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The Paradox of Small Changes in Bone Density and Reductions in Risk of Fracture with Raloxifene

As its name implies, antiresorptive therapy decreases the resorption of bone tissue by osteoclasts. Slowing the resorption of bone in postmenopausal women slows the loss of bone. Raloxifene and tamoxifen act in this fashion to improve bone mass in postmenopausal women, but this effect is of modest magnitude—about a 2% higher bone density in women randomly assigned to SERMs compared to those assigned to placebo. It is surprising, therefore, to see that raloxifene reduces the risk of vertebral fractures by 35–45% in women with osteoporosis. This implies that there must be some other mechanisms for reducing the risk of vertebral fracture that are not captured by standard measurements of hip and spine bone density. Theoretically, slowing bone resorption may directly increase the biomechanical strength of bone. At sites of bone resorption, trabeculae are temporarily thinner. Increasing the number of resorption sites in trabecular bone would be expected to weaken the bone, probably to a degree that is out of proportion to what would be expected from the relatively small amount of bone that is lost. In addition, bone is constantly undergoing stress—forces placed on bone by ordinary daily activity. These forces are not evenly distributed in bone: they tend to focus at defects or depressions in a bone. So, theoretically, bone resorption both decreases mechanical strength at resorption sites and causes mechanical forces to increase disproportionately at the same sites. Therefore, slowing down the rate of resorption and the number of places that this occurs may improve the strength of bone to a degree that is not reflected in commensurate improvement in bone density. However, this remains theoretical and there is no direct proof or easy way to test this hypothesis. Studies of the relationship between change in markers of bone turnover and reduction in risk of fractures have not yet provided solid support for this hypothesis, but these markers are imperfect and highly variable indicators of bone resorption.