fracture per 1 SD decrease in BMD or in bone mineral apparent density (BMAD), by skeletal site, for individuals in Rochester, Minnesota. Men were 22–90 years of age (n = 348) and women were 21–98 years of age (n = 351). Per 1 SD decrease in BMD at total hip, for 29- to 49-year-old men, odds ratios were 1.38, 1.12, and 1.17 for total hip, femoral neck, and anterior–posterior (AP) spine scans, respectively, and 1.45 for total wrist. For women ages 20–49 years, odds ratios were 2.44, 1.72, and 1.59 for total hip, femoral neck, and AP spine scans, respectively, and 1.56 for total wrist. It is noted that unlike the astronaut crew, the baseline fracture rate was relatively increased in this community-based population. For men, total hip BMD was 1.061–0.140 g/cm2, and for premenopausal women, mean total hip BMD was 0.941–0.124 g/cm2. After adjusting for age, total hip BMD was the best predictor of fracture risk in women. In men, BMD at the wrist was the stronger predictor of fracture risk. This experience should be compared to existing data on baseline BMD and bone loss in the astronaut population.
It is widely appreciated that an assessment of fracture risk based solely on DXA BMD measurement is inadequate. Susceptibility to fracture at any level of bone mass is determined by bone density measurement and by the structural integrity of bone, which cannot be measured accurately with existing technology. The application of engineering principles to derive indirect assessments of bone strength—bone section modulus and buckling ratio (Beck et al., 2001; Kaptoge et al., 2003; Melton et al., 2005)—represents an advance in the field. Quantitative computed tomographic (QCT)– derived measurements of bone density and bone geometry in 14 astronauts after four to six months on the International Space Station (ISS) indicated a decline in bone density and parameters of bone strength (i.e., strength index, compressive strength indexes) in proximal hip and vertebral body (Lang et al., 2004). Consistent with earlier DXA studies involving Mir cosmonauts, bone was lost at rates of 0.8–0.9 percent per month at the spine and 1.2–1.5 percent per month at the hip. Although both cortical and trabecular bone declined, the percent loss in trabecular bone was greatest in the hip: proximal femur, 2.2–2.7 percent per month. As noted above, Lang et al. pointed out that the various bone strength indexes have not been validated as predictors of fracture risk and that the number of ISS subjects was small. Although presenting data were derived by a sensitive technique—volumetric QCT—it is likely that more accurate data relating bone loss to strength parameters will be derived in the near future from