Microdamage accumulation in the monkey vertebra does not occur when bone turnover is suppressed by 50% or less with estrogen or raloxifene

Abstract

Long-term suppression of bone turnover with alendronate has previously been shown to increase the degree of mineralization and accumulation of microdamage in animal bones. In an effort to ascertain if other suppressors of bone resorption can also affect mineralization and microdamage accumulation, we evaluated bones from cynomolgus macaques treated with raloxifene or conjugated equine estrogens (CEE). Cynomolgus monkeys (Macaca fascicularis) were randomized, ovariectomized (except for Sham controls), and orally treated each day for 2 years with vehicle (Sham and Ovx controls), 1 mg/kg raloxifene (R1), 5 mg/kg raloxifene (R5), or 0.04 mg/kg CEE. The functional quality of the mineralized matrix was analyzed postnecropsy by biomechanical testing, histomorphometry, biochemistry, and nanoindentation. Failure testing of the whole vertebra showed no significant differences in vertebral strength among groups. Similarly, failure testing of a beam of pure bone that was machined from the femoral diaphysis also showed no differences in material strength (ultimate stress) between groups. Histomorphometry of the L2 centrum showed that Ovx tended to increase activation frequency relative to Sham controls. Estrogen (CEE) treatment for 2 years at about four times the clinical exposure tended to reduce activation frequency (Ac.f) by 41% compared to Ovx. Treatment with raloxifene at either approximately the clinical dose or five times higher nonsignificantly lowered Ac.f by 34% and 23%, respectively, relative to Ovx. Raloxifene had similar effects on serum osteocalcin, a biochemical measure of systemic bone turnover. Analysis of microcrack surface density in the cancellous bone of L3 showed a 40% reduction for Ovx relative to Sham. CEE microcrack surface density was not different than Sham whereas the R5 crack density was significantly less than Sham and CEE. R1 microcrack surface density was not significantly different from Sham or Ovx. No significant differences in crack length were observed among the groups. Hardness, which is a measure of the state of mineralization, and elastic modulus were measured for both trabecular bone on a micron scale by nanoindentation. No significant differences between groups were observed. In summary, differences in functional bone quality of the lumbar spine were not observed between Sham, Ovx, or treated monkeys. CEE increased microcracks from Ovx to Sham levels, whereas raloxifene had no effect on microdamage accumulation. We conclude that suppressing bone turnover by 40% or less offers protection against microdamage accumulation that could result in an increased risk of vertebral fracture.