Automatic regulation of occlusal force because of hardness-change of the bite object.

It is considered that the information of chewed food, such as size and texture, is important for smooth mastication. In this study, we analysed aspects of the control of occlusal force, by experimentally reproducing situations in which the hardness of food changed unpredictably during mastication, using a device that utilized a 3-sectioned urethane rubber piece with different hardness values. Seven healthy subjects were instructed to perform repetitive jaw open-close movements paced by a metronome (1.0 Hz) and to maintain constant occlusal force throughout the trial. Using the device, the following parameters were measured during the first to fifth strokes after changing the hardness, peak value, impulse, duration and time to peak of occlusal force in the waveform of occlusal force, cycle time of open-close jaw movements, jaw gape and maximum speed in the closing phase in the waveform of the jaw movements. Each parameter value was statistically analysed by anova with Fisher's least significant difference method (P < 0.05). Peak occlusal force, impulse and jaw gape were significantly affected by the change in hardness, while an increase in hardness caused increases in the values for peak occlusal force and impulse against the instruction, after which those values remained constant while the subjects occluded the same level of hardness. Our results indicate that the level of the resulting occlusal force is regulated automatically according to the hardness of the chewed food. We concluded that occlusal force was adapted for efficient mastication when the hardness of foods changes unpredictably.