Relationship between axial and bending behaviors of the human thoracolumbar vertebra

Study Design. The authors studied the mechanical behavior of vertebrae through the use of finite element analyses. Objectives. To determine the relation between axial and bending rigidity, and to determine the geometric and densitometric factors that affect this relation. Summary of Background Data. Metrics of vertebral body mechanical properties in bending have not been established despite evidence that anterior bending loads play a significant role in osteoporotic vertebral fracture. Methods. Voxel-based finite element models were generated using quantitative computed tomography (QCT) scans of 18 human cadaveric vertebral bodies, and both axial and bending rigidities of the vertebra were computed. Both rigidity measures and their ratio were correlated with vertebral geometric and densitometric factors obtained from the QCT scans. Results. Bending rigidity was moderately correlated with axial rigidity (r(2)=0.69) and strongly correlated with the product of axial rigidity and vertebral anteroposterior depth squared (r(2)=0.88). The ratio of bending to axial rigidity was independent of bone mineral density (P=0.20) but was moderately correlated with the square of vertebral depth (r(2)=0.69). Conclusions. Vertebral anteroposterior depth plays an important role in bending rigidity. The scatter in the correlation between bending and axial rigidity suggests that some individuals can have vertebrae with a normal axial stiffness but an abnormally low bending stiffness. Because whole-bone stiffness is indicative of bone strength, these results support the concept that use of more than one metric of vertebral strength, for example, compression and bending strengths, may improve osteoporotic fracture risk prediction.