Bite force, endurance and masseter muscle fatigue in healthy edentulous subjects and those with TMD

It is well known that bite force and EMG activity are considerably reduced in edentulous patients, but the susceptibility of their jaw-closing muscles to localized fatigue is less certain. This information is even less clear for edentulous subjects who have TMD. Eleven healthy edentulous subjects an d 10 edentulous subjects with TMD participated in this study. Maximum bite force was measured first, with the transducer placed on the canine-first pr emolar region bilaterally, and then two rapid relaxations were made from a brief voluntary clench to 50% of maximum. A sustained voluntary clench of 5 0% of maximum was then maintained and endurance time was noted. EMG was rec orded from both masseter muscles and the median frequency of the power spec trum of the EMG from 2 s at the beginning of the sustained clench and 2 s a t the end was subsequently calculated. Two more rapid relaxations from brie f clenches were performed immediately after the sustained clench. The mean maximum bite force in the healthy group was 115 N (SD +/- 41) and in the TM D group was 75 N (SD +/- 22), this difference being significant (P = 0.0013 ). The mean endurance time in the healthy group was 86 s (SD +/- 51) and in the TMD group was 63 s (SD +/- 20). The percentage change in the median fr equency in the healthy group as a result of the sustained contraction was 6 % (left) and 8.6% (right) and in the TMD group was 13.9% (left) and 12.8% ( right). The percentage change in the mean relaxation half time for the heal thy group was 28.5% and for the TMD group was 72%, a significant difference (P = 0.0046). It was apparent that the maximum bite force was low in edent ulous subjects and was further reduced in edentulous TMD subjects; enduranc e time was reduced in TMD subjects; fatigue resistance of the masseter musc les was reduced in TMD subjects.