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Load transfer characteristics between posterior fusion devices and the lumbar spine under shear loading : an in vitro investigation Melnyk, Angela Dorothy


Introduction: Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spondylolisthesis (DS). The optimal stiffness of these fusion systems to enhance load-sharing and fusion rate while minimizing adjacent segment stresses is unknown. To our knowledge, posterior instrumentation for DS has not been tested under anterior shear loads, a highly relevant loading direction for DS. Objectives: To determine the amount of shear load supported by posterior lumbar fusion devices of varying stiffness under shear loading. Methods: The effect of implant stiffness and specimen condition on implant load was assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression load and a cyclic anterior shear load (5-250 N). Implants (High-Stiffness (HS): ∅ 5.5 mm Titanium, Medium-Stiffness (MS): ∅ 6.35 x 7.2 mm Oblong PEEK, Low-Stiffness (LS): ∅ 5.5 mm Round PEEK, and Ultra-Low-Stiffness (ULS): ∅ 5.5 mm Rod X), instrumented with strain gauges to measure loads, were tested in each of three specimen conditions simulating degenerative changes: intact, facet destabilization and disc destabilization. Results: Transducers measured implant shear loads to within ±5 N. All implants supported significantly greater shear loads as the specimen was destabilized. The LS and ULS implants supported significantly less load than the HS and MS implants for all specimen conditions. Mean implant loads as a percent of the applied shear load in order of increasing specimen destabilization for the HS implant were: 43, 67 and 76%, for the MS implant were: 32, 56 and 77%, for the LS implant were: 18, 35 and 50%, and for the ULS implant were: 16, 39 and 42%. Standard errors were below 8%. Discussion: An accurate shear load transducer was developed; the methodology is adaptable to many implant designs and materials. Implant shear stiffness significantly affected the shear load-sharing characteristics of the fusion devices. Low-stiffness implants transferred significantly greater loads to the spine, and may possibly enhance the transition to the adjacent, uninstrumented spine in vivo.

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