Changes in the masticatory cycle following treatment of posterior unilateral crossbite in children.

In a previous study, we established that young children with unilateral posterior crossbite have a longer mandibular ramus and more superiorly and posteriorly positioned condyles on the crossbite side. In this study, we evaluated chewing cycle shape and duration in 14 of the patients before treatment, and we looked for changes in cycle shape and duration 6 months after treatment with rapid palatal expansion. Mandibular kinematics was recorded while chewing gum using an optoelectric recording system at 100 Hz. Subjects were asked to chew normally for 20 cycles, chew on the crossbite side only for 20 cycles, and chew on the noncrossbite side only for 20 cycles. A special computer program selected the 10 most representative cycles from each series and computed an average duration and an average maximum excursion along 3 orthogonal axes. Multilevel linear models were used to generate an 8th-order polynomial describing average cycle shape and to test for statistically significant differences in shape between the patients and the controls and between the patients before and after treatment. Before treatment, the patients chewed more slowly than did the controls. Treatment shortened their cycle duration to equal control values. Before treatment, the patients also had larger maximum excursions than did the controls and exhibited a reverse-sequence cycle shape when chewing on the crossbite side. Treatment did not alter the patients' abnormal cycle shape. These results suggest that some features of the masticatory kinematics respond to orthodontic treatment alone, but others do not.