Electromyograph attachment

Masseter muscle activity was measured using an electromyograph (PowerLab; ADInstruments, Nagoya, Japan). Silver disk electrodes of 10 mm in diameter with bipolar leads (Duotrode; Morita Corp., Osaka, Japan) were adhered on both sides of the central part of the masseter muscle. The distance between electrodes was 21 mm, and the electrodes were placed parallel to the direction in which the masseter muscle fibers run.

The activity of the masseter muscle, when subjects clenched their teeth at full strength with nothing interposed between the upper and lower teeth, was defined at 100 % maximum voluntary contraction (MVC), and subjects were able to see the amount of muscle force displayed in numerical values through visual feedback.

Measurement and analysis of subject dentition

Measurement of occlusal loading and occlusal contact area using Occluzer

Pressure-sensitive film (Dental Prescale 50H type R; Fuji Photo Film Co., Tokyo, Japan) for occlusal force diagnostic use was used to examine the occlusal contact area and occlusal load in the intercuspal position together. Masseter muscle activity (clenching strength) was set at 40, 60, 80, and 100 % MVC, and subjects were measured three times at each of these clenching intensities using visual feedback. Subjects remained in a seated position, and the head on the headrest of the dental unit with the occlusal plane is parallel to the floor.

Subjects were instructed to open their mouths one finger width and the Prescale was inserted, and subjects were then instructed to slowly close their mouths and clench their teeth. The Prescale was interposed between the full dentition between the second molars on either side, and subjects were instructed to bite in the intercuspal position. Clenching on the Prescale was carried out for 3 s, and considering the content of muscle fatigue, measurements were taken at 5-min intervals.

The Prescale was kept in a cool, dark place for 24 h; after which, the colored parts, their surface area, and their color density were analyzed using a dedicated analyzer (Occluzer FPD707®; Fuji Photo Film Co., Tokyo, Japan). Occlusal force was analyzed by a software (DePROS-PC; GC, Tokyo, Japan), and these data were converted to pressure values. The occlusal loading and occlusal contact area for molars were then calculated.