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A New Bone Surrogate Model for Testing Interbody Device Subsidence

Au, Anthony G., PhD*; Aiyangar, Ameet K., MSc*; Anderson, Paul A., MD†,‡; Ploeg, Heidi-Lynn, PhD*,‡

doi: 10.1097/BRS.0b013e31820bffe9

Study Design. An in vitro biomechanical study investigating interbody device subsidence measures in synthetic vertebrae, polyurethane foam blocks, and human cadaveric vertebrae.

Objective. To compare subsidence measures of bone surrogates with human vertebrae for interbody devices varying in size/placement.

Summary of Background Data. Bone surrogates are alternatives when human cadaveric vertebrae are unavailable. Synthetic vertebrae modeling cortices, endplates, and cancellous bone have been developed as an alternative to polyurethane foam blocks for testing interbody device subsidence.

Methods. Indentors placed on the endplates of synthetic vertebrae, foam blocks, and human vertebrae were subjected to uniaxial compression. Subsidence, measured with custom-made extensometers, was evaluated for an indentor seated either centrally or peripherally on the endplate. Failure force and indentation stiffness were determined from force-displacement curves.

Results. Subsidence measures in human vertebrae varied with indentor placement: failure forces were higher and indentors subsided less with peripheral placement. Subsidence measures in foam blocks were insensitive to indentor size/placement; they were similar to human vertebrae for centrally placed but not for peripherally placed indentors. Although subsidence measures in synthetic vertebrae were sensitive to indentor size/placement, failure force and indentation stiffness were overestimated, and subsidence underestimated, for both centrally placed and peripherally placed indentors.

Conclusion. The synthetic endplate correctly represented the human endplate geometry, and thus, failure force, stiffness, and subsidence in synthetic vertebrae were sensitive to indentor size/placement. However, the endplate was overly strong and thus synthetic vertebrae did not accurately model indentor subsidence in human cadaveric vertebrae. Foam blocks captured subsidence measures more accurately than synthetic vertebrae for centrally placed indentors, but because of their uniform density were not sufficiently robust to capture changes generated from different indentor sizes/placements. The current bone surrogates are not accurate enough in terms of material property distribution to completely model subsidence in human cadaveric vertebrae.

A new bone surrogate for testing interbody device subsidence is introduced: a geometrically realistic synthetic L5 vertebra containing cortices, endplates, and cancellous bone. The efficacy of the synthetic vertebra was investigated by comparing indentor subsidence with human cadaveric vertebrae and the commonly used polyurethane foam block.

* Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI

Department of Orthopedics and Rehabilitation, University of Wisconsin-Madison, Madison, WI

Department of Biomedical Engineering, University of Wisconsin-Madison, Madison, WI.

Address correspondence and reprint requests to Anthony Au, PhD, 3046 Mechanical Engineering Building, University of Wisconsin-Madison, Madison, WI 53706; E-mail:

Acknowledgment date: August 16, 2010. Revision date: October 26, 2010. Acceptance date: December 13, 2010.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Federal and Institutional funds were received in support of this work.

Although one or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript, benefits will be directed solely to a research fund, foundation, educational institution, or other nonprofit organization which the author(s) has/have been associated.

Financial support was received from the Natural Science and Engineering Research Council of Canada and the University of Wisconsin Graduate School. The synthetic vertebrae prototypes were provided by Sawbones, Vashon Island, Washington, developed as part of an NIH SBIR grant 5R44 AR054289-03. An institutional review board exemption from the University of Wisconsin was received for this study (protocol #M-2008–1473).

© 2011 Lippincott Williams & Wilkins, Inc.