Mandibular Resting Posture : An Electromyographic Evaluation
Mandibular resting posture is the positional expression of cranial-facial morphology and the myotatic reflex of its contiguous musculature. Alteration of sensory input to the main sensory, mesencephalic and motor nucleus of the Fifth nerve in a myotalic reflex system, theoretically should result in changes or alteration in motor response. The purpose of this study was then to examine post interference phenomena as noted by amplitude variations within muscles during centric occlusion and cyclic chewing and to block its expression via mandibular nerve anesthesia. A Faraday cage was utilized to completely enclose the experimental subject and a 5P5 polygraph was used to conduct, amplify and record the bioelectric potentials elicited from the contracting muscles. Electrodes of the surface type, silver disc, monopolar, were attached to the middle belly of temporalis and the masseter muscles bilaterally. An experimental sequence from normal to shimming, removal of shim, mandibular block, shim inserted, shim removed, was instituted to discover if these treatments caused variations in amplitude, frequency and duration of muscular activity. This tested the hypothesis that an interference in occlusion, upon removal will cause an enhancement or an increased amplification of muscular activity as compared with prior amplitudes. A shim of . 003 inch thickness was chosen as a proximal separator which is analgous to an orthodontic force, as it causes the teeth to move in opposite directions. Following a random selection of observations of each treatment, these were measured with a map measurer by two observers to obtain objective, quantitative values. The values obtained were statistically analyzed using an Analysis of variance with the following results: A very highly significant difference in the response of subjects and muscles to different treatments, and of the muscles of an individual to the same treatment. There was no significant difference in the response pattern of one kind of chewing as compared with another. A wide range of muscular activity is noted between patients although treatments and sequences are the same. The analysis of a graph representing total muscular activity of each subject during each treatment indicated the individuality of response. The mean of each observation indicated, however, the presumed physiologic response to the treatments. Anesthesia reduced amplitude and frequency in myograms from resting Temporalis and Masseter muscles. Anesthesia resulted in altered myogram response in all subjects indicating the presence of a feedback mechanism involved in mastication. The direction and magnitude of the change is highly variable.
Orthodontics and Orthodontology, Mandible; Vertical Dimension
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