The biomechanical basis of vertebral body fragility in men and women

The aim of this study was to quantify the biomechanical basis for vertebral fracture risk in elderly men and women. A bone is likely to fracture when the loads imposed are similar to or greater than its strength. To quantify this risk, we developed a fracture risk index (FRI) based on the ratio of t he vertebral body compressive load and strength. Loads were determined by u pper body weight, height, and the muscle moment arm, and strength was estim ated from cross-sectional area (CSA) and volumetric bone mineral density (v BMD). With loads less than the strength of the bone, the FRI remains < 1. F or any given load, once bone strength diminishes due to a failing vBMD, the FRI will increase. Should FRI approach or exceed unity, structural failure of the vertebra is likely. We measured vertebral body CSA vBMD of the midd le zone of third lumbar vertebra by lateral and posteroanterior (PA) scanni ng using dual-energy X-ray absorptiometry (DXA) and calculated vertebral co mpressive stress (load per unit area) in 327 healthy men and 686 healthy wo men and 26 men and 55 postmenopausal women with vertebral fractures. Activi ties that require forward bending of the upper body caused similar to 10-fo ld more compressive stress on the vertebra compared with standing upright. Men and women had similar peak vBMD in young adulthood. Because men have gr eater stature than women, the loads imposed on the vertebral body are highe r (3754 +/- 65 N vs. 3051 +/- 31 N; p < 0.001). However, because CSA also w as higher in men than women, peak load per unit CSA (stress) did not differ by gender (317.4 +/- 4.7 N/cm(2) vs. 321.9 +/- 3.3 N/cM(2), NS). The FRI w as similar in young men and women and well below unity (0.42 +/- 0.02 vs. 0 .43 +/- 0.01; NS). Gender differences emerged during aging; CSA increased i n both men and women but more so in men, so load per unit area (stress) dim inished but more so in men than in women. vBMD decreased in both genders bu t less so in men. These changes were captured in the FRI, which increased b y only 21 % in men and by 102 % in women so that only 9 % of elderly men bu t 26 % of elderly women had an FRI greater than or equal to 1. Men and wome n with vertebral fractures had an FRI that was greater than or equal to uni ty (1.03 +/- 0.13 vs. 1.35 +/- 0.13; p < 0.05) and was 2.04 SD and 2.26 SD higher than age-matched men and women, respectively. In summary and conclus ion, young men and women have a similar vBMD, vertebral stress, and FRI. Du ring aging, CSA increases more, and vBMD decreases less in men than in wome n. Thus, fewer men than women are at risk for fracture because fewer men th an women have these structural determinants of bone strength below a level at which the loads exceed the bone's ability to tolerate them. Men and wome n with vertebral fractures have FRIs that are equal to or exceed unity. The results show that a fracture threshold for vertebrae can be defined using established biomechanical principles; whether this approach has greater sen sitivity and specificity than the current BMD T score of -2.5 SD is unknown .