The imbalance of masticatory muscle activity affects the asymmetric growth of condylar cartilage and subchondral bone in rats.

OBJECTIVE

To examine the effects of imbalance of masticatory muscle activity of the rat mandible on the condylar cartilage and subchondral bone during the growth period.

DESIGN

Forty 5-week-old male Wistar rats were randomly divided into experimental (n=20) and control (n=20) groups. In the experimental group, the left masseter muscles were resected. The rats were sacrificed at 7 or 9 weeks of age in both groups. Microcomputed tomography was used to determine the three-dimensional morphology and cancellous bone structure. For histological and histochemical examination, 5-μm-thick serial frontal sections of the condyle were stained with toluidine blue and immunostained with asporin and TGF-β1 to evaluate the promotion and inhibition of chondrogenesis.

RESULTS

In the experimental group, microcomputed tomography analysis showed asymmetric growth; the resected side condyles showed degenerative changes. Histological analysis showed that the total cartilage in the central region of the resected side was significantly thinner than in the non-resected side in the experimental group, as well as in the control group. Compared with the control group, the expression of asporin was significantly higher in the resected side, and significantly lower in the non-resected side. In contrast, the expression of TGF-β1-immunopositive cells in the non-resected side was significantly higher than in the resected side and the control group.

CONCLUSIONS

These findings imply that lateral imbalance of masseter muscle activity lead to inhibition of chondrogenesis and induce asymmetric formation of the condyle during the growth period.