Influence of extraoral lateral force loading on the mandible in the mandibular development of growing rats.

INTRODUCTION The objective of this study was to elucidate the influence of extraoral lateral force loading of the mandible on mandibular development. METHODS Thirty growing Wistar rats were divided into 3 groups: control, sham, and experimental. To determine longitudinal developmental changes, each animal was placed under anesthesia and immobilized in a custom-built body retainer to allow microcomputed tomography to be performed before and after the experiment. In the experimental group, a fixing device of aluminum was fitted to the zygomatic arch. Lateral force was then applied to the mandible with an open coil for 2 weeks. Stereoscopic images were constructed from 3-dimensional microcomputed tomography images. Absolute lengths and perpendicular heights from the baseline of the lower border of the mandible were measured. The images were superimposed at the baseline planes in each animal. After the final observation, the rats were killed, and the bilateral condyles excised. The condyles were decalcified by the standard method, stained with hematoxylin and eosin and toluidine blue, and observed under a microscope. RESULTS Lateral force induced the mandible to shift toward the nonloaded side; absolute mandibular length at menton-condylion on the side where the load was applied was greater than that in the control group. No difference from the control group was noted on the nonloaded side, and there were no significant differences in perpendicular heights. Lateral loading on the mandible resulted in histopathologic changes: (1) on the side where the load was applied, the cartilaginous zone hypertrophied in the highest margin of the condylar head, the erosive zone expanded, and the width of the mandibular neck decreased; (2) the chondrocyte layer shifted to the medial side on the nonloaded side, and cartilaginous ossification occurred in the lateral direction immediately below the chondrocyte layer, which deformed the mandibular neck toward the medial side and caused asymmetric development of the mandible. CONCLUSIONS These results demonstrate that extraoral lateral force loading during the growth stage causes asymmetrical mandibular development.