[Muscular equilibrium and orthognathic surgery. A preliminary electromyographic study].

Few studies have been written about the effects of orthodontic preparation and of the consequences of surgical movements on mandibular motricity. The aim of this preliminary report is to evaluate the modifications of muscular activity between the pre-surgical phase and its evolution during the year after surgery, together with the skeletal modifications found. The muscular activity of six patients with serious vertical discrepancies requiring rehabilitation of the lower facial height has been studied using electromyographical recordings. A longitudinal study of the EMG data has been established in order to discover a muscular adaptation method towards physiological equilibrium following the realization of a new facial diagram. The reduction of the lower facial height obtained through a surgical procedure including a maxillary impaction, appears to indicate that, at rest, the masseter muscles activity tends to normalize, whereas that of the temporal muscles may sometimes increase. During maximal contraction, facial hyper-divergence reduction would lead to a sharp decrease in temporal and masseter muscular activity which would subsequently return to normal. The increase in vertical dimension would also cause significant changes in muscular tonus depending on the sagittal direction of the associated mandibular osteotomy. It would take longer to achieve muscular activity stability. These few results show: The existence of significant modifications following orthognathic surgery, possible reduction of high rest muscular activities, often revealing a cranio mandibular dysfunction, after vertical typology rehabilitation. The electromyographic recordings during treatment then enable to perfect the therapeutic re-evaluation of these major vertical discrepancies.