Kinematics of total facet replacement (TFAS-TL) with total disc replacement

BACKGROUND Total disc replacement (TDR) and total facet replacement (TFR) have been the focus of recent kinematics evaluations. Yet their concurrent function as a total joint replacement of the lumbar spine's 3-joint complex has not been comprehensively reported. This study evaluated the effect of a TFR specifically designed to replace the natural facets and supplement the function with the natural disc and with TDR. The ability to replace degenerated facets to complement a pre-existing or simultaneously implanted TDR may allow surgeons to completely address degenerative pathologies of the 3-joint complex of the lumbar spine. We hypothesized that TFR would reproduce the biomechanical function of the natural facets when implanted in conjunction with TDR. METHODS Lumbar spines (L1-5, 51.3 ± 14.2 years, N = 6) were tested sequentially as follows: (1) intact, (2) after TDR implantation, and (3) after TFR implantation in conjunction with TDR, all at L3-4. Specimens were tested in flexion-extension (+ 8 Nm to - 6 Nm), lateral bending (± 6 Nm), and axial rotation (± 5 Nm). A 400 N compressive follower preload was applied during flexion-extension tests. Three-dimensional segmental motion was recorded and analyzed using analysis of variance in Systat (Systat Software Inc., Chicago, Illinois) and multiple comparisons with Bonferroni correction. RESULTS The TDR implantation (TDR + natural facets) allowed similar lateral bending (P = .66), but it generally increased flexion-extension (P = .06) and axial rotation (P < .05) range of motion (ROM) at the implanted level compared to intact. The TFR + TDR (following replacement of the natural facets with TFR) decreased ROM to levels similar to intact in lateral bending (P = .70) and axial rotation (P = .23). The TFR + TDR flexion-extension ROM was reduced in comparison to intact and TDR + natural facets (P < .05). CONCLUSIONS The TFR with TDR was able to restore stability to the lumbar segment after bilateral facetectomy, while allowing near-normal motions in all planes.