ANALYSIS OF THE BIOELECTRICAL ACTIVITY OF THE MASTICATORY MUSCLES, DEFINED ACCORDING TO BERNOULLI'S PRINCIPLE AS "STRONG" OR "FAST", FOR THE PREDICTION OF PATHOLOGICAL TOOTH ABRASION (ACCORDING TO MAGNETIC RESONANCE IMAGING AND ELECTROMYOGRAPHIC STUDIES)

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Pathological abrasion is a well-known disease, but it is topical issue of hard dental tissues. The principle of D. Bernoulli and its numerical expression in the form of the coefficient “k” were used. It is calculated by the formula , where k is the calculated coefficient; l is the length of the muscle in this case, the masseter separately on the right and left; s is the cross-sectional area of the muscle. The masseter muscles' length, width, and thickness were determined using SIEMENS MAGNETOM Skyra 3T magnetic resonance imager, 2018.
 The classical method was used for the electromyographic study of the masseter muscles. We analysed 64 MRI scan results, of which 15 were control subjects with intact dentitions, and 49 were patients with varying degrees of tooth abrasion. After determining the physical parameters as length, width, and thickness of the masseter muscles on the right and left sides, the cross-sectional area of the muscles was determined and the author's coefficient of the numerical expression of the Bernoulli principle for each masseter was proposed.
 Electromyograms of the masseter muscles in the mode of free chewing were recorded for all subjects with subsequent software analysis. Extrapolation of the obtained coefficients of the numerical expression of the Bernoulli principle of the left and right masseter muscles to the values of quantitative and qualitative indicators of electromyograms of the same muscles revealed a direct dependence of the bioelectrical activity of muscles on the ratio of muscle length to cross-sectional area. Higher values of this coefficient correspond to higher values of bioelectrical bursts of muscle activity, faster contraction and relaxation, shorter chewing time, and longer resting time of the muscle, which is a sign of the biomechanical property "speed". Masseter muscles with such properties are observed in individuals without pathological tooth abrasion. In the groups of patients with pathological tooth abrasion, a decrease in the ratio of the length of the masseter to its crosssectional area is observed in proportion to the increase in the severity of the pathological process. The amplitude of muscle bioelectrical activity bursts decreases, the speed of excitation and relaxation processes decreases, the chewing time is prolonged, and the resting time is reduced, which indicates that the muscle acquires the biomechanical property of "strength". "Fast" masseter muscles do not cause pathological tooth abrasion, while "strong" muscles contribute to the development of pathological tooth abrasion. Employing both Bernoulli's principle and MRI studies on the muscles responsible for mandibular movements will enable a comprehensive investigation into the progression of localized pathological tooth abrasion and provide a more intricate understanding of the muscles' impact on the development of dentoalveolar anomalies.