High Concentrations of Pamidronate in Bone Weaken the Mechanical Properties of Intact Femora in a Rat Model

PURPOSE Bisphosphonates have been used to treat osteoporosis for more than ten years. However, complications associated with long-term administration of bisphosphonates, such as nonunion after pelvic insufficiency fracture or osteonecrosis of the jaw, have been recently reported in the literature. We investigated the relationships among the mechanical properties of the intact rat femur as well as healing fracture calluses and the intraosseous concentration of pamidronate (ICP), after long-term administration of pamidronate in a rat osteoporosis model. MATERIALS AND METHODS We performed bilateral ovariectomy in 25 3-month-old female Sprague-Dawley rats. Beginning three months after ovariectomy, disodium pamidronate (0.5 mg/kg) was injected every month. After the six-month administration period, the left femoral shaft was fractured using the closed fracture technique. Five weeks after fracture, 23 rats were euthanized and both femora were removed. We checked the mechanical properties of the intact (right) and fractured (left) femora using a three-point bending technique. Intraosseous concentration of pamidronate was checked by high-performance liquid chromatography. RESULTS The mean ICP was 61.8+/-15.7 ng/mg of bone. High ICP decreased the ultimate load to failure, stiffness, and ultimate stress of the intact femora (p=0.015, 0.027, 0.039, respectively). There was a tendency to decrease the ultimate load to failure in the healing callus when the ICP increased (p= 0.183). High ICP decreased the bone mineral density of the femoral head (p=0.005). CONCLUSION High concentrations of pamidronate in intact bone decreased the bone mineral density and weakened the mechanical strength of the rat femora. The mechanical strength of the early healing callus was not correlated with concentration of pamidronate in the bone.