Determinants and heterogeneity of mechanical competence throughout the thoracolumbar spine of elderly women and men.

Vertebral fractures represent the hallmark of osteoporosis. Here, we test the hypotheses that (sub)cortical bone strength and density predict failure better than trabecular core strength and density, and that elderly women display lower failure stress of thoracic vertebrae than men. We examined the vertebral bodies T3 to L5 in 39 spines from elderly donors (23 women; 16 men; age 79 +/- 11 years). Peripheral quantitative computed tomography was used to measure total, trabecular, and (sub)cortical bone density. Mechanical tests were performed in functional spinal units, planoparallel sections of vertebrae, trabecular cores, and (sub)cortical ring specimens. The failure stress decreased with descending vertebral level. Failure stress was highest for the (sub)cortical rings and planoparallel sections and lowest for the trabecular core. The failure stress did not differ significantly between men and women. Mechanical strength of the functional unit was more strongly correlated with the strength of the (sub)cortical ring (r = 0.78) than with that of the trabecular core (r = 0.62). However, total density was more highly correlated with mechanical strength of the same and remote vertebrae (r = 0.63) than trabecular (r = 0.50) or (sub)cortical density (r = 0.36), respectively. The results show that vertebral strength is similar in elderly women and men. Strength of (sub)cortical bone provides significantly better prediction of strength of functional spinal units than that of the trabecular core. However, total density predicts functional segment failure stress with higher accuracy than (sub)cortical or trabecular density and is thus recommended for predicting fracture strength clinically.