was also shown to be much greater in VMD subjects than in VME patients. This finding led the investigators to conclude that interocclusal distance *Associate Professor, Department of Oral and Maxillofacial Surgery; Co-Director, Dentofacial Deformities Center. **Associate Professor, Department of Oral and Maxillofacial Surgery; Director of Research. ***Orthodontic resident, University of Iowa, Dental School. ****Second year dental student. 578 at the physiologic rest position is inversely related to vertical dentofacial morphology. Several plausible factors may account for the different results in these two investigations and hence their contrasting conclusions. The purpose of this article was to evaluate two of those factors, head position and facial morphology, in an attempt to resolve this important clinical issue. In the present study, EMG rest position of the mandible was assessed in a group of long-faced subjects while controlling head position. Finding a reduction in EMG rest-vertical relation when subjects were allowed to select their own head inclination would help explain the differences in the earlier investiga- tions. MATERIAL AND METHODS Twelve subjects from the Dentofacial Deformity Cen- ter at the University of Texas Health Science Center at San Antonio were selected on the basis of cephalometric standards for VME. VME, also called the long-face syndrome, may occur with or without an open bite (anterior occlusion).6 Generally, both groups can be identified cephalometrically by an increased anterior facial height, excessive vertical development of the max- illa, a high mandibular plane, and a normal lip length with excessive showing of the anterior maxillary teeth.6 This method was chosen to classify subjects because it closely paralleled the selection technique chosen by Wessberg et al. 2 Three subjects had open anterior occlusion and nine did not. There were six men and six women. One 5 mm silver-silver chloride electrode was placed over each masseter muscle with a ground under the chin. This electrode placement has been previously used and shown to provide a nonspecific electromyo- graphic signal with input from several masticatory and facial muscles.‘, 3, ‘. a Vertical opening was measured on a K-5 Kinesiograph (Myo-Tronics Research, Inc. Seattle, Wash.). A Myotron model-220 (Enting Instruments, Dorst, The Netherlands) electromyographic feedback instrument provided auditory and visual feedback of jaw and facial musculature activity. Prior to application of the electrodes, all subjects were OCTOBER 1985 VOLUME 54 NUMBER 4