Active mandibular protrusion in the rational treatment of skeletal Class II malocclusions

BACKGROUND: Aim of the present study is to assess the ideal mandibular protrusion exercises, which should always be associated with functional treatment of skeletal Class II malocclusions, characterized by mandibular hypoplasia. METHODS: The study was carried out on a sample including 8 growing subjects (mean age 10.6 years +/- 0.6), similar in age and biotypology (mandibular hypoplasia; normodivergent growth pattern). The electromyography (EMG) activity of the superficial bundle of the masseter (considered as an accessory protrusive muscle) was bilaterally recorded at rest and, during voluntary contraction, at different degrees of protrusion (minimum, medium, maximum), condition dynamically analyzed at various times of contraction (initial, 5, 10, 30 seconds). The mean values of the recruitment pattern during the whole contraction, the number of turns/sec and the ratio turns/mean amplitude were evaluated. RESULTS: The data obtained by means of a precise electromyographic analysis suggest that the ideal exercise for an active mandibular protrusion should be a medium degree contraction lasting 10 seconds. These results are consistent with the linear relationship between muscle force and EMG amplitude, correlation effective only up to 60% of the exerted force, because of the muscular fatigue appearing beyond this threshold. During such a muscular fatigue, due to maximum contraction, it is possible to observe a progressive decrease in the EMG activity, owing in particular to the motor-neuron reduced discharge-frequency. CONCLUSIONS: This result suggests a small benefit of long-lasting muscular contractions, both in maximum and in medium protrusion movements, for re-educational purposes. Only during the medium-degree, 10 sec-lasting contraction it's possible to point out a turns-number variation significant of an active and constant contraction, which decreases if long-lasting (30 sec).