A patient-specific finite element methodology to predict damage accumulation in vertebral bodies under axial compression, sagittal flexion and combined loads.
ABSTRACT Due to the inherent limitations of DXA, assessment of the biomechanical properties of vertebral bodies relies increasingly on CT-based finite element (FE) models, but these often use simplistic material behaviour and/or single loading cases. In this study, we applied a novel constitutive law for bone elasticity, plasticity and damage to FE models created from coarsened pQCT images of human vertebrae, and compared vertebral stiffness, strength and damage accumulation for axial compression, anterior flexion and a combination of these two cases. FE axial stiffness and strength correlated with experiments and were linearly related to flexion properties. In all loading modes, damage localised preferentially in the trabecular compartment. Damage for the combined loading was higher than cumulated damage produced by individual compression and flexion. In conclusion, this FE method predicts stiffness and strength of vertebral bodies from CT images with clinical resolution and provides insight into damage accumulation in various loading modes.
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ABSTRACT: Vertebral fractures may result in pain, loss of height, spinal instability, kyphotic deformity and ultimately increased morbidity. Fracture risk can be estimated by vertebral bone mineral density (BMD). However, vertebral fractures may be better defined by more selective methods that account for micro-architecture. Our aim was to quantify regional variations in bone architecture parameters (BAPs) and to assess the degree with which regional variations in BAPs affect vertebral fracture strength. The influence of disc health and endplate thickness on fracture strength was also determined. The soft tissue and posterior elements of 20 human functional spine units (FSU) were removed (T9 to L5, mean 74.45+/-4.25 years). After micro-CT scanning of the entire FSU, the strength of the specimens was determined using a materials testing system. Specimens were loaded in compression to failure. BAPs were assessed for 10 regions of the vertebral cancellous bone. Disc health (glycosaminoglycan content of the nucleus pulposus) was determined using the degree of binding with Alcian Blue. Vertebrae were not morphologically homogeneous. Posterior regions of the vertebrae had greater bone volume, more connections, reduced trabecular separation and more plate-like isotropic structures than their corresponding anterior regions. Significant heterogeneity also exists between posterior superior and inferior regions (BV/TV: posterior superior 12.6+/-2.8%, inferior 14.6+/-3%; anterior superior 10.5+/-2.2%, inferior 10.7+/-2.4%). Of the two endplates that abutted a common disc, the cranial inferior endplate was thicker (0.44+/-0.15 mm) than the caudal superior endplate (0.37+/-0.13 mm). Our study found good correlations between BV/TV, connective density and yield strength. Fracture risk prediction, using BV/TV multiplied by the cross sectional area of the endplate, can be improved through regional analysis of the underlying cancellous bone of the endplate of interest (R(2) 0.78) rather than analysis of the entire vertebra (R(2) 0.65) or BMD (R(2) 0.47). Degenerated discs lack a defined nucleus. A negative linear relationship between disc health and vertebral strength (R(2) 0.70) was observed, likely due to a shift in loading from the weaker anterior vertebral region to the stronger posterior region and cortical shell. Our results show the importance of considering regional variations in cancellous BAPs and disc health when assessing fracture risk.Bone 01/2008; 41(6):946-57. · 3.82 Impact Factor
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ABSTRACT: Biomechanical properties within cadaveric vertebral bodies were parametrically studied using finite element analysis after calibration to experimental data. To develop and validate three-dimensional finite element models of the human thoracolumbar spine based on quantitative computed tomography scans. Specifically, combine finite element modeling together with biomechanical testing circumventing problems associated with direct measurements of shell properties. Finite element methods can help to understand injury mechanisms and stress distribution patterns within vertebral bodies as an important part in clinical evaluation of spinal injuries. Because of complications in modeling the vertebral shell, it is not clear if quantitative computed tomography-based finite element models of the spine could accurately predict biomechanical properties. We developed a novel finite element modeling technique based on quantitative computed tomography scans of 19 radiographically normal human vertebra bodies and mechanical property data from empirical studies on cylindrical trabecular bone specimens. Structural properties of the vertebral shell were recognized as parametric variables and were calibrated to provide agreement in whole vertebral body stiffness between model and experiment. The mean value of the shell properties thus obtained was used in all models to provide predictions of whole vertebral strength and stiffness. Calibration of n = 19 computer models to experimental stiffness yielded a mean effective modulus of the vertebral shell of 457 +/- 931 MPa ranging from 9 to 3216 MPa. No significant correlation was found between vertebral shell effective modulus and either the experimentally measured stiffness or the average trabecular modulus. Using the effective vertebral shell modulus for all 19 models, the predicted vertebral body stiffness was an excellent predictor of experimental measurements of both stiffness (r2= 0.81) and strength (r2 = 0.79). These findings indicate that modeling of the vertebral shell using a constant thickness of 0.35 mm and an effective modulus of 457 MPa, combined with quantitative computed tomography-based modeling of trabecular properties and vertebral geometry, can accurately predict whole vertebral biomechanical properties. Use of this modeling technique, therefore, should produce substantial insight into vertebral body biomechanical behavior and may ultimately improve clinical indications of fracture risk of this cohort.Spine 04/2003; 28(6):559-65. · 2.16 Impact Factor
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ABSTRACT: Although the trabecular bone of the human vertebral body has been well characterized, the thin "cortical" shell and endplate that surround the trabecular centrum have not. In addition, the accuracy of estimating the thickness of the shell and endplate using computed tomography (CT) has not been evaluated directly. To address these issues, we measured the thickness of the vertebral shell and endplate in the mid sagittal plane of 16 human L1 vertebral bodies using direct and CT based methods. Specimens were assigned to four equal sized groups based on age (middle-aged, mean age = 49 years; old, mean age = 84) and gender. We investigated the dependence of the shell and endplate thicknesses on age, gender, and anatomic region. Our findings indicate that the shell and endplate in vertebrae over age 45 are porous and often irregular, with an average thickness of approximately 0.35 mm. However, when measured from CT images, the vertebral shell and endplate appear significantly thicker, indicating that measurements based on clinical CT scans overestimate the thickness by a factor of at least two. In addition, our data indicated that, in the midsagittal plane, the anterior shell is thicker than the posterior shell or either endplate. Although these data indicated that thickness did not depend on age or gender, these particular findings are inconclusive given the small and heterogeneous sample we examined. We conclude that the so-called cortical shell and endplate of the vertebral body are thin (less than one-half of a millimeter) and porous, and perhaps are better thought of as thin membranes of fused trabeculae than as true cortices.Bone 01/1994; 15(4):409-14. · 3.82 Impact Factor