The effect of trabecular structure on DXA-based predictions of bovine bone failure

The objective of this study was to determine the influence of structural organization on the relationship between clinically assessed bone mineral density (BMD) and strength of trabecular bone. Accurate assessment of bone strength or fracture risk is a critical need as the population ages and the incidence of fractures increases. While ex vivo tests have demonstrated that BMD measured with dual-energy X-ray absorptiometry (DXA) correlates with failure load of whole bones, it is also known that the strength of trabecular bone cannot be explained by density alone. Cylindrical cores of bovine trabecular bone, harvested in a variety of orientations, were scanned with DXA to determine density, assessed with micro-MRI (magnetic resonance imaging) to measure orientation, and then loaded them to failure in bending. Measures of trabecular architecture included the angle between the specimen longitudinal (Z) axis and the principal material axis, the maximum mean intercept length (MILmax), the mean intercept length in the specimen primary axis (MILz), the degree of anisotropy, and the ratio between MILz and the length of the principal material axis. MILz was strongly associated with failure stress (r2 = 0.85, P < 0.001). BMD was also moderately associated with failure stress (r2 = 0.44, P = 0.004). Using a stepwise linear multiple regression analysis, the strongest predictor of failure stress was a combination of BMD, angle, and MILz (R2 = 0.91). When only longitudinal specimens were analyzed, the strongest predictor of failure stress was a combination of BMD and angle (R2 = 0.95). Therefore, trabecular orientation plays a significant and important role in failure of trabecular bone. Accounting for this may improve the associations between DXA-based density measures and patient fractures.