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TECHNICAL BRIEFS

Prediction of Femoral Head Collapse in Osteonecrosis

[+] Author and Article Information
K. Y. Volokh1

Department of Orthopedic Surgery, Johns Hopkins University, Baltimore, MD

H. Yoshida

Department of Orthopedic Surgery, Johns Hopkins University, Baltimore, MD

A. Leali, J. F. Fetto

Department of Orthopaedic Surgery, New York University, New York, NY

E. Y. Chao2

Department of Orthopedic Surgery, Johns Hopkins University, Baltimore, MDeyschao@yahoo.com

It should be mentioned that the use of the shell elements for the cortical bone modeling is not mandatory. Brick elements can also be used for the cortical shell; however, one layer of the brick elements will not be enough for estimating the critical buckling pressure of the femoral head under various necrotic conditions.

In the case where there is no lesion in the cortical shell.

1

On leave of absence from the Technion-Israel Institute of Technology, Israel.

2

Corresponding author. Present address: EYS Chao, The Ross Research Building, 720 Rutland Avenue, Room 235, Baltimore, MD 21205.

J Biomech Eng 128(3), 467-470 (Dec 05, 2005) (4 pages) doi:10.1115/1.2187050 History: Received July 18, 2005; Revised December 05, 2005

The femoral head deteriorates in osteonecrosis. As a consequence of that, the cortical shell of the femoral head can buckle into the cancellous bone supporting it. In order to examine the buckling scenario we performed numerical analysis of a realistic femoral head model. The analysis included a solution of the hip contact problem, which provided the contact pressure distribution, and subsequent buckling simulation based on the given contact pressure. The contact problem was solved iteratively by approximating the cartilage by a discrete set of unilateral linear springs. The buckling calculations were based on a finite element mesh with brick elements for the cancellous bone and shell elements for the cortical shell. Results of 144 simulations for a variety of geometrical, material, and loading parameters strengthen the buckling scenario. They, particularly, show that the normal cancellous bone serves as a strong supporting foundation for the cortical shell and prevents it from buckling. However, under the development of osteonecrosis the deteriorating cancellous bone is unable to prevent the cortical shell from buckling and the critical pressure decreases with the decreasing Young modulus of the cancellous bone. The local buckling of the cortical shell seems to be the driving force of the progressive fracturing of the femoral head leading to its entire collapse. The buckling analysis provides an additional criterion of the femoral head collapse, the critical contact pressure. The buckling scenario also suggests a new argument in speculating on the femoral head reinforcement. If the entire collapse of the femoral head starts with the buckling of the cortical shell then it is reasonable to place the reinforcement as close to the cortical shell as possible.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 3

Left: A typical buckling mode in the form of the surface cavity. ABAQUS simulation with 6000 continuum brick elements for the cancellous bone and 1000 thin shell elements for the cortical bone. The nodal points of the contacting brick and shell elements were properly adjusted not allowing for the displacement jump across the contact surface. Right: Sample femoral head after collapse.

Grahic Jump Location
Figure 2

Left: The intact femoral head model used in the finite element analysis. The head is fixed at the plane separating it from the femoral neck. Right: The femoral head with the necrotic zone in the form of a cone with the base angle of 2π∕3rad. The necrotic zone has a lower value of the Young modulus than the surrounding intact cancellous bone.

Grahic Jump Location
Figure 1

Left: Discrete element mesh including 4000 unilateral linear springs, which mimic the cartilage, for the contact pressure derivation. Right: A schematic distribution of the contact pressure derived from the discrete element analysis. This pressure distribution is the initial load for the subsequent eigenvalue buckling analysis with the finite element method.

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