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Research Papers

Interbody Spacer Material Properties and Design Conformity for Reducing Subsidence During Lumbar Interbody Fusion

[+] Author and Article Information
Lillian S. Chatham, Christopher M. Yakacki, R. Dana Carpenter

Department of Mechanical Engineering,
University of Colorado Denver,
Denver, CO 80204

Vikas V. Patel

Department of Orthopaedic Surgery,
University of Colorado Denver,
Anschutz Medical Campus,
Aurora, CO 80045

Manuscript received October 5, 2016; final manuscript received March 14, 2017; published online April 5, 2017. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 139(5), 051005 (Apr 05, 2017) (8 pages) Paper No: BIO-16-1396; doi: 10.1115/1.4036312 History: Received October 05, 2016; Revised March 14, 2017

There is a need to better understand the effects of intervertebral spacer material and design on the stress distribution in vertebral bodies and endplates to help reduce complications such as subsidence and improve outcomes following lumbar interbody fusion. The main objective of this study was to investigate the effects of spacer material on the stress and strain in the lumbar spine after interbody fusion with posterior instrumentation. A standard spacer was also compared with a custom-fit spacer, which conformed to the vertebral endplates, to determine if a custom fit would reduce stress on the endplates. A finite element (FE) model of the L4–L5 motion segment was developed from computed tomography (CT) images of a cadaveric lumbar spine. An interbody spacer, pedicle screws, and posterior rods were incorporated into the image-based model. The model was loaded in axial compression, and strain and stress were determined in the vertebra, spacer, and rods. Polyetheretherketone (PEEK), titanium, poly(para-phenylene) (PPP), and porous PPP (70% by volume) were used as the spacer material to quantify the effects on stress and strain in the system. Experimental testing of a cadaveric specimen was used to validate the model's results. There were no large differences in stress levels (<3%) at the bone–spacer interfaces and the rods when PEEK was used instead of titanium. Use of the porous PPP spacer produced an 8–15% decrease of stress at the bone–spacer interfaces and posterior rods. The custom-shaped spacer significantly decreased (>37%) the stress at the bone–spacer interfaces for all materials tested. A 28% decrease in stress was found in the posterior rods with the custom spacer. Of all the spacer materials tested with the custom spacer design, 70% porous PPP resulted in the lowest stress at the bone–spacer interfaces. The results show the potential for more compliant materials to reduce stress on the vertebral endplates postsurgery. The custom spacer provided a greater contact area between the spacer and bone, which distributed the stress more evenly, highlighting a possible strategy to decrease the risk of subsidence.

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Figures

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Fig. 1

Meshed model with CAD-developed spacer, posterior instrumentation (pedicle screws and rods), and urethane loading blocks

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Fig. 2

Comparison of standard spacer and custom fit spacer in the model

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Fig. 3

Experimental setup. Compressive loads were applied to the L4–L5 unit via compression platens with the proximal and distal ends potted in urethane blocks. Strains were measured during loading using a strain recorder and stored on a PC for analysis.

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Fig. 4

Locations of strain gages used in experimental tests. Anterior view of L4–L5 unit with locations of strain rosettes at the spacer and L4 vertebral body (left). Posterior view of L4–L5 unit with location of uniaxial strain gages at the rods (right).

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Fig. 5

Mesh convergence results for L4–L5 model stiffness. Differences in overall model stiffness converged to less than 2% at a mesh density of 93,790 nodes.

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Fig. 6

Box-and whisker plots of stress distribution in each region of the model under a compressive load of 730 N using the standard spacer. Horizontal lines represent the median stress in each region; boxes extend from the lower quartile to the upper quartile of all stress values in the region; whiskers indicate the maximum and minimum stress in the region. The stresses for bone–spacer interfaces include the entire region of contact between the endplates and spacer surfaces. Other regions correspond to the measurement locations used in mechanical tests (see Fig. 3), with the addition of an L5 anterior location analogous to that used for L4. The stress in the posterior rods (to the right of the dashed line) is plotted on a separate scale due to the higher stress magnitudes.

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Fig. 7

Box-and-whisker plots of endplate stress distributions at the bone–spacer interfaces with the standard and custom spacer

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Fig. 8

Stress distribution at the L5 bone–spacer interfaces with the 70% porous PPP standard spacer (left) and 70% porous PPP custom spacer (right)

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Fig. 9

Box-and-whisker plots of the stress distribution in posterior rods with the standard spacer and custom spacer

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