Anatomic Facet Replacement System (AFRS) Restoration of Lumbar Segment Mechanics to Intact: A Finite Element Study and In Vitro Cadaver Investigation

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.

Facet Joints of the Spine: Structure–Function Relationships, Problems and Treatments, and the Potential for Regeneration

“…The anatomic facet replacement system (AFRS) is composed of two separate implants that are fixated with pedicle screws to the vertebra and with a cross-linking component to one another. As shown with a finite-element model, the implant demonstrated kinematics similar to those of the intact facet joint (135). No clinical studies have been reported so far (135).…”

“…As shown with a finite-element model, the implant demonstrated kinematics similar to those of the intact facet joint (135). No clinical studies have been reported so far (135).…”

Biomechanical Evaluation of a New Fixation Type in 3D-Printed Periacetabular Implants using a Finite Element Simulation

“…These fixation types reportedly incur high traumatic fracture risks under compressive loading conditions [16,17]. Enhancements of the mechanical stability using spinal rod connectors, which have a bridge component between two straight connectors, have been reported in spine surgery publications [18,19]. Thus far, this bridge component design has not been utilized for pelvic implant fixation.…”

Biomechanical Evaluation of an Endplate-Conformed Polycaprolactone-Hydroxyapatite Intervertebral Fusion Graft and Its Comparison With a Typical Nonconformed Cortical Graft

“…The objective of this study is to identify the effect of vertebral endplate-conformed graft surface on endplate stress distribution and on the degree of motions using a finite element (FE) model. To accomplish the research objective, an experimentally validated 3D intact ligamentous lumbar spine (L4-L5) motion segment FE model [18][19][20] was used. The FE model was run with two case studies to identify the difference between endplate-conformed geometry versus commercial cortical bone graft on spinal unit (FSU) stability, endplate stress distribution, and maximum endplate stresses.…”