The effects of masseter muscle paralysis on facial bone growth.

BACKGROUND Understanding the effects of muscle function on facial bone growth may help us treat children with facial anomalies. Facial bone growth is known to be a result of both genetic and epigenetic influences. One of the main epigenetic factors controlling growth is thought to be muscle action. The purpose of this study was to establish a model of single facial muscle paralysis and to identify the effects masseter muscle paralysis has on mandible and zygoma growth. METHODS Twenty New Zealand white rabbits were divided into control, paralysis, and sham groups. Masseter muscle paralysis was achieved with botulinum toxin A (BTX). Computed tomographic and single-photon emission computed tomography (SPECT) scans and cephalometric measurements were performed. Masseter weights and mandible and zygoma volumes, shapes, and metabolism were measured. RESULTS Eighteen animals completed the study. Significant decreases in zygoma and mandible volumes with minimal changes in shape were seen on the paralyzed sides. SPECT showed a decrease in bone production in both zygomas and mandibles on the paralyzed sides. CONCLUSIONS An animal model has been created in which the effects of single muscle paralysis on bone growth can be studied. Masseter muscle function may be responsible in maintaining mandible and zygoma volume by controlling bone production. Masseter function alone has less influence on mandible and zygoma shape.