Effects of acetylsalicylic acid and naproxen on the mechanical properties of intact femora in rats.

The influence of acetylsalicylic acid (ASA) and naproxen on growing bones was studied. Young male rats were used. The drugs were administered via gastric gavage twice a day for 9 or 18 days. Drug doses giving serum concentrations corresponding to ordinary anti-inflammatory steady-state levels in humans were used. There was a drug-related influence on the strength of the growing femur. After 9 days the ultimate bending moment of the distal femoral epiphyseal plate and ultimate torsional moment and stress of the femoral diaphysis increased by about 10% in the rats treated with 150 mg/kg/12 h of ASA as compared with controls. After 18 days there were no differences. The ultimate metaphyseal bending moment of the distal femur was not influenced after 9 days with this dose, but was reduced by about 10% compared with controls after 18 days. Doses of 100 mg/kg/12 h of ASA and 20 mg/kg/12 h of naproxen did not change the bone strength. The doses used were well tolerated and did not influence the bone growth or body weight of the rats. A naproxen dose of 40 mg/kg/12 h was lethal; rats that received this dose succumbed to jejunal perforations. The results indicate that ASA influences the remodeling of normally growing bones.