Bone marrow fat cell enlargement and a rise in intraosseous pressure in steroid-treated rabbits with osteonecrosis

The etiology of steroid-induced osteonecrosis (ON) is unclear. This study was designed to determine whether bone marrow fat cell size, intraosseous pressure, and blood flow rate differed between steroid-treated rabbits with ON and those without. Twenty-nine rabbits were intramuscularly injected once with 20 mg/kg of methylprednisolone acetate (MPSL), and five rabbits were injected once with physiologic saline (PS) as a control. Intraosseous pressure and blood flow rate in the proximal femur were determined before and at 2 weeks after the injection. After these measurements, both femora and humeri were histopathologically examined for the presence of ON, and size of bone marrow fat cells were morphologically examined. At 2 weeks after steroid injection, the intraosseous pressure was significantly higher in rabbits with ON than in those without (p = 0.0251), and the blood flow rate had decreased significantly more in rabbits with ON than in those without (p = 0.0051). The size of the bone marrow fat cells was significantly (p = 0.0004) larger in rabbits with ON (diameter, 63.5 ± 5.8 μm) than in those without (diameter, 53.3 ± 6.9 μm). Injection of PS (5 rabbits), 1 (10 rabbits), 5 (10 rabbits), and 20 (10 rabbits) mg/kg of body weight of MPSL showed that a larger dose of steroid increased both fat cell size and prevalence of ON. These results suggest that bone marrow fat cell enlargement and a rise in intraosseous pressure may be important when considering the pathophysiology of steroid-induced ON in rabbits.