Cortical and Trabecular Load Sharing in the Human Vertebral Body

The biomechanical role of the vertebral cortical shell remains poorly understood. Using high-resolution finite element modeling of a cohort of elderly vertebrae, we found that the biomechanical role of the shell can be substantial and that the load sharing between the cortical and trabecular bone is complex. As a result, a more integrative measure of the trabecular and cortical bone should improve noninvasive assessment of fracture risk and treatments. A fundamental but poorly understood issue in the assessment of both osteoporotic vertebral fracture risk and effects of treatment is the role of the trabecular bone and cortical shell in the load-carrying capacity of the vertebral body. High-resolution ( mu CT-based finite element models were developed for 13 elderly human vertebrae (age range: 54-87 years; 74.6 plus or minus 9.4 years), and parameter studies - with and without endplates - were performed to determine the role of the shell versus trabecular bone and the effect of model assumptions. Across vertebrae, whereas the average thickness of the cortical shell was only 0.38 plus or minus 0.06 mm, the shell mass fraction (shell mass/total bone mass) - not including the endplates - ranged from 0.21 to 0.39. The maximum load fraction taken by the shell varied from 0.38 to 0.54 across vertebrae and occurred at the narrowest section. The maximum load fraction taken by the trabecular bone varied from 0.76 to 0.89 across vertebrae and occurred near the endplates. Neither the maximum shell load fraction nor the maximum trabecular load fraction depended on any of the densitometric or morphologic properties of the vertebra, indicating the complex nature of the load sharing mechanism. The variation of the shell load-carrying capacity across vertebrae was significantly altered by the removal of endplates, although these models captured the overall trend within a vertebra. The biomechanical role of the thin cortical shell in the vertebral body can be substantial, being about 45% at the midtransverse section but as low as 15% close to the endplates. As a result of the complexity of load sharing, sampling of only midsection trabecular bone as a strength surrogate misses important bio-mechanical information. A more integrative approach that combines the structural role of both cortical and trabecular bone should improve noninvasive assessment of vertebral bone strength in vivo.