1999 Volume 62 Issue 2 Pages 77-89
We investigated the effects of changes in occlusal contacts on mandibular movement and electromyographic activities of the jaw muscles, Electromyographs (EMG) were recorded simultaneously from the inferior head of the right lateral pterygoid (Lpt), the anterior bellies of the digastric (Dig) and masseter muscles (Mm) with mandibular movement. We prepared a mandibular biteplate that was adjusted so that all the maxillary premolars and molars contacted the splint (AL). We then eliminated occlusal contacts on the bilateral first premolars, leaving contacts on 7__-6__-5__-∣5__-6__-7__-. Contacts were next eliminated on the second premolars, leaving 7__-6__-∣6__-7__- contacting. When contacts on the second molars were eliminated, only 6__-∣6__- were in occlusion. Then contact on the left first molar were eliminated, leaving occlusion only on 6__-∣. Subjects wearing each splint were instructed to perform 20 open-close-clench cycles (OCC) repetitively. Two sessions were carried out. Changes in time parameters and velocity of mandibular movement, opening distance, EMG activities and mandibular position during the open-close movement were observed following changes in the occlusal contacts. As the number of occlusal contacts decreased, the cycle time of mandibular movement and the duration and interval of jaw muscle activity decreased. The average acceleration of opening movement significantly increased from AL to 6__-∣6__- and significantly decreased from 6__-∣6__- to 6__-∣. The coefficient of variation (CV) for duration in AL was lower than for other contacts in both the jaw depressors and the Mm. Integrated EMGs of the jaw depressors and Mm significantly decreased following decreases in the occlusal contacts. Standard deviation (SD) of the lateral direction of the terminal closing position in 6__-∣ was greater than with other occlusions. These results suggest that the rhythm of OCC in AL was slower than for other contacts, although the regulation of rhythm was best for the full set of contacts. A decrease in the number of occlusal contacts may disrupt the rhythm of mandibular movement and mandibular position.
