A comparative study of the masseter muscle activities after clenching task in mandibular rest position before and after treatment in TMD patients Nori

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1 A comparative study of the masseter muscle activities after clenching task in mandibular rest position before and after treatment in TMD patients Noriko Inoue, Tohru Imai*, Takaaki Yamamoto*, Shinji Nakamura* Shin Sapporo Anne Orthodontic Clinic *D epartment of Orthodontics, School of Dentistry, Hokkaido University (Director: Prof. Shinji Nakamura) (Received: October 30, 1998.) Key words: temporomandibular disorders, masseter muscle activities, mandibular rest position, clenching task, a multi-channel analyzing system Abstract: The purpose of this study was to investigate the electromyographic features of masseter muscle activities before and after a clenching task in the mandibular rest position in patients with temporomandibular disorders (TMD) by a multi-channel analyzing system. The subjects included eight patients with TMD (TMD group) and eight normal subjects (Normal group). The following results were obtained. 1) Masseter muscle activities in the mandibular rest position (MMA) increased after the clenching task in four of the Normal group, and such increase in the activity was observed in nine of the 18 masseter muscle sites examined. In the TMD group before treatment for TMD, MMA increased after clenching task in seven patients, and this increase in the activity was observed in all of the 18 sites. In the TMD group after treatment for TMD, MMA increased after clenching task in four patients, and the increase in the activity was observed in seven of the 18 sites. The two groups showed no differences as far as the areas of masseter muscles were concerned. 2 ) MMA in the Normal group had been significantly lower from the 2nd frame after clenching task on the preferred chewing side (PCS) and from the 3rd frame after clenching task on the non-preferred chewing side (NPCS) than in the 5th frame before clenching task (p<0.05). MMA in the TMD group before treatment were significantly higher on both sides in the 1st frame after clenching task than in the 5th frame before clenching task (p<0.05). MMA in the TMD group before treatment had decreased from the 1st frame after clenching task, but even MMA in the 5th frame after clenching task did not get to MMA level in the 5th frame before clenching task. MMA in

2 the TMD group after treatment were similar to those in the Control group. 3) Before the clenching task, there were no significant differences between average masseter muscle activities (AMMA) in the Normal group and in the TMD group before treatment. However, after clenching task, AMMA in the TMD group before treatment were significantly higher than those in the Normal group. There were no significant differences between AMMA in the Normal group and in the TMD group after treatment both before and after clenching task. 4) AMMA on the NPCS before clenching task and those on both sides after clenching task were significantly lower in the TMD group after treatment than in the TMD group before treatment. 5) There were no relationships between the masseter muscle activities in the rest position and the seventy of clinical symptoms. The results showed that in TMD patients who have not been treated, a clenching task causes an increase in masseter muscle activities in the mandibular rest position.

3 Table 1 Clinical symptoms and preferred chewing side in the TMD group : Clinical symptoms of the subject

4 Fig. 1 The position at which the electrodes plate was attached on the masseter muscle and channel numbers The anterior side of the electrodes plate was set at the anterior border of the masseter muscle and the upper side of the plate set at the arcus zygomaticus. Fig. 2 Analysis frames of masseter muscle activities in the mandibular rest position The numbers on the EMG wave indicate each analysis frame of the EMG taken over a 13-sec period form 20 sec after swallowing every minute from 5 minutes before to 5minutes after clenching task.

5 œ Fig. 3 Calculation of avaerage activity M : Average activity in the analysis frame (ƒêv) X (i) : activity (ƒêv) N : Number of data in the analysis frame Fig. 4 The sites in which Masseter muscle activities increased after clenching task in the Normal group and the TMD group : Subjects who showed and increase in MMA PCS : preferred chewing side, NPCS : non-preferred chewing side

6 Fig. 5 Change of Masseter muscle activities before and after clenching task in the Normal group and the TMD group PCS : preferred chewing side, NPCS : non-preferred chewing side Table 2 Comparison of masseter muscle activites before and after the clenching task in the Normal group and TMD group (Wilcoxon signed rank test) ª: higher muscle activities after clenching task «: lower muscle activities after clenching task

7 Table 3 Comparison of average masseter muscle activities in the Normal group and in the TMD group before and after treatment (Mann-Whitney U test, Wilcoxon signed rank test) PCS : preferred chewing side, NPCS : non-preferred chewing side

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