Free radical scavengers versus methylprednisolone in the prevention of experimentally induced heterotopic ossification.

The etiology of heterotopic ossification (HO) is still obscure, it is difficult to devise an effective preventive or therapeutic approach. The options for the prevention of HO are still limited. The prophylactic effect of nonsteroidal anti-inflammatory drugs (NSAIDs) is insufficient. Moreover, NSAIDs increase the risk of nonunion and loosening in patients with multiple joint injuries. The present experimental study was designed to compare methylprednisolone with free radical scavengers for the prevention of HO. The model of Michelsson et al. was used to induce HO in the hind legs of 30 female New Zealand albino rabbits, weighing approx. 4 kg. The animals were randomized into three groups of 10 animals each, and received daily either placebo, a free radical scavenger cocktail [allopurinol and N-acetylcysteine (A/A)], or methylprednisolone in a randomized, double-blind fashion. Every four days, X-rays were obtained to measure the thickness and the length of new bone formation at the thigh. A statistically significant difference in thickness and length of newly formed bone was found between the three groups. In the placebo group HO increased from day 16 toward a medium length of 6 mm and a median thickness of 1.5 mm. In the A/A group, no signs of HO were found. In the methylprednisolone group, only one animal presented minor HO from day 32. Both free radical scavengers and methylprednisolone were found to inhibit HO, and may be considered effective measures for the prevention of heterotopic bone formation. However, it could not be demonstrated which of the two had the strongest inhibitory effect.