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Research Papers

Cartilage Thickness Distribution Affects Computational Model Predictions of Cervical Spine Facet Contact Parameters

[+] Author and Article Information
Wesley Womack, Ugur M. Ayturk

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

Christian M. Puttlitz1

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

1

Corresponding author.

J Biomech Eng 133(1), 011009 (Dec 23, 2010) (10 pages) doi:10.1115/1.4002855 History: Received May 06, 2010; Revised September 20, 2010; Posted October 25, 2010; Published December 23, 2010; Online December 23, 2010

With motion-sparing disk replacement implants gaining popularity as an alternative to anterior cervical discectomy and fusion (ACDF) for the treatment of certain spinal degenerative disorders, recent laboratory investigations have studied the effects of disk replacement and implant design on spinal kinematics and kinetics. Particularly relevant to cervical disk replacement implant design are any postoperative changes in solid stresses or contact conditions in the articular cartilage of the posterior facets, which are hypothesized to lead to adjacent-level degeneration. Such changes are commonly investigated using finite element methods, but significant simplification of the articular geometry is generally employed. The impact of such geometric representations has not been thoroughly investigated. In order to assess the effects of different models of cartilage geometry on load transfer and contact pressures in the lower cervical spine, a finite element model was generated using cadaver-based computed tomography imagery. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include four different geometric representations of the articular cartilage. Model predictions indicate that the two most common representations of articular cartilage geometry result in significant reductions in the predictive accuracy of the models. The two anatomically based geometric models exhibited less computational artifact, and relatively minor differences between them indicate that contact condition predictions of spatially varying thickness models are robust to anatomic variations in cartilage thickness and articular curvature. The results of this work indicate that finite element modeling efforts in the lower cervical spine should include anatomically based and spatially varying articular cartilage thickness models. Failure to do so may result in loss of fidelity of model predictions relevant to investigations of physiological import.

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Figures

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Figure 1

Cartilage thickness has little effect on ROM in the three principal spinal rotation directions

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Figure 2

Total contact force was nominally affected by cartilage representation for the three principal spinal rotation directions. A reduced gap for the flat model results in slightly higher forces.

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Figure 3

Total facet contact area for the different model variants. The flat model greatly overpredicted the contact area as compared with the other models. The experimental mean (Tekscan) ±1 standard deviation (shaded area) are shown for reference.

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Figure 4

Contact pressure distribution at the left C6/C7 facet articulation under lateral bending loading at 2 N m. Peak pressures and contact areas are exaggerated and underrepresented for the flat and constant thickness models, respectively.

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Figure 5

Mean contact pressure is generally higher for the constant thickness model and lower for the flat model in the three principal spinal rotation directions

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Figure 6

Peak facet contact pressure for the four model variants. These data represent the third quartile pressure for nodes exceeding 0.01 MPa.

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Figure 7

Center of pressure tracks—flexion/extension. (a) Baseline, (b) k=0.75, (c) flat, and (d) constant thickness. Range for constant thickness is −0.8 to +2 N m. Bubble size represents the relative contact force, and the variation of force with respect to the applied moment is indicated in Fig. 2. Coordinates are normalized to (±5,0) at the neutral position in the baseline model with respect to the caudal vertebrae. Averages for left and right are given, with the extension limit indicated by the gray circles. Neutral position centers of pressure are marked (gray circles).

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Figure 8

Center of pressure tracks—lateral bending. (a) Baseline, (b) k=0.75, (c) flat, and (d) constant thickness. Bubble size represents the relative contact force, and the variation of force with respect to the applied moment is indicated in Fig. 2. Coordinates are normalized to (±5,0) at the neutral position in the baseline model with respect to the caudal vertebrae. Averages for left and right are given with the right lateral bending limit indicated by the gray circles. Neutral position centers of pressure are marked (gray circles).

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Figure 9

Center of pressure tracks—axial torsion. (a) Baseline, (b) k=0.75, (c) flat, and (d) Constant thickness. Bubble size represents the relative contact force, and the variation of force with respect to the applied moment is indicated in Fig. 2. Coordinates are normalized to (±5, 0) at the neutral position in the baseline model with respect to the caudal vertebrae. Averages for left and right are given with the right axial torsion limit marked (gray circles). Neutral position centers of pressure are indicated by the gray circles. Constant thickness data are for 0 to +0.3 N m.

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Figure 10

Minimum principal strain at the C6/C7 left articulation reveals the effects of cartilage representation on joint congruence and contact pressures. Data are shown for lateral bending loading at 2 N m.

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Figure 11

The distribution of thickness as a function of radius (rratio=r/rp) (left) and the resultant mapping in three dimensions (right). Color surface and dots represent measured data and the black grid represents the analytical thickness fitting function (reproduced with permission from Elsevier).

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