Bone formation in experimental myositis ossificans. Light and electron microscopy study.

The development of ectopic ossification in experimental myositis ossificans of the rabbit thigh was studied. The right hind limb of 25 rabbits was immobilized with the knee in extension. Once a day the limb was passively mobilized for 2-3 minutes. The animals were killed 3, 5, 7, 14, 21, 28 and 35 days after the beginning of the experiment. Specimens for light and electron microscopy were obtained from both hind limbs. Extensive necrosis and fibrosis were observed in the right vastus intermedius muscle during the first week. Proliferation of chondrocytes and osteoblasts with newly formed woven bone and cartilage formation were found in the periosteum within 7 and 14 days after the beginning of the experiment. Intensive enchondral ossification and hard calcified tissue were observed later. It was evident that bone formation in this experimental model started in the periosteum after necrosis of the adjacent skeletal muscle. Therefore the temporal and spatial relationship of traumatic changes in the periosteum and muscle seems important for the development of myositis ossificans.