Experimental studies on the influences of physical properties of foods on the appearance of silent period in discharge of masticatory muscles

It is considered that the appearance of Silent Period (SP) during food crushing may relieve tissue injury caused by sudden change of maxillo-mandibular impactive force. In connection with this phenomenon, the relation between changes in vertical velocity of mandibular movement and the appearance of SP during food crushing was investigated. In addition, efforts were made to clarify the influences of lack of periodontal pressoreceptive information on the appearance of SP. Subjects were ten males (aged 24 to 31) with natural dentition and no masticatory dysfunction. Electromyograms were recorded on 4 lead locations: the central parts of each masseter and the anterior bundles of each temporal muscle. A Mandibular Kinesiograph and electromyograph were used to record simultaneously mandibular movement and myoelectric discharge during food crushing for the sake of comparing changes in mandibular position, vertical velocity of mandibular movement and muscular activity. Eight kinds of foods of various textures were selected for use in the examinations: dry macaroni, rice cracker, raw carrot, pickled thistle root, pickled radish, raisin, synthetic rubber block and softened chewing gum. The food was positioned on the occlusal table of the lower first molar on the habitual masticatory side of each subject. Then the subject was required to close his mouth rapidly to crush the food at a single masticatory stroke. Two experimental conditions were established in order to clarify the significance of periodontal pressoreceptive information on the appearance of SP: (1) normalcy and (2) infiltration anesthesia of the periodontal tissues of the upper and lower first-molar region on the habitual masticatory side. Results 1. SP appeared during a single masticatory stroke in response to sudden decreases or increases of vertical velocity of mandibular movement. 2. The frequency of the appearance of SP depends on the crushing characteristics of the food which is tested. For softened chewing gum and the synthetic rubber block, only one SP occurred; two SPs occurred in the cases of pickled radish and raisin; three SPs in the cases of raw carrot and pickled thistle root; and more than four SPs in the cases of dry macaroni and rice cracker. 3. Lack of periodontal pressoreceptive information resulting from anesthesia had no effect on the appearance of SP. Conclusions: 1. Sudden changes in vertical velocity of mandibular movement may depend on the crushing characteristics of the food which is crushed.(ABSTRACT TRUNCATED AT 400 WORDS)