CHANGES IN MASSETER ACTIVITY AND COORDINATION AFTER CORRECTION OF UNILATERAL POSTERIOR CROSSBITES: FUNCTIONAL REASONS

AIM Unilateral posterior crossbite is a serious asymmetric malocclusion that may become clinically evident in early childhood, involving structures that are actively developing, including motor control of masticatory function. In fact, the significant presence of reverse chewing cycles has been well established in patients with unilateral posterior crossbite, during chewing on the crossbite side only. As a result, there is a serious asymmetry of the masticatory function. Mirroring the kinematic pattern, the activation of the masseter muscles is altered as well. During chewing on the crossbite side, patients show decreased activity of the masseter on the crossbite side and increased masseter activation on the contralateral side during reverse cycles, resulting in a reduced side to side difference in masseter muscle activity, whereas normally, unilateral chewing is characterized by a significant difference in activation between the ipsilateral and contralateral masseter muscles. The aim of this work was to describe the effects of functional therapy on masseter activity and chewing kinematic in patients with unilateral posterior crossbite. METHODS Fifty children (9.1 ± 2.3 years) with unilateral posterior crossbite (34 on the right side, 16 on the left side) and twenty children (9.5 ± 2.6 years) with normal occlusion were selected for the study. The mandibular motion and the muscular activity during chewing soft and hard boli were simultaneously recorded with a K7 kinesiograph (Myotronics, USA) before and after correction of unilateral posterior crossbite with the appliance Function Generating Bite (mean treatment time of 7.3 ± 2.4 months plus the retention time of 5–6 months). Chewing cycles were divided into non-reverse and reverse, based on the vectorial direction of closure. Since the amplitude of the EMG signal is influenced by individual anatomical characteristics, the percent difference between ipsilateral and contralateral peaks of the masseter amplitude was computed together with the percentage of reverse cycles. Significance level was set at P < 0.05. RESULTS Before therapy, the percentage of reverse cycles on the crossbite side was greater in patients than in controls (P < 0.001) and significantly reduced after therapy (P < 0.001) towards the reference normal value (soft bolus; pre: 57 ± 30%, post:12 ± 17%; hard bolus; pre: 65 ± 34%, post: 12 ± 13%; reference value: soft bolus 4 ± 2%, hard bolus 5 ± 3%). Before therapy the percent difference between electromyography envelope peaks in patients was lower than in controls (P < 0.01) and significantly increased after therapy (P < 0.05) becoming similar to the reference normal value. The normalization of coordination between the bilateral masseters after therapy was due to a significant reduction of the activity of the contralateral masseter muscle for both bolus types. CONCLUSIONS The altered muscular activation and the altered kinematics in presence of unilateral posterior crossbite might be considered a useful indicator of the severity of the masticatory function involvement. The correction of the malocclusion with a functional appliance induced a favorable change in the neuromuscular control of chewing of patients, who recovered a significant reduction of the reverse chewing patterns and a normal-like coordination between the masseter muscles. Interestingly, results showed that the normal-like coordination of masseters between sides was reached thanks to the lowering of the hyperactivity of the normal side. In comparison, the hypoactivity of the masseter of the crossbite side showed a limited improvement, meaning that a longer time is required to restore the hypotrophy of a muscle.