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TECHNICAL PAPERS: Bone/Orthopedic

The Biomechanics of Human Femurs in Axial and Torsional Loading: Comparison of Finite Element Analysis, Human Cadaveric Femurs, and Synthetic Femurs

[+] Author and Article Information
M. Papini1

Department of Mechanical and Industrial Engineering, Ryerson University, 350 Victoria Street, Toronto, Ontario, Canada M5B 2K3mpapini@ryerson.ca

R. Zdero, E. H. Schemitsch

Martin Orthopedic Biomechanics Laboratory, St. Michael’s Hospital, 30 Bond Street, West Annex, Toronto, Ontario, Canada M5B 1W8

P. Zalzal

Department of Surgery, Faculty of Health Sciences, McMaster University, 1200 Main Street West, Hamilton, Ontario, Canada L8S 4L8

1

Corresponding author.

J Biomech Eng 129(1), 12-19 (Jun 25, 2006) (8 pages) doi:10.1115/1.2401178 History: Received June 27, 2005; Revised June 25, 2006

To assess the performance of femoral orthopedic implants, they are often attached to cadaveric femurs, and biomechanical testing is performed. To identify areas of high stress, stress shielding, and to facilitate implant redesign, these tests are often accompanied by finite element (FE) models of the bone/implant system. However, cadaveric bone suffers from wide specimen to specimen variability both in terms of bone geometry and mechanical properties, making it virtually impossible for experimental results to be reproduced. An alternative approach is to utilize synthetic femurs of standardized geometry, having material behavior approximating that of human bone, but with very small specimen to specimen variability. This approach allows for repeatable experimental results and a standard geometry for use in accompanying FE models. While the synthetic bones appear to be of appropriate geometry to simulate bone mechanical behavior, it has not, however, been established what bone quality they most resemble, i.e., osteoporotic or osteopenic versus healthy bone. Furthermore, it is also of interest to determine whether FE models of synthetic bones, with appropriate adjustments in input material properties or geometric size, could be used to simulate the mechanical behavior of a wider range of bone quality and size. To shed light on these questions, the axial and torsional stiffness of cadaveric femurs were compared to those measured on synthetic femurs. A FE model, previously validated by the authors to represent the geometry of a synthetic femur, was then used with a range of input material properties and change in geometric size, to establish whether cadaveric results could be simulated. Axial and torsional stiffnesses and rigidities were measured for 25 human cadaveric femurs (simulating poor bone stock) and three synthetic “third generation composite” femurs (3GCF) (simulating normal healthy bone stock) in the midstance orientation. The measured results were compared, under identical loading conditions, to those predicted by a previously validated three-dimensional finite element model of the 3GCF at a variety of Young’s modulus values. A smaller FE model of the 3GCF was also created to examine the effects of a simple change in bone size. The 3GCF was found to be significantly stiffer (2.3 times in torsional loading, 1.7 times in axial loading) than the presently utilized cadaveric samples. Nevertheless, the FE model was able to successfully simulate both the behavior of the 3GCF, and a wide range of cadaveric bone data scatter by an appropriate adjustment of Young’s modulus or geometric size. The synthetic femur had a significantly higher stiffness than the cadaveric bone samples. The finite element model provided a good estimate of upper and lower bounds for the axial and torsional stiffness of human femurs because it was effective at reproducing the geometric properties of a femur. Cadaveric bone experiments can be used to calibrate FE models’ input material properties so that bones of varying quality can be simulated.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Experimental configuration for determination of axial and torsional stiffness of cadaveric and third generation composite femurs (3GCFs)

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Figure 2

Mesh of third generation composite femur (3GCF) showing configuration for measurement of torsional and axial stiffnesses

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Figure 3

Axial versus torsional rigidity: ⟨◆⟩ measured cadaver results; (▵) predicted large femur FE results at value of Young’s modulus indicated in parenthesis; (◻) predicted small femur FE results at value of Young’s modulus indicated in parenthesis; and (×) measured third generation composite femur results. Solid line is linear fit of cadaver results (R2=0.65). Dashed lines are a linear fit of FE results (R2=0.99).

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