No abstract

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.

This study involved a biomechanical examination of the first and second tar-sometatarsal (TMT) joints. In the in vitro testing protocol, physiologic joint moments were applied to the first and second TMT joints of 10 cadaver specimens, which had been dissected to yield only the midfoot components comprising the tarsals, metatarsals, and associated ligaments. The isolated joints were placed in a 37 °C water bath and were independently cycled into and out of dorsiflexion, while angular displacement and resultant joint moments were collected. The specimens were sequentially cycled between zero and peak moment levels of 2.5, 5.0, 7.5, and 10 Nm, after which mean moment-angle curves were plotted for each TMT joint at each loading condition. Least-squares curve-fitting procedures, employing a root mean square error threshold of 0.005 Nm, were used to describe the average overall moment vs. angle relationship of each joint. Curves for the first and second TMT joints exhibited similar behavior. The joints displayed reasonably constant stiffness at low angles of dorsiflexion, followed by rapidly increasing stiffness at higher angles of dorsiflexion. These data provide new insight into the behavior of the TMT joints under load and are valuable for use in numerical models of the foot, as well as in the understanding and treatment of certain foot pathologies.