Studies of nontraumatic osteonecrosis. Manometric and histologic studies of the femoral head after chronic steroid treatment: an experimental study in rabbits.

High-dose methylprednisolone was administered to adult rabbits over a five- to nine-week period, according to previously described experimental models of nontraumatic osteonecrosis (NON). Manometric studies were performed using a sensitive computer-directed microvolume flow system. Osteocyte viability within the femoral head was assessed by counting empty osteocyte lacunae in five random high-power fields of hematoxylin- and eosin-stained sections. There was no significant elevation of intramedullary pressure or venous outflow resistance in femoral heads of animals receiving steroids, and a significant random loss of osteocytes was observed in these femoral heads. The clinical relevance of these studies is as follows. NON associated with steroids may not simply be the result of progressive ischemia. Ischemia and elevated intramedullary pressure may be epiphenomena, and a direct cytotoxic effect of steroids on osteocytes may be the earliest event in the development of nontraumatic osteonecrosis. The stress test, as it is currently being used in the clinical setting, may be unreliable.