If bone adapts to maintain constant strains and if on-axis yield strains in trabecular bone are independent of apparent density, adaptive remodeling in trabecular bone should maintain a constant safety factor (yield strain/functional strain) during habitual loading. To test the hypothesis that yield strains are indeed independent of density, compressive (n=22) and tensile (n=22) yield strains were measured without end-artifacts for low density (0.18±0.04 g cm-3) human vertebral trabecular bone specimens. Loads were applied in the superior–inferior direction along the principal trabecular orientation. These ‘on-axis’ yield strains were compared to those measured previously for high-density (0.51±0.06 g cm-3) bovine tibial trabecular bone (n=44). Mean (±S.D.) yield strains for the human bone were 0.78±0.04% in tension and 0.84±0.06% in compression; corresponding values for the bovine bone were 0.78±0.04 and 1.09±0.12%, respectively. Tensile yield strains were independent of the apparent density across the entire density range (human p=0.40, bovine p=0.64, pooled p=0.97). By contrast, compressive yield strains were linearly correlated with apparent density for the human bone (p<0.001) and the pooled data (p<0.001), and a suggestive trend existed for the bovine data (p=0.06). These results refute the hypothesis that on-axis yield strains for trabecular bone are independent of density for compressive loading, although values may appear constant over a narrow density range. On-axis tensile yield strains appear to be independent of both apparent density and anatomic site.
Keywords: Trabecular bone; Yield strain; Safety factor; Remodeling; Spine