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Total Disc Replacement Positioning Affects Facet Contact Forces and Vertebral Body Strains

Rundell, Steven A. MS*†; Auerbach, Joshua D. MD; Balderston, Richard A. MD§; Kurtz, Steven M. PhD*†

doi: 10.1097/BRS.0b013e318186b258
Biomechanics

Study Design. A validated nonlinear three-dimensional finite element (FE) model of a single lumbar motion segment (L3-L4) was used to evaluate the effects of total disc replacement (TDR). The model was implanted with a fixed-bearing TDR (ProDisc-L) at 2 surgically relevant positions and exercised about the 3 anatomic axes. Facet forces, range of motion (RoM), and vertebral body strains were evaluated.

Objective. The objective of the current study was to evaluate how TDR implantation and positioning affects facet joint forces and vertebral body strains. We hypothesized that facet contact forces (FCFs) would increase with TDR to compensate for the loss of periprosthetic load-bearing structures, and that vertebral body strains would increase in the region around the metallic footplates.

Summary of Background Data. TDR has the potential to replace fusion as the gold standard for the treatment of painful degenerative disc disease. However, complications after TDR include index level facet arthrosis and implant subsidence. Alterations in facet and vertebral body loading after TDR and their dependence on implant positioning are not fully understood.

Methods. An FEM of L3-L4 was created and validated using RoM, disc pressure, and bony strains from previously published data. A TDR was incorporated into the L3-L4 spine model. All models were subjected to a compressive follower load of 500 N and moments of 7.5 Nm about the 3 anatomic axes.

Results. Overall RoM and FCFs tended to increase with TDR. FCFs increased by an order of magnitude during flexion. Posterior placement of the device resulted in an unloading of the facets during extension. Areas of strain maxima were observed in the anterior portion of the vertebral body during flexion after TDR. The area of initial bone resorption signal under the metal footplate was greater when the device was anteriorly placed.

Conclusion. The current study predicted a decrease in segmental rotational stiffness resulting from TDR. This resulted from the removal of load bearing soft tissue structures, and caused increased loading in the facets. Additionally, vertebral body strains were generally higher after TDR, and tended to increase with decreased rotational stiffness. Posterior placement of the device provided a more physiologic load transfer to the vertebral body.

The current study used a finite element model of L3-L4 to evaluate the effects of TDR implantation and positioning. Results indicate a reduction in rotational stiffness with TDR. Also, positioning resulted in alterations in the facet contact forces, cancellous bone strains, and initial bone remodeling stimulus.

From the *Exponent Inc; †Drexel University; ‡Department of Orthopaedic Surgery, The University of Pennsylvania; and §Booth, Bartolozzi, Balderston Orthopaedics, Pennsylvania Hospital, Philadelphia, PA.

Acknowledgment date: January 11, 2008. First revision date: May 13, 2008. Acceptance date: June 6, 2008.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Corporate/Industry 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 reprints requests to Steven Rundell, Exponent, Inc., 3401 Market St., Suite 300, Philadelphia, PA 19104; E-mail: srundell@exponent.com

© 2008 Lippincott Williams & Wilkins, Inc.