Spine. Volume 34(19), 1 September 2009, pp 2022-2032

Effect of Nucleus Replacement Device Properties on Lumbar Spine Mechanics

Rundell, Steven A. MS*†; Guerin, Heather L. PhD*; Auerbach, Joshua D. MD‡; Kurtz, Steven M. PhD*†

From the *Exponent Inc, Philadelphia, PA; †Drexel University, Philadelphia, PA; and ‡Department of Orthopaedic Surgery, Bronx-Lebanon Hospital Center, Albert Einstein College of Medicine, Bronx, NY.
Acknowledgment date: October 21, 2008. Revision date: January 19, 2009. Acceptance date: March 13, 2009.
The manuscript submitted does not contain information about medical device(s)/drug(s).
No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.
Address correspondence and reprint requests to Steven A. Rundell, MS, Exponent, Inc., 3401 Market St, Suite 300, Philadelphia, PA 19104; E-mail: srundell (at) exponent.com

Study Design. A validated nonlinear three-dimensional finite element model of a single lumbar motion segment (L3–L4) was used to evaluate a range of moduli for ideally conforming nucleus replacement devices.

Objective. The objective of the current study was to determine the biomechanical effects of nucleus replacement technology for a variety of implant moduli. We hypothesized that there would be an optimal modulus for a nucleus replacement that would provide loading in the surrounding bone and anulus similar to the intact state.

Summary of Background Data. Nucleus pulposus replacements are interventional therapies that restore stiffness and height to mildly degenerated intervertebral discs. Currently a wide variety of nucleus replacement technologies with a large range of mechanical properties are undergoing preclinical testing.

Methods. A finite element model of L3–L4 was created and validated using range of motion, disc pressure, and bony strains from previously published data. The intact model was altered by changing the mechanical properties of the nucleus pulposus to represent a wide range of nucleus replacement technologies (E = 0.1, 1, 4, and 100 MPa). All of the models were exercised in compression, flexion, extension, lateral bending, and axial rotation. Vertebral body strain, peak anulus fibrosus shear strain, initial bone remodeling stimulus, range of motion, and center of rotation were analyzed.

Results. A nucleus replacement modulus of 1 and 4 MPa resulted in vertebral body strains similar to the intact model. The softest device indicated increased loading in the AF and bone resorption adjacent to the implant. Areas of strain maxima and bone formation were observed adjacent to the implant for the stiffest device.

Conclusion. The current study predicted an optimal nucleus replacement of 1 to 4 MPa. An overly stiff implant could result in subsidence, which would preclude the benefit of disc height increase or restoration. Conversely, an overly soft implant could accelerate a degenerative cascade in the anulus.

Copyright: © 2009 Lippincott Williams & Wilkins, Inc.