BackgroundMany decompression procedures involve complete or partial facetectomy. Spinal fusion usually stabilizes the motion segment after complete facetectomies. However, problems with fusion, such as adjacent-level degeneration, have increased interest in motionpreservation technologies. Facet arthroplasty may become an important posterior motion-preservation device, but its biomechanical literature is sparse. MethodsWe conducted an in vitro investigation and À nite element study to compare the biomechanical effects of an artiÀ cial facet system to the intact spine. In the in vitro study, we tested human osteo-ligamentous segments (L3-S1) in intact, injured, and artiÀ cial facet-repaired conditions. For the À nite element study, we used a 3-dimensional ligamentous L3-S1 segment model. We simulated destabilization in the intact model by removing the facets across the L4-L5 functional unit, then repaired it with appropriately sized facet implants and compared the ranges of motion, facet loads, disc pressures, and device loads. We also analyzed a À nite element model with a rigid posterior pedicle-rod À xation system. We subjected the cadaveric specimens and the models to 400 N of follower load plus a 10 Nm moment in extension, Á exion, bending, and rotation. We used a novel technique to apply the follower load in the À nite element models such that preload induced minimal vertebral rotation during the range of motion. ResultsThe predicted ranges of motion for the intact and implanted models were consistent with cadaver data. After destabilization and facet replacement, the artiÀ cial facet system restored motion in all loading modes to intact values. The implant facet loads were similar to intact facet loads in extension and axial rotation, but less in lateral bending. The intradiscal pressure at the implanted level for the facet replacement device was similar to the intact pressure, whereas with the rigid system the intradiscal pressure was up to 70% less than the intact pressure. The maximum von-Mises stress predicted in the facet replacement construct was 85 MPa in extension at the bone-pedicle screw interface, compared with 174 MPa in the rigid system. Contact stresses at implant mating surfaces were minimal. ConclusionsThe artiÀ cial facet system replicated natural facet kinematics. The cadaveric ranges of motion and the predicted À nite element-based data indicated that the implant can "restore" the normal function of the segment after artiÀ cial facet replacement. Clinical RelevanceCompared to rigid posterior pedicle-rod À xation, the artiÀ cial facet system restored the intact mechanics at the implanted level and may prevent adjacent-level degeneration.

BackgroundMany decompression procedures involve complete or partial facetectomy. Spinal fusion usually stabilizes the motion segment after complete facetectomies. However, problems with fusion, such as adjacent-level degeneration, have increased interest in motion- preservation technologies. Facet arthroplasty may become an important posterior motion-preservation device, but its biomechanical literature is sparse.MethodsWe conducted an in vitro investigation and finite element study to compare the biomechanical effects of an artificial facet system to the intact spine. In the in vitro study, we tested human osteo-ligamentous segments (L3-S1) in intact, injured, and artificial facet–repaired conditions. For the finite element study, we used a 3-dimensional ligamentous L3-S1 segment model. We simulated destabilization in the intact model by removing the facets across the L4-L5 functional unit, then repaired it with appropriately sized facet implants and compared the ranges of motion, facet loads, disc pressures, and device loads. We also analyzed a finite element model with a rigid posterior pedicle-rod fixation system. We subjected the cadaveric specimens and the models to 400 N of follower load plus a 10 Nm moment in extension, flexion, bending, and rotation. We used a novel technique to apply the follower load in the finite element models such that preload induced minimal vertebral rotation during the range of motion.ResultsThe predicted ranges of motion for the intact and implanted models were consistent with cadaver data. After destabilization and facet replacement, the artificial facet system restored motion in all loading modes to intact values. The implant facet loads were similar to intact facet loads in extension and axial rotation, but less in lateral bending. The intradiscal pressure at the implanted level for the facet replacement device was similar to the intact pressure, whereas with the rigid system the intradiscal pressure was up to 70% less than the intact pressure. The maximum von-Mises stress predicted in the facet replacement construct was 85 MPa in extension at the bone–pedicle screw interface, compared with 174 MPa in the rigid system. Contact stresses at implant mating surfaces were minimal.ConclusionsThe artificial facet system replicated natural facet kinematics. The cadaveric ranges of motion and the predicted finite element–based data indicated that the implant can “restore” the normal function of the segment after artificial facet replacement.Clinical RelevanceCompared to rigid posterior pedicle-rod fixation, the artificial facet system restored the intact mechanics at the implanted level and may prevent adjacent-level degeneration.

Lumbar spondylolisthesis is an extension of spondylolysis in which breakage of the vertebrae occurs at the pars interarticularis causing the vertebrae to slip forward. Spondylolisthesis is seen in both younger and older populations with most lesions occurring at the L4-L5 or L5-S1 level. Although the forward slippage of the vertebra does not usually exceed 30% of the body’s anterior-posterior width, possible spinal stenosis and nerve impingement may lead to severe pain and other complications. The purpose of this study is to determine the effect of single level and bi-level dynamic stabilization in reducing the spondylolisthesis. We used the finite element model for this purpose since it is not practical to procure specimens with spondylolisthesis for an experimental investigation.

Problems associated with spinal fusion such as adjacent level degeneration and donor site pain have shifted the focus to motion preservation technologies. The Anatomic Facet Replacement System (AFRS™) (Facet Solutions, Inc., Logan, Utah) attempts to address posterior lumbar spine pathologies while preserving stability and natural biomechanics thereby mitigating any potential adjacent level effects resulting from the reduction or elimination of motion as seen in semi-constrained dynamic stabilization and fusion devices. The AFRS™ is comprised of a precision instrumentation set whose design is based upon a comprehensive CT morphology study of the facet joint. It utilizes traditional pedicle screw fixation of its superior and inferior facet implants and is manufactured from a wear resistant alloy called cobalt-chromium-molybdenum. An experimentally validated finite element model was used for the quantification of facet loads and stresses in various components of the facet replacement system and also in the model stabilized using a pedicle screw rigid rod fixation system.

An experimentally validated three-dimensional (3D) finite element (FE) model of the ligamentous L3–S1 segment was used to study the effects of artificial facet designs on the segment biomechanics (motion, facet loads, and stresses). The intact model was modified to simulate several artificial facet designs across the L4–L5 segment including capping with and without screws and pedicle screw based designs with sliding articulating surfaces. For the pedicle screw based design, the effect of increasing the connecting shaft thickness and increasing width surrounding the pedicle screw, butted against the vertebral pedicle for further support, was studied. All of the FE models were evaluated in response to 6 Nm moment in extension, flexion, bending, and rotation. The predicted increases in motion, compared to the intact case, were smaller. The predicted facet loads decreased up to 25.7% in extension and 25.1% in bending at the implanted level as compared to intact spine segment. For all of the loading modes, the stresses in both implant designs were less than the yield stress of titanium. Therefore, the implants are unlikely to fail. Additional cadaver and other experimental protocols are essential for the evaluations of the most appropriate designs identified through the FE investigations.