Age-dependence of femoral strength in white women and men

Abstract

Although age-related variations in areal bone mineral density (aBMD) and the prevalence of osteoporosis have been well characterized, there is a paucity of data on femoral strength in the population. Addressing this issue, we used finite-element analysis of quantitative computed tomographic scans to assess femoral strength in an age-stratified cohort of 362 women and 317 men, aged 21 to 89 years, randomly sampled from the population of Rochester, MN, and compared femoral strength with femoral neck aBMD. Percent reductions over adulthood were much greater for femoral strength (55% in women, 39% in men) than for femoral neck aBMD (26% in women, 21% in men), an effect that was accentuated in women. Notable declines in strength started in the mid-40s for women and one decade later for men. At advanced age, most of the strength deficit for women compared with men was a result of this decade-earlier onset of strength loss for women, this factor being more important than sex-related differences in peak bone strength and annual rates of bone loss. For both sexes, the prevalence of "low femoral strength" (<3000 N) was much higher than the prevalence of osteoporosis (femoral neck aBMD T-score of -2.5 or less). We conclude that age-related declines in femoral strength are much greater than suggested by age-related declines in femoral neck aBMD. Further, far more of the elderly may be at high risk of hip fracture because of low femoral strength than previously assumed based on the traditional classification of osteoporosis.

Fig. 1

Finite-element models for six subjects: a typical young and old woman and man, as well as the strongest and weakest women in the cohort. The images show local regions of high (red) and low (blue) strength bone. The bone is virtually loaded in a typical sideways fall configuration through the virtual PMMA plates (colored orange) shown at the head and greater trochanter.

Fig. 2

Mean (±95% CI) values of femoral strength (A) and femoral neck (FN) aBMD (B) by decade of age for Rochester, MN, women and men. Data for subjects in the 20- to 39-year age range and over age 80 were pooled to account for the smaller sample size in those groups (see Table 1 for sample sizes). Linear regression analysis of these mean data over the range of the best-fit lines was used to estimate age dependent rates of loss (for women over age 45 years and men over age 55 years).

Fig. 3

Comparison with published manufacturer/reference values(24) for femoral neck (FN) aBMD and FN aBMD as measured in this study for the Rochester cohort (mean ± 95% CI). The Hologic data, derived from NHANES III,(25) were converted to Lunar-equivalent (L-equiv) values using the following equation: Lunar = 0.142 + 1.013 × Hologic.(24) Trends lines are shown for the Hologic and Lunar data sets. For both sexes, both manufacturer data sets fall within the 95% CI of the Rochester figures.

Fig. 4

Estimated annualized percent change in femoral strength and femoral neck aBMD for Rochester, MN, women and men, as calculated from linear regression analysis of the data shown in Fig. 2.

Fig. 5

Age-specific prevalence of “low femoral strength” (femoral strength < 3000 N) and osteoporosis (femoral neck aBMD T-score ≤ –2.5) for Rochester, MN, women and men.