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Lateral functional shift of the mandible: Part I. Effects on condylar cartilage thickness and proliferation.

Lateral functional shift of the mandible is characterized by transverse rotation of the entire mandible about a vertical axis toward 1 side of the head. The effect of this shift is that the condyle on the side opposite the direction of the shift is displaced anteriorly, or protruded, while the condyle on the side toward the shift is more stable positionally and is likely to be slightly retruded. According to the view that growth of the mandibular condylar cartilage (MCC) adapts to its local functional-biomechanical environment, differential changes in metabolic activity of the MCC would be expected on the nonprotruded and the protruded sides. To evaluate this hypothesis, 21 rats (28 days old) were fitted with intraoral positioners designed to shift the mandibular postural position asymmetrically. Cartilage thickness and BrdU labeling index in the MCC 3, 7, and 14 days after placement of the positioners were compared with those in age-matched controls that received no positioners. Cartilage thickness in the MCC on the protruded side was significantly greater than that in the controls at each time interval, with the difference increasing slightly with time. The labeling index for the protruded MCC was significantly greater than the controls at both 7 and 14 days after positioner placement. Trends on the nonprotruded side were generally opposite, culminating in reduced thickness and proliferation after 14 days. Thickness and labeling index were greater on the protruded side at every time interval except 3 days (labeling index). These trends in cartilage thickness and proliferation are consistent with previous studies of MCC changes after bilateral functional protrusion or retrusion. These data suggest that changes in MCC thickness and proliferative activity might accompany a lateral functional shift of the mandible in growing persons.