Selective Modification of Bone Quality by PTH, Pamidronate, or Raloxifene

Bone strength, a determinant of resistance to fracture, depends on BMD, geometry, microarchitecture, bone turnover rates, and properties of the bone at the material level. Despite comparable antifracture efficacy, anti-catabolics and bone anabolic agents are likely to modify the various determinants of bone strength in very different ways. Eight weeks after ovariectomy (OVX), 8-mo-old osteoporotic rats received pamidronate (APD; 0.6 mg/kg, 5 days/mo, SC), raloxifene (3 mg/kg, 5/7 days, tube feeding), PTH(1-34) (10 mu g/kg, 5/7 days, SC), or vehicle for 16 wk, and we measured vertebral BMD, maximal load, stiffness and energy, microarchitecture, and material properties by nanoindentation, which allows the calculation of the elastic modulus, tissue hardness, and working energy. Markers of bone turnover., plasma osteocalcin, and urinary deoxypyridinoline (Dpd) were also determined. PTH induced greater maximal load than APD or raloxifene, as well as greater absorbed energy, BMD, and increased bone turnover markers. PTH markedly increased trabecular bone volume and connectivity to values higher than sham. Animals treated with APD had BV/TV values significantly higher than OVX but lower than sham, whereas raloxifene had no effect. Tissue hardness was identical in PTH-treated and OVX untreated controls. In contrast, APD reversed the decline in strength to levels not significantly different to sham, reduced bone turnover, and increased hardness. Raloxifene markedly increased material level cortical hardness and elastic modulus. These results show the different mechanisms by which anti-catabolics and bone anabolics reduce fracture risk. PTH influences microarchitecture, whereas bisphosphonates alter material-level bone properties, with probable opposite effects on remodeling space. Raloxifene primarily improved the material stiffness at the cortical level.