Influence of tooth contacts during mandibular retrusion on terminal jaw relations and EMG activity during habitual opening and closing movements]

The purpose of this study was to clarify the influence of tooth contacts during mandibular retrusion on terminal jaw relations (TJR) and the surface EMG activity during habitual opening and closing movements. The age of five healthy subjects were 25 to 28 years. The maxillary stabilization splints with steep retrusive contacts (S-SP) and with flat retrusive contacts (F-SP) were prepared for each subject who was instructed to wear S-SP for one week and then to wear F-SP for one week after an interval of two weeks without S-SP. The EMG activity of the masseter and temporal muscles and the TJR during habitual opening and closing movements using a device for recording jaw movements in 6 degrees of freedom were simultaneously measured by an intraoral central bearing device. The results were as follows: 1. The TJR recorded after wearing F-SP shifted more posteriorly than that of wearing S-SP. 2. The posterior temporal muscle activity after wearing F-SP, compared with that of wearing S-SP, resulted in a significant increase. In conclusion, the elimination of the retrusive guidance caused the predominance of posterior temporal muscle activity, and posterior displacement of the mandible.