Osteoporosis in men and women: a story about bone mineral density thresholds and hip fracture risk.

In postmenopausal women, the T score for bone mineral density (BMD) is a well-accepted diagnostic criterion for osteoporosis. It is also used to assess fracture risk. However, it is unclear whether in elderly men similar BMD thresholds should be used. Different hypotheses have been proposed for the relation of BMD with hip fracture risk in men. In this study, we tested those hypotheses using a mathematical model and we compared the calculated results with observed prospective data from the Rotterdam study. In the model, we combined the observed femoral neck BMD distribution for men and women with previously derived hip fracture risk functions based on age and BMD. For men, we tested different hypotheses for the relation of BMD with hip fracture risk. The relation of BMD with hip fracture risk is similar in men and women (scenario 1) or the relative risk (RR) per standard deviation (SD) decrease of BMD is either larger or smaller in men than in women (scenario 2a and 2b), or, at a similar absolute fracture risk, men have a higher BMD (scenario 3). In the prospective data, men with a hip fracture had an average BMD that was 0.070 g/cm2 higher than women with a hip fracture. The calculated results from the first scenario were consistent with those data and were also consistent with the observed hip fracture incidence and the observed female-to-male (F/M) risk ratio (1.7). When the RR for each SD decrease of BMD was assumed to be either larger or smaller in men than in women (second scenario), the calculated average BMD difference in men and women became respectively smaller or larger than observed. When men would have a higher fracture risk at similar BMD levels (third scenario), the calculated total number of hip fractures increased and even exceeded that in women, with an F/M risk ratio of 0.94 in our example. In women, a larger proportion of hip fractures occurs at a T score below -2.5 than in men using the same absolute BMD threshold, but using a male-specific T score largely solves this diagnostic problem. Taken together, the average hip fracture risk in men is much lower than in women but appeared to be similar at the same BMD. Therefore, we propose the use of the same absolute BMD thresholds for decisions about interventions.