ABSTRACT The purpose of this study was to evaluate the electromyographic (EMG) activity of the masseter and temporalis muscles and the skeletal and dental behavior of children having slow maxillary expansion with a quadhelix appliance. The study sample consisted of 30 children (mean age = 7 years 4 months; 17 girls/13 boys) diagnosed with a unilateral posterior crossbite. Lateral cephalometric tracing and EMG analyses were carried out before orthodontic treatment (T1) and 1 month after removal of the appliance (T2). The EMG activity of the masseter and temporalis muscles was analyzed in two clinical situations: at muscular rest and during habitual chewing. Differences in the measurements were evaluated by using paired t tests. Maxillary expansion with the quadhelix appliance did not promote significant anteroposterior and vertical skeletal alterations. Among the cephalometric variables related to dental pattern, only 1-ANS-PNS values presented a significant increase at T2. At muscular rest, EMG analysis indicated a significant increase in the activity of the temporalis muscle. During habitual chewing, EMG activity decreased significantly for both evaluated muscles after removal of the quadhelix appliance. These alterations suggest an adaptation and reprogramming of the musculature to the new occlusal condition.
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