Effects of therapy on masseter activity and chewing kinematics in patients with unilateral posterior crossbite

Objective: To describe the effects of therapy on masseter activity and chewing kinematic in patients with unilateral posterior crossbite (UPC). Design: Fifty children (age: mean +/- SD: 9.1 +/- 2.3 years) with UPC (34 on the right side, 16 on the left side) and twenty children (age: 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, before and after correction with function generating bite, after a mean treatment time of 7.3 +/- 2.4 months plus the retention time of 5-6 months. The percentage of reverse cycles and the percent difference between ipsilateral and contralateral peaks of the masseter electromyography envelopes were computed. Results: Before therapy, the percentage of reverse cycles during chewing 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. Conclusions: The correction induced a normal-like coordination of masseter muscles activity together with a significant reduction of the reverse chewing patterns. The previous altered muscular activation corresponded to the altered kinematics of reverse chewing cycles that might be considered a useful indicator of the severity of the masticatory function involvement.