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Research Papers

Finite Element Lumbar Spine Facet Contact Parameter Predictions are Affected by the Cartilage Thickness Distribution and Initial Joint Gap Size

[+] Author and Article Information
Daniel J. Woldtvedt, Wesley Womack, Benjamin C. Gadomski, Dieter Schuldt

Orthopaedic Bioengineering Research Laboratory, Department of Mechanical Engineering and School of Biomedical Engineering,  Colorado State University, 1374 Campus Delivery, Fort Collins, CO 80523-1374

Christian M. Puttlitz1

Orthopaedic Bioengineering Research Laboratory, Department of Mechanical Engineering and School of Biomedical Engineering,  Colorado State University, 1374 Campus Delivery, Fort Collins, CO 80523-1374puttlitz@engr.colostate.edu

1

Corresponding author: Christian M. Puttlitz, Ph.D., Director, Orthopaedic Bioengineering Research Laboratory, Associate Professor, Department of Mechanical Engineering.

J Biomech Eng 133(6), 061009 (Jul 05, 2011) (7 pages) doi:10.1115/1.4004287 History: Received November 30, 2010; Revised May 11, 2011; Posted May 25, 2011; Published July 05, 2011; Online July 05, 2011

Current finite element modeling techniques utilize geometrically inaccurate cartilage distribution representations in the lumbar spine. We hypothesize that this shortcoming severely limits the predictive fidelity of these simulations. Specifically, it is unclear how these anatomically inaccurate cartilage representations alter range of motion and facet contact predictions. In the current study, cadaveric vertebrae were serially sectioned, and images were taken of each slice in order to identify the osteochondral interface and the articulating surface. A series of custom-written algorithms were utilized in order to quantify each facet joint’s three-dimensional cartilage distribution using a previously developed methodology. These vertebrae-dependent thickness cartilage distributions were implemented on an L1 through L5 lumbar spine finite element model. Moments were applied in three principal planes of motion, and range of motion and facet contact predictions from the variable thickness and constant thickness distribution models were determined. Initial facet gap thickness dimensions were also parameterized. The data indicate that the mean and maximum cartilage thickness increased inferiorly from L1 to L5, with an overall mean thickness value of 0.57 mm. Cartilage distribution and initial facet joint gap thickness had little influence on the lumbar range of motion in any direction, whereas the mean contact pressure, total contact force, and total contact area predictions were altered considerably. The data indicate that range of motion predictions alone are insufficient to establish model validation intended to predict mechanical contact parameters. These data also emphasize the need for the careful consideration of the initial facet joint gap thickness with respect to the spinal condition being studied.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 3

Mean contact pressure predictions. The constant thickness cartilage model variant was not as sensitive to changes in the facet joint gap width as compared to the variable thickness cartilage representation.

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Figure 4

Total facet contact force results. In general, smaller initial facet gaps resulted in higher predicted contact forces.

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Figure 5

Total facet contact area predictions. The data mirror the contact force predictions; facet gaps resulted in higher contact areas.

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Figure 2

Range of motion predictions were minimally affected by differences in the facet cartilage thickness distribution and gap width

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Figure 1

Cross-sectional view of the finite element cartilage representations at the L4–L5 level. The constant thickness model (left) was created by extruding 1.25 mm of cartilage from the subchondral surfaces. The variable thickness model (right) utilized a mathematical fit function to describe the shape of the cartilage layer on each facet surface.

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