Comparison of Vertebral Strengths Derived from FE models of the Vertebral Body and the Vertebral Body with Posterior Elements

Abstract

Vertebral fracture is the most common type of osteoporotic fracture and is associated with significant health and economic costs. In recent years, patient-specific vertebral body finite element (FE) models based on quantitative computed tomography (QCT) have been developed to investigate vertebral strength. Current models consider the vertebral body alone with loading restricted to pure compression. However, osteoporotic vertebral fractures are often associated with forward bending which concentrates loading on the anterior vertebral body leading to the typical wedge fractures observed in life. The purpose of this study is to compare the vertebral strengths derived from FE models with those measured experimentally under forward bending conditions. Sixteen cadaveric spinal units, each consisting of two vertebrae and an intervertebral disc (IVD), were scanned using clinical QCT. Specimens were then potted in dental cement and loaded on a materials testing machine to generate a wedge fracture. Custom codes developed and validated previously in MATLAB were used to generate FE models with an 8-node voxel mesh: one of the vertebral body alone (”VB”), and another of the vertebral body with posterior elements intact (”VB w/PE”). Artificial padding was generated at the inferior and superior endplates to represent the dental cement and facilitate the simulation of the forward-bending condition. Transverseisotropic linear-elastic material properties were assigned to the vertebra, and the FE model was solved using ANSYS. Vertebral strength was defined in two different ways; i) the load that caused maximum Von Mises strain exceeding a yield strain of 0.78% (VM_ER), and ii) the load that caused maximum Von Mises stress exceeding a yield stress (VM_SR) in contiguous elements that occupied at least 244.14 mm3. Linear regression was used to compare FE- and experiment-derived vertebral strength data. This showed that there was a positive correlation between the strength measured experimentally and the strength derived from both FE models; VM_SR (r2= 0.78 VB, r2= 0.75 VB w/PE), VM_ER (r2= 0.75 VB, r2= 0.68 VB w/PE). The results of this study indicate that a simple vertebral body FE model under forward-bending conditions may be sufficient for the estimation of vertebral strength.