Study Design. In vitro biomechanical study investigating L4–L5 kinematics and intradiscal pressure (IDP) with a facet replacement system.
Objective. To assess the influence of the Auxiliary Facet System (AFS).
Summary of Background Data. Posterior dynamic systems are used in the treatment of low back pain to avoid adjacent segment degeneration. Facet replacement systems are supposed to stabilize a lumbar segment after facetectomy and neural decompression, and to provide an intersegmental range of motion (ROM).
Methods. The AFS is fixed by 4 pedicle screws, linked by 2 angulated rods, a polyaxial connector, and a crosslink. Flexibility tests were conducted on 6 human cadaver specimens (L3–S1) using a load testing device and the Polaris system. The specimens were loaded by steps of 1 Nm to 10 Nm in flexion/extension, lateral bending, and axial rotation. The following configurations were investigated: intact segment, instrumented, instrumented plus medial facetectomy, and facetectomy alone. The sagittal mean center of rotation (MCR) was calculated, and IDPs were measured in flexion/extension.
Results. The ROM of the intact segment was 10.9° (9.4°–15.5°) in flexion/extension, 9.5° (6.8°–12.1°) in lateral bending, and 4.7° (3.4°–6.0°) degrees in axial rotation. Medial facetectomy and instrumentation led to −6% of ROM in flexion/extension and +1% lateral bending. Medial facetectomy without implant led to +106% of axial rotation (P = 0.028). The instrumentation reduced axial rotation to −38% (P = 0.028). This decrease was because of the presence of the cross-link. The MCR was located around the middle of the superior L5 endplate in intact and instrumented specimens. It moved cranial after facetectomy without instrumentation. The implant decreased the maximal IDP during flexion/extension to −17% (P = 0.028).
Conclusion. The AFS had a minor influence on flexion/extension and lateral bending, and the MCR kept physiologic. Bilateral facetectomy yielded an increase in axial rotation, which was stabilized by the implant. The AFS seemed to reduce IDPs.