THE EFFECT OF PARTICLE-SIZE REDUCTION ON THE JAW GAPE IN HUMAN MASTICATION

Eight subjects participated in chewing experiments to examine how the jaw gape in consecutive chewing cycles depends on the dimensions and the volume of the food bolus. Different volumes of chewing-gum and a silicone rubber (Optosil(R)) were used. The initial particle size of Optosil was also varied. With the aid of a sieving procedure, the size distribution of Optosil particles was determined after different numbers of chewing cycles. The maximum size of a chewing-gum bolus along three orthogonal axes was determined after a random number of chewing cycles for all the volumes of chewing-gum offered. The jaw gape was measured by means of an optical motion analysis system and calibrated at the level of the first molars. The maximum jaw gape and the gape at the first fall in mandibular velocity after the onset of jaw-closing were determined for different chewing cycles from the position signal and its time derivative. For each volume of chewing gum offered to the subject, the average of the maximum gapes attained in the subsequent cycles of jaw movement was related to the characteristic height of a bolus of that volume; maximum velocity of closing appeared to occur while the antagonistic teeth were already penetrating the gum bolus. The increase in the maximum jaw gape, the gape at the velocity maximum, and the height of the gum bolus, as a function of the volume of chewing-gum, could be well-described by a power function of the volume. The exponent of the power function for both jaw gapes was only slightly smaller than the exponent obtained for the bolus height. When a hard but comminutable test food (Optosil) was used with large initial particle sizes (> 4.8 mm), the gape at the first fall in velocity closely approached the calculated height of the largest particles in early chewing cycles, regardless of the volume of food offered. The maximum gape was correlated with the subsequent gape at the first fall in velocity and therefore was, indirectly, also related to the height of the largest Optosil particles. In a later phase of the chewing process, the gape at the velocity maximum was always larger than the height of the largest particles.