Jaw protruder muscles and condylar cartilage growth in the rat.

Many studies have explored the role of the protrusive musculature in promoting growth at the condylar cartilage and the overall lengthening of the lower jaw, with emphasis on the lateral pterygoid muscle (LPM). The largely anteroposterior orientation of the superficial part of the masseter muscle (SM) in the rat suggests that it may also function as a protruder of the lower jaw. Accordingly, it is possible that the action of the SM may play a part in the regulation of growth of the condylar cartilage and the lower jaw. To examine this hypothesis, bilateral resection of the superficial portion of the masseter muscle was performed in male Sprague-Dawley rats at 26 days of age. At 5 days after surgery, [3H]-thymidine incorporation in the condylar cartilage was increased (F = 6.93, p less than or equal to 0.01) in the SM myectomy group relative to the surgical control and unoperated control groups. However, by 20 days after surgery no differences were present. At this sacrifice interval, lower jaw dimensions relating to areas of muscle attachment, as well as ramus height, were significantly reduced in the SM myectomy group, but overall jaw length (mental foramen to condyle) was unaffected. In contrast, myotomy of the LPM resulted in a significant decrease in mitotic activity of the cartilage 4 days after surgery. This decrease was present, but not more pronounced, in animals subjected to both SM myectomy and LPM myotomy. Hence, myotomy or myectomy of these two muscles, each with a protrusive orientation, produces opposite effects on proliferative activity at the condylar cartilage.