General Session: Biomechanics
Presented by: A. Freeman - View Audio/Video Presentation (Members Only)
A. Freeman(1), J. Walker(2), M. Fenn(2), M. Bushelow(2), C. Cain(3), A. Tsantrizos(2)
(1) Excelen, Center for Bone and Joint Research and Education, Minneapolis, MN, United States
(2) DePuy Synthes, West Chester, PA, United States
(3) University of Colorado Hospital, Aurora, CO, United States
Introduction: Stand-alone anterior interbody fusion devices are becoming a popular treatment option for degenerative conditions in the lumbar spine. There are several distinct categories of integrated fixation that are currently available including fixed angle screws, variable angle screws and blade type fixation. There are currently no published biomechanical studies comparing the stability provided by each of the unique integrated fixation options. The purpose of this cadaveric biomechanical study was to evaluate range of motion (ROM) from the same test laboratory for stand-alone interbody spacers utilizing three different integrated fixation methods.
Methods: In this study, ROM data was evaluated on lumbar motion segments implanted with stand-alone spacers using fixed angle screws (SynFix-LR), variable angle screws (Sovereign) and blade fixation (ROI-A). All tests were performed at a single laboratory, although data for the variable angle screws and blade fixation devices were obtained from published literature (Beaubien 2010, Freeman 2012). Human lumbar spines were acquired, scanned using DEXA, dissected into L23 and L45 motion segments and potted. There were slight variations in the loading parameters with flexibility testing being performed by applying pure moments of ±6-7.5 Nm with compressive preloads of 0-100 N. The use of smaller bending moments and/or compressive preloads were used when evaluating the variable angle screw and blade fixation devices, which have been shown to reduce specimen ROM. Note also that the SynFix-LR and the Sovereign evaluated the cage-only condition which allows for a better understanding of the increased stability provided by the fixation.
Results: The intact ROM was similar between the three fixation groups with average values varying by 1.1°, 0.7° and 1.4° in flexion-extension, lateral bending and axial rotation, respectively. The fixed angle screw device provided the greatest reduction in ROM compared to the intact condition in all test directions. In flexion-extension the blade fixation and variable angle screw devices reduced ROM by ~60% whereas the fixed angle screw device provided a 73% reduction.
Conclusion: In this study, a stand-alone lumbar spacer with fixed-angle screws consistently provided greater stability in each anatomic plane than spacers using blade fixation or variable angle screws. Therefore, despite the fixed angle device being subjected to the most aggressive testing conditions (±7.5 Nm, no preload) it provided greater stability than blade and variable screw fixation. Note also that the SynFix-LR and the Sovereign were subjected to a challenging environment as the cage only condition was in excess of 50% of the intact flexion/extension motion, which requires a lot of the fixation. Depending on the anatomy and fit, the cage itself can provide a significant amount of stability to the segment (up to and exceeding 80% of intact motion). 1. Freeman AL, et al. Spine Week 2012, Amsterdam. 2. Beaubien BP, et al. J Spinal Disord Tech, Volume 23 (5), July 2010.