Nocturnal bite force during sleep associated bruxism was measured in ten subjects. Hard acrylic dental appliances were fabricated for each subject's upper and lower jaws. Miniature strain-gauge transducers were mounted on the right and left sides in the first molar regions of the upper dental appliance. Thin metal plates that contact strain-gauge transducers were also attached to the lower dental appliance. Nocturnal bite force was measured for three nights at each subject's home after one-week familiarization of these appliances. Bite force signals were obtained with dynamic data logger, and then transferred to a computer. In this study, we defined bruxism event by using the following criteria: 1) Elevations of bite force signal above 5 kgf were selected as bruxism events; 2) When the inter-events interval was equal to or less than 2 seconds, these events were linked and treated as a single event ; and 3) When the duration of the event was less than 2 seconds, the event was rejected.
Four hundred ninety-nine bruxism events that satisfied the above criteria were selected from recordings for a total of 30 nights. The maximum voluntary bite force during the daytime was also measured with this system. The mean amplitude of detected bruxism events was 22.5 kgf (s.d. 13.0 kgf), and the mean duration was 7.1 sec (s.d. 5.3 sec). The mean amplitude of the highest nocturnal bite force in individual subjects was 42.3 kgf (range 15.6-81.2 kgf). The mean maximum voluntary bite force during the daytime was 79.0 kgf (range 51.8-99.7 kgf) and the mean ratio of nocturnal/daytime maximum bite force was 53.1%(range 17.3-111.6%). These data suggested that nocturnal bite force during bruxism can exceed the amplitude of maximum voluntary bite force during the daytime.