Bone Quality: An Empty Term

A lthough the concept of " bone quality " is at least 15 years old [1], the term has recently sparked much discussion and debate among clinicians and clinical researchers [2–5]. At a recent National Institutes of Health conference on bone quality, the term was defi ned as: " The sum total of characteristics of the bone that infl uence the bone's resistance to fracture " [6]. This defi nition arose from the results of multicenter clinical trials that evaluated the effects of two classes of drugs—antiresorptive bisphosphonate therapy (alendronate and risedronate) and selective estrogen receptor modulator therapy (raloxifene)—on the prevention of osteoporotic fragility fractures [7,8]. While these studies reported consistent reductions in the incidence of fractures, the treatment effects could not be explained by contemporary changes in dual X-ray absorptiometric bone mineral density (BMD), the present clinical standard of bone fragility. These confl icting fi ndings led to speculation that the antiresorptive drugs had additional skeletal effects upon a feature of the bone called " bone quality " [7–12]. The idea of bone quality, and the explanation for the confl icting results, linked together two important notions: (1) antiresorptive drugs acted by suppressing bone turnover through inhibiting bone resorption, and (2) increased bone turnover (mainly the increased bone resorption, as detected by bone markers) compromises the bone strength through deteriorated bone microarchitecture (a trait that cannot be captured by BMD measurement but could potentially be improved by antiresorptive treatment) [4]. Bone quality is now a widely embraced concept that seems to offer a solution to the classic paradox of osteoporosis: while low BMD values are associated with increased relative risk of fracture at the population level, the predictive value of BMD in an individual patient remains quite marginal [13–15]. And to further support the concept of bone quality, inclusion of increased bone turnover in fracture-predicting models has somewhat improved the ability to predict fracture risk independently of BMD [8,16–19]. Although the concept of bone quality might seem attractive for all of the reasons discussed above, nevertheless the notion has three major conceptual fl aws. BMD and bone quality do not explain fractures. First, although BMD indeed shows a strong correlation with whole bone strength in the laboratory setting (r up to ~0.9) [20], in the clinical setting, paradoxically, the overall proportion of various fragility fractures attributable to low BMD (indicating reduced bone strength) remains modest (from 0% to 44%) …