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The Restoration of Lumbar Intervertebral Disc Load Distribution: A Comparison of Three Nucleus Replacement Technologies

Dahl, Michael C. PhD*; Ahrens, Michael MD; Sherman, John E. MD; Martz, Erik O. MS*

doi: 10.1097/BRS.0b013e3181bef192
Biomechanics

Study Design. A validated L3–L4 nonlinear finite element model was used to evaluate strain and pressure in the surrounding structures for 4 nucleus replacement technologies.

Objective. The objective of the current study was to compare subsidence and anular damage potential between 4 current nucleus replacement technologies. It was hypothesized that a fully conforming nucleus replacement would minimize the risk of both subsidence and anular damage.

Summary of Background Data. Nucleus pulposus replacements are emerging as a less invasive alternative to total disc replacement and fusion as a solution to degenerative intervertebral discs. Multiple technologies have been developed and are currently undergoing clinical investigation.

Methods. The testing conditions were applied by excavating the nucleus of the intact model and virtually implanting models representing the various nucleus replacement technologies. The implants consisted of a conforming injectable polyurethane (E = 4 MPa), soft hydrogel (E = 4 MPa), stiff hydrogel (E = 20 MPa), and polyether-etherketone (PEEK) on PEEK articulating designs. The model was exercised in flexion, extension, lateral bending, axial rotation (7.5 Nm with 450 N preload), and compression (1000 N). Vertebral body strain, anular maximum shear strain, endplate contact pressure, anulus-implant contact pressure, and bone remodeling stimulus were reported.

Results. The PEEK implant induced strain maxima in the vertebral bodies with associated endplate contact pressure concentrations. For the PEEK and hydrogel implants, areas of nonconformity with the endplate indicatedadjacent bone resorption. Lack of conformity between the implant and inner anulus for the PEEK and hydrogel implants resulted in inward anular bulging with associated increased maximum shear strain. The conforming polyurethane implant maintained outward bulging of the inner anular wall and indicated no bone resorption or stress shielding adjacent to the implant.

Conclusion. A fully conforming nucleus replacement resulted in a decreased propensity for subsidence, anular bulging, and further degeneration of the anulus when compared with nonconforming implants.

A finite element model evaluated strain and pressure in the surrounding structures of 4 nucleus replacement technologies. Lack of implant nuclear cavity conformity caused inward anular bulging, increased shear strain, high endplate pressure concentrations, and nonbiofidelic bone resorption. Implant conformance resulted in decreased propensity for subsidence and further anular degeneration.

From the *Disc Dynamics Inc., Eden Prairie, MN; †Clinic for Spine Surgery and Scoliosis Center, University of Lübeck, Neustadt, Germany; and ‡Twin Cities Orthopedics, Edina, MN.

Acknowledgment date: March 27, 2009. First revision date: June 30, 2009. Second revision date: August 26, 2009. Acceptance date: August 29, 2009.

The device(s)/drug(s) that is/are the subject of this manuscript is/are being evaluated as part of an ongoing FDA-approved investigational protocol (IDE) or corresponding national protocol for this indication. The Disc Dynamics DASCOR (r) Disc Arthroplasty Device is designed to replace the disc nucleus in patients with degenerative disc disease (DDD) requiring surgical treatment at a single level of the lumbar spine, from L2 to the sacrum. For this study DDD is defined as discogenic back pain with degeneration of the disc as confirmed by medical history, physical examination, and radiographic studies as measured by: CT, MRI, plain film, myelography, CT discography, etc. At least one of the following radiographic findings must be coupled with the history and physical findings.

Corporate 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 Michael C. Dahl, PhD, Disc Dynamics Inc., 9600 W. 76th St, Suite T, Eden Prairie, MN 55344; E-mail: mdahl@discdyn.com

© 2010 Lippincott Williams & Wilkins, Inc.