Jaw movements and patterns of mandibular bone strain during mastication in the monkey Macaca fascicularis

Small amalgam fillings were placed in maxillary and mandibular second molar and canine teeth for cine-radiographic analysis. The rosette strain gauges were bonded bilaterally to mandibular cortical bone below the second or third molars. The monkeys were placed in a restraining chair that did not restrict normal head, neck or jaw movements; they were fed various foods and the bone-strain data recorded. Simultaneous jaw movements were recorded with cine-radiographic apparatus synchronized with the bone-strain recordings. During vigorous mastication, the transition between fast close and the power stroke was correlated with a sharp increase in masticatory force. In most instances, the jaws were maximally-loaded prior to maximum intercuspation, i.e. during the buccal phase (phase I) of occlusion. The macaques swallowed frequently throughout a chewing sequence and these swallows were intercalated into the chewing cycle toward the end of the power stroke. Such swallows had little effect on the magnitude or direction of peak principal strains during the power stroke. Bone-strain data suggested that unloading patterns during the power stroke of mastication were largely a function of the relaxation time of the jaw adductors. The period from 100 per cent peak strain to 50 per cent peak strain during unloading closely approximated to the half-relaxation time of the whole adductor jaw muscles.