Effect of Specimen Specific Anisotropic Material Properties in Quantitative Computed Tomography Based Finite Element Analysis of the Vertebra

Abstract

Intraand interspecimen variations in trabecular anisotropy are often ignored in quantitative computed tomography (QCT)based finite element (FE) models of the vertebra. The material properties are typically estimated solely from local variations in bone mineral density (BMD), and a fixed representation of elastic anisotropy (“generic anisotropyâ€‌) is assumed. This study evaluated the effect of incorporating specimenspecific, trabecular anisotropy on QCTbased FE predictions of vertebral stiffness and deformation patterns. Orthotropic material properties estimated from microcomputed tomography data (“specimenspecific anisotropyâ€‌), were assigned to a large, columnar region of the L1 centrum (n = 12), and genericanisotropic material properties were assigned to the remainder of the vertebral body. Results were compared to FE analyses in which genericanisotropic properties were used throughout. FE analyses were also performed on only the columnar regions. For the columnar regions, the axial stiffnesses obtained from the two categories of material properties were uncorrelated with each other (p = 0.604), and the distributions of minimum principal strain were distinctly different (p ≤ 0.022). In contrast, for the whole vertebral bodies in both axial and flexural loading, the stiffnesses obtained using the two categories of material properties were highly correlated (R2 > 0.82, p < 0.001) with, and were no different (p > 0.359) from, each other. Only moderate variations in strain distributions were observed between the two categories of material properties. The contrasting results for the columns versus vertebrae indicate a large contribution of the peripheral regions of the vertebral body to the mechanical behavior of this bone. In companion analyses on the effect of the degree of anisotropy (DA), the axial stiffnesses of the trabecular column (p < 0.001) and vertebra (p = 0.007) increased with increasing DA. These findings demonstrate the need for accurate modeling of the peripheral regions of the vertebral body in analyses of the mechanical behavior of the vertebra.