BIOMECHANICS SPINE Volume 37, Number 18, pp E1126–E1133 ©2012, Lippincott Williams & Wilkins E1126 www.spinejournal.com August 2012 Load Transfer Characteristics Between Posterior Spinal Implants and the Lumbar Spine Under Anterior Shear Loading An In Vitro Investigation Angela D. Melnyk, MASc,*†‡ Tian Lin Wen, MD ,§ Stephen Kingwell, MD, FRCSC,¶ Jason D. Chak, MEng,*†‡ Vaneet Singh, MS,** Peter A. Cripton, PhD ,*†‡ Charles G. Fisher, MD, MHSc, FRCSC,†‡  Marcel F. Dvorak, MD, FRCSC,†‡  and Thomas R. Oxland, PhD*†‡ Study Design. A biomechanical human cadaveric study. Objective. To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model. Summary of Background Data. Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis. Methods. The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5–250 N). Implants (high- P osterior instrumented fusion is the most common surgi- cal treatment of degenerative lumbar spinal conditions with associated instability. Although fusion rates are high, 1 researchers postulate that rigid instrumentation may impede fusion by stress-shielding the spine and may accel- erate adjacent segment degeneration 2 ,3 possibly by causing increased mobility at the adjacent intervertebral joint. 4 –6 In response to the increasing numbers of fusion surgeries 7 and revision surgeries due to adjacent segment disease, 3 ,8 low- stiffness posterior spinal implants have been developed. 9 –12 The theoretical goals of new spinal devices are to optimize load sharing through the instrumented spinal segments while minimizing adjacent segment stresses. 9 ,13 Devices are implanted to achieve fusion, 14 to restore normal spinal motion, 11 ,12 or to extend rigid fusion instrumentation to nonfusion segments From the Departments of *Mechanical Engineering; †Orthopaedics, University of British Columbia, Vancouver, British Columbia, Canada; ‡International Collaboration on Repair Discoveries (ICORD), University of British Columbia, Vancouver, British Columbia, Canada; §Beijing Army General Hospital, Beijing, China; ¶University of Ottawa Hospital, Ottawa, Ontario, Canada; Division of Spine, Department of Orthopaedics, University of British Columbia, Vancouver, British Columbia, Canada; and **Medtronic Inc., Memphis, TN. Acknowledgment date: December 16, 2011. First revision date: March 30, 2012. Acceptance date: April 15, 2012. The device(s)/drug(s) is/are FDA approved or approved by corresponding national agency for this indication. Natural Sciences and Engineering Research Council of Canada and Medtronic Inc. funds were received to support this work. 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: e.g., honoraria, gifts, consultancies, royalties, stocks, stock options, decision-making position. Address correspondence and reprint requests to Angela D. Melnyk, MASc, University of British Columbia, 818 West 10th Ave., Vancouver, BC V5Z 1M9 Canada; E-mail: toxland@icord.org stiffness [HSI]: ø 5.5-mm titanium, medium-stiffness [MSI]: ø 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: ø 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system. Results. As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization. Conclusion. Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants. Key words: lumbar spine, implant, low-stiffness, load sharing, shear, biomechanical. Spine 2012;37:E1126–E1133 DOI: 10.1097/BRS.0b013e31825b528d Copyright © 2012 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.