Quantitative Computed Tomography-Based Finite Element Models of the Human Lumbar Vertebral Body: Effect of Element Size on Stiffness, Damage, and Fracture Strength Predictions

2003 ◽  
Vol 125 (4) ◽  
pp. 434-438 ◽  
Author(s):  
R. Paul Crawford ◽  
William S. Rosenberg ◽  
Tony M. Keaveny

This study investigated the numerical convergence characteristics of specimen-specific “voxel-based” finite element models of 14 excised human cadaveric lumbar vertebral bodies (age: 37–87; M=6, F=8) that were generated automatically from clinical-type CT scans. With eventual clinical applications in mind, the ability of the model stiffness to predict the experimentally measured compressive fracture strength of the vertebral bodies was also assessed. The stiffness of “low”-resolution models (3×3×3 mm element size) was on average only 4% greater p=0.03 than for “high”-resolution models (1×1×1.5 mm) despite interspecimen variations that varied over four-fold. Damage predictions using low- vs high-resolution models were significantly different p=0.01 at loads corresponding to an overall strain of 0.5%. Both the high r2=0.94 and low r2=0.92 resolution model stiffness values were highly correlated with the experimentally measured ultimate strength values. Because vertebral stiffness variations in the population are much greater than those that arise from differences in voxel size, these results indicate that imaging resolution is not critical in cross-sectional studies of this parameter. However, longitudinal studies that seek to track more subtle changes in stiffness over time should account for the small but highly significant effects of voxel size. These results also demonstrate that an automated voxel-based finite element modeling technique may provide an excellent noninvasive assessment of vertebral strength.

1999 ◽  
Vol 121 (6) ◽  
pp. 629-635 ◽  
Author(s):  
G. L. Niebur ◽  
J. C. Yuen ◽  
A. C. Hsia ◽  
T. M. Keaveny

The convergence behavior of finite element models depends on the size of elements used, the element polynomial order, and on the complexity of the applied loads. For high-resolution models of trabecular bone, changes in architecture and density may also be important. The goal of this study was to investigate the influence of these factors on the convergence behavior of high-resolution models of trabecular bone. Two human vertebral and two bovine tibial trabecular bone specimens were modeled at four resolutions ranging from 20 to 80 μm and subjected to both compressive and shear loading. Results indicated that convergence behavior depended on both loading mode (axial versus shear) and volume fraction of the specimen. Compared to the 20 μm resolution, the differences in apparent Young’s modulus at 40 μm resolution were less than 5 percent for all specimens, and for apparent shear modulus were less than 7 percent. By contrast, differences at 80 μm resolution in apparent modulus were up to 41 percent, depending on the specimen tested and loading mode. Overall, differences in apparent properties were always less than 10 percent when the ratio of mean trabecular thickness to element size was greater than four. Use of higher order elements did not improve the results. Tissue level parameters such as maximum principal strain did not converge. Tissue level strains converged when considered relative to a threshold value, but only if the strains were evaluated at Gauss points rather than element centroids. These findings indicate that good convergence can be obtained with this modeling technique, although element size should be chosen based on factors such as loading mode, mean trabecular thickness, and the particular output parameter of interest.


Author(s):  
Yener N. Yeni ◽  
Do-Gyoon Kim ◽  
Roger R. Zauel ◽  
Evan M. Johnson ◽  
Dianna D. Cody

Vertebral fractures are among the most common and debilitating fractures. Structural organization of cancellous and cortical bone in a vertebra and their local properties are important factors that determine the strength of a vertebra. Linear finite element models utilizing Quantitative Computed Tomography (QCT) images have proven useful for predicting vertebral strength and are potentially useful in predicting risk of fracture in a clinical setting [1]. However, the amount of architectural detail in these models is not sufficient for studying trabecular stress and strains, and their relationship with the microscopic structure, which is important for understanding the mechanisms behind vertebral fragility.


2004 ◽  
Vol 126 (1) ◽  
pp. 122-125 ◽  
Author(s):  
Xiang Wang, ◽  
Xiangyi Liu, and ◽  
Glen L. Niebur

The Orientation of trabecular bone specimens for mechanical testing must be carefully controlled. A method for accurately preparing on-axis cylindrical specimens using high-resolution micro-CT imaging was developed. Sixteen cylindrical specimens were prepared from eight bovine tibiae. High-resolution finite element models were generated from micro-CT images of parallelepipeds and used to determine the principal material coordinate system of each parallelepiped. A cylindrical specimen was then machined with a diamond coring bit. The resulting specimens were scanned again to evaluate the orientation. The average deviation between the principal fabric orientation and the longitudinal axis of the cylindrical specimen was only 4.70±3.11°.


2000 ◽  
Vol 33 (12) ◽  
pp. 1575-1583 ◽  
Author(s):  
Glen L Niebur ◽  
Michael J Feldstein ◽  
Jonathan C Yuen ◽  
Tony J Chen ◽  
Tony M Keaveny

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