Bone loss and bone size after menopause

Background: Bone loss increases after menopause. However, bone strength also depends on structural characteristics such as bone size. Whether bone size increases as a result of periosteal apposition and whether a strength index accounting for both bone density and bone size might predict the risk of fracture better than bone density alone are unclear. Methods: Bone mass and the skeletal structure of the distal radius were evaluated by single-photon absorptiometry every other year in 108 women, all of whom were followed from the time of menopause for a mean period of 15 years. Postmenopausal serum estradiol levels and fractures of the distal radius were noted. Results: The mean (+/-SD) annual decrease in bone mineral density was 1.9+/-0.7 percent. The medullary bone diameter increased annually by 1.1+/-0.9 percent, and the periosteal diameter by 0.7+/-0.3 percent; the strength index decreased by 0.7+/-0.7 percent. The expansion of the medullary diameter and the expansion of the periosteal diameter were correlated with one another (r = 0.54, P<0.001), and women in the highest quartile of medullary expansion had more loss of bone mineral density and greater periosteal apposition than women in the lowest quartile (P<0.001 for both comparisons). The postmenopausal serum estradiol level was correlated with changes in the periosteal diameter (r = -0.25, P=0.009) and with changes in bone mineral density (r = 0.34, P<0.001). A 1-SD decrement in the strength index at base line was associated with a risk ratio for fracture of the distal radius of 3.8 (95 percent confidence interval, 1.8 to 8.0). Conclusions: Increased bone loss after menopause is associated with increased periosteal apposition, which partially preserves bone strength. A strength index may be a helpful predictor of the risk of fracture.