BACKGROUND:Clinical fol ow-up studies have shown that alendronate is effective to prevent femoral head col apse fol owing traumatic osteonecrosis of the femoral head, but its mechanism remains poorly understood. OBJECTIVE:To analyze the effects and mechanisms of alendronate on prevention of col apse in traumatic osteonecrosis of the femoral head. METHODS:Total y 45 Sprague-Dawley rats were randomly divided into three groups, with 15 rats in each group. Placebo group received physiological saline after establishing models of avascular necrosis of the femoral head. Alendronate group received treatment of alendronate after model induction. Sham surgery group received an equal volume of physiological saline. At 5 weeks after model establishment, the rats were sacrificed. Femoral samples at the modeling side were col ected for general observation, X-ray irradiation, Micro-CT and histological detection. RESULTS AND CONCLUSION:General observation revealed that the femoral head was obviously deformed in the placebo group, but mild deformity was visible in the alendronate group. The ratio of height to width of the femoral head ranked as fol ows:sham surgery group>alendronate group>placebo group, showing significant differences. Micro-CT scanning results demonstrated that the mean number of bone trabecula was more in the alendronate group than in the placebo group, but less than sham surgery group, showing significant differences. The mean thickness of bone trabecula was less in the alendronate group than in the placebo group, but no significant difference was detectable as compared with the sham surgery group. The mean spacing of bone trabecula was less in the alendronate group than in the placebo group, but larger than in the sham surgery group, showing significant differences. Bone volume, bone surface area and bone mineral density were larger in the alendronate group than in the placebo group, but less than sham surgery group, showing significant differences. Histological detection results demonstrated that apparent sequestrum existed and osteoclasts were obviously inhibited in the alendronate group. The number of osteoclasts was noticeably less in the alendronate group than in the placebo group. Osteoblasts and new vessels were suppressed to some degrees. Results suggested that alendronate can inhibit curing reaction through inhibiting osteoclast and osteoblast activity and vessel formation, which can final y slow down the absorption of necrotic bone and preserve femoral head mass and shape. Thus, alendronate can be used as a preventive against femoral head col apse in rats with traumatic osteonecrosis of the femoral head.