Methylprednisolone has a preventive effect on heterotopic bone formation and on knee thickening and stiffening following manipulation of immobilized rabbit legs.

The effect of methylprednisolone administered following manipulation of the immobilized knee of 35 adult rabbits was investigated. In the legs of 15 control rabbits which received no methylprednisolone, heterotopic cartilage and bone in the periarticular tissues and in the vastus intermedius muscle, and extensive knee stiffening and thickening regularly developed. In the other rabbits, methylprednisolone showed a marked preventive effect on the development of all these pathological signs.

Methylprednisolone has a preventive effect on the development of radiological changes, thickening and stiffening of the rabbit knee following immobilization.

The effect of methylprednisolone (MP), Depo-Medrol, on the development of radiologically detectable changes, i.e. thickening and stiffening in rabbit knees following a standard immobilization method, was investigated. The right knees of 34 rabbits were immobilized by a splint and an elastic bandage in extension for five weeks, whereafter the splint was removed. Twelve of the rabbits were given MP in the back muscles once a week during the immobilization period only and eight other rabbits were given diminishing doses during the immobilization period and also during the remobilization period. Marked radiologically detectable changes, stiffening, and thickening developed regularly in the immobilized knees of the rabbits which were not given MP. In the other rabbits MP had a significant preventive effect on the development of all these pathological signs. The degree of the effect of MP was related to the dose and to the administration period. The optimal effect was registered when MP was given initially in large but then gradually diminishing doses during the immobilization and the remobilization periods.

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.