STANDARDIZED ELECTROMYOGRAPHIC INDEXES ALLOW A RELIABLE MEASUREMENT OF MASTICATORY MUSCLES FUNCTION decreases and the conduction velocity is biased towards higher values 27. Moreover we have to underline that surface electrodes, in many cases, capture electrical signals generated from several muscles-crosstalk effect- preventing a single muscle measurement. In facial cutaneous areas, the mimic muscles and sometimes anatomical variability, could complicate the univocal sEMG signal-muscle matching( as previously reported platysma to masseter partial coverage) 32. Indeed it is well known that some muscles functional compartmentalization due to their anatomical and innervation complexity thwarts a simple function assessment; in particular Temporalis muscles is a composite morphologic-functional entity not yet fully understood 33, 34. In any case, taking into account the technical complications, it appears reasonable to apply sEMG analysis to understand the functional areas central management instead of focusing on single muscle function measurement. Notwithstanding the amount of experimental and theoretical studies, no global consensus was achieved in the sEMG raw signal interpretation. For this reason, studies that could increase the understanding of sEMG advantages and limits in order to allow a more aware clinical application are required. The most revealing standardization effort occurred in 1997 – 1999 within the European Project on‘‘ Surface EMG for Non Invasive Assessment of Muscles” where a fine analysis was presented for a number of sEMG acquisition technical factors 35, 36. Yet, when comparing several EMG studies published in the year 2004, Armijo-Olivo et al. stated that 60 % of the analysed studies did not report a normalization procedure, making comparisons of the EMG signal difficult due to anthropomorphic differences between recording sites and individuals 37. They underlined that comparison of muscle function evaluating the absolute values of the recorded potentials( that is, microvolt values) provides inaccurate results. As a consequence of this inaccuracy, comparing the EMG activities between subjects and under different conditions requires a normalization process. The efficacy of intra-subjects standardization procedure was investigated also for the jaw stretch reflex. In fact, Koutris et al. demonstrated that when the amplitude of the masseter muscle during the jaw stretch reflex is normalized relative to the prestimulus EMG amplitude, it becomes independent from the location of the electrodes over the muscle 38. In the present study, the repeatability of standardized( normalized) indexes calculated in independent sEMG acquisitions was evaluated. Indeed, as suggested by Ferrario et al., sEMG could be usefully employed for the control of the muscular modifications induced by variations in the occlusal conditions, either natural( for instance, different occlusal classes, crossbite occlusion), or artificial( for instance, dental prostheses, fillings, or even orthodontic treatment) 26, 39. The use of sEMG can help in the correct construction of oral devices 40. To simplify the understanding of occlusal induced muscular modifications, easy-tointerpret indexes could be helpful in the clinical device / prosthesis-adapting procedures. In order to perform a correct evaluation of these indexes in the everyday clinical practice, their reliability should be accurately understood. No significant differences in the standardized indexes elaborated from sEMG signals of T1 and T2 acquisition sessions were found, strengthening the effectiveness of the standardization procedure. The small sample analysed in the present study does not allow the evaluation of the values in an epidemiologic perspective. The average POC of Temporalis Anterior and Masseter, obtained during this study, resulted slightly lower than the ones reported by De Felicio et al. and by Ferrario et al., probably due to differences in the selection of the sample 9, 16, 24. As regards the standardization of masticatory muscles sEMG signal, a unique agreement within the scientific community does not exist 28. Different sEMG studies have made use of the protocol and related indexes proposed by Ferrario et al. to evaluate the masticatory muscles 16, 24, 41-44. The only difference between two acquisitions( on cotton rolls and in IP) is dental contact, thus limiting the technical and biological noise inherent in this instrumental analysis. This way, through the estimate of standardized indexes, it is possible to evaluate the muscular reaction to dental proprioception. Other stimuli, that can influence the muscular activity in both tests( for instance, intramuscular pain), cannot always be identified by evaluating the standardized indexes( supposing that they can influence the standardization task and the teeth clenching in the same way). For this reason, an effective dental occlusion instrumental evaluation applying sEMG protocol needs healthy supporting tissues( bones, teeth, periodontium). Applying these clinical recommendations, the proposed standardization approach allows dental proprioception effect on functional areas management isolation, reducing the crosstalk and technical signal confounding factors relevance. Anyway, instrumental information about muscular functional adaptation at dental contact can be useful for the classification of dental patients, and the functionalization of therapeutic procedures as removable and fixed prostheses. As an example, patient L. D., a woman aged 26, needed rehabilitation of incongruous composite fillings( Fig. 2). An initial sEMG test could aid the Dentist to estimate if the reduced posterior vertical dimension, due to filling wear, can cause altered muscular performance. As shown in Figure 3, the initial sEMG test revealed that the pre-operative occlusal condition did not allow a maximal muscular activity. Masseter POC was lower than 80 % as the result of a no-synchronous and asymmetric Masseter
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