The relationship between asymmetry index of masticatory muscles and occlusal positions

Abstract

To evaluate masticatory muscle activity bilaterally during submaximal clenching, an asymmetry index (A.I.) has been recently introduced by Naeije et.al. However the characteristics of A.I. both in the temporal and masseter muscle activity have not been cleared yet. Therefore in the present study, the influence of clenching efforts and occlusal positions on the A.I. of the temporal and masseter muscles was investigated.
Fourty healthy subjects with normal occlusion (Aged 20 to 37 yrs.) were asked to clench at 50 percent of the maximum voluntary clenching (MVC) level of the masseter muscles at 5 different clenching tasks as follows: (1) clenching in the intercuspal position (ICP), (2) intentional vertical clenching on the right and left side in the ICP (Int.VR and VL), (3) lateral shift clenching to the right and leftt side (Lat.R and L) . Bite checks were obtained in ICP and Lat. R and L, with green bite wax, and the number of post-canine tooth contacts were calculated bilaterally.
Results were as follows; As to the temporal muscle, the A.I. of the ipsilateral side changed positively as compared with the contralateral side in all subjects. However, three patterns were recognized in the masseter muscle A.I., by evaluating the predominancy of the ipsi- and contralateral side activity at Int. VR and VL, and Lat. R and L. A positive correlation was recognized between the A.I. value of the masseter muscle and the asymmetry of tooth contact number both in the ICP and Lat. R and L, while it was not found in the temporal muscle. These results suggested that the A.I. values were related to the clenching direction and occlusal positions in the temporal musle, but related to the number of post-canine tooth contacts in the masseter muscle.