Effects of lateral pterygoid muscle hyperactivity on differentiation of mandibular condyles in rats

BACKGROUND: The effects of biomechanical stress on the growth and development of the mandibular condyle have been studied by many investigators. However, the role of the lateral pterygoid muscle in this development is not clear. METHODS: Hyperfunction of the lateral pterygoid muscles of male 3-week-old Sprague-Dawley rats was induced by electrical stimulation, and the responses of the mandibular condyles were compared to control tissues by a double-fluorescent staining technique using polyclonal antibodies against type I and type II collagen. Electrical stimulation consisted of repeated application (5 seconds on/5 seconds off) of a 5 Hz current for up to 7 days. RESULTS: In the first 2 days, cartilaginous tissues rich in type II collagen disappeared in the anterior and posterior areas, which were loaded by tensional force due to direct and indirect attachment of the lateral pterygoid muscles. Tissues in these areas were replaced by intramembranous bone that was reactive for type I collagen at 7 days. By the end of the experiment, the trabecula of the condyle was remodeled more perpendicularly, thus resisting the compressive force due to hyperfunction of the lateral pterygoid muscles. CONCLUSIONS: These results suggest that the activity of the lateral pterygoid muscle might play a significant role in the differentiation of progenitor cells and in the maturation and calcification of chondrocytes in mandibular condyles.