Effect of Tempromandibular Joint Sounds on Timing of the Masseter Muscle Activity in the Open-close-clench Cycle

The aim of the present study was to investigate the effect of tempromandibular joint (TMJ) sounds and distribution of malocclusion on the timing pattern of the masseter muscle in the open-close-clench (OCC) cycle. A total of 39 adult women were participated in two part of study. They completed OCC cycles in the functional condition, with 80 cycles in a minute. Two separated recording for 15 sec with an interval 2-3 min from each side were done. The results of comparing the synchronized signals of motion in sagittal plan and electromyography in first part demonstrated the earlier onset of masseter activity in two side and longer duration of right masseter in group with TMJ sounds (21 subjects) compare to controled group (18 subjects). In second part ten subjects were selected which they had only one joint with sound, the ipsilateral masseter (side with TMJ sound) contracted sooner and for a longer duration in comparison with contralateral side. Also in the first part that distribution of occlusion was significantly different between two groups, the two-way ANOVA showed that it had an interaction effect on the increase of the duration of masseter activity. In conclusion, TMJ sounds in accompany with malocclusion without obvious symptoms such as pain and limitation can change timing pattern of masseter activity in the functions same a mastication. This abnormal pattern may affect TMJ and other structures to produce vicious cycle and other problems.