The relationship between dentofacial morphology and the isometric jaw-opening and closing muscle function as evaluated by electromyography

The purpose of this study was to clarify the relationship between the dentofacial morphology and masticatory muscle function in both isometric jaw-opening and closing. The subjects consisted of 26 male adults without masticatory dysfunction. Electromyograms were obtained from the digastric and masseter muscles to evaluate their muscle activities. The sum energy, median frequency, and equilibrium index were calculated as parameters of the muscle activity. A correlation analysis was performed between the parameters for the muscle activity and the dentofacial morphology on lateral roentgenographic cephalograms. The sum energy in the jaw-closing muscle showed a significant relationship with the genial angle, mandibular length, and facial height; but not in the jaw-opening muscle. A median frequency in the jaw-opening muscle showed a significant relationship with size and position of the mandible, and also that in the jaw-closing muscle with the mandibular plane angle, facial height, and mandibular body length. Furthermore, the equilibrium index of the median frequency was related to the facial height, mandibular length, and inclination of the mandibular incisor. In conclusion, the dentofacial morphology was found to be closely related to the muscle activity not only of the jaw-closing muscle but also of the jaw-opening muscle during isometric muscle contraction.