Biomechanical Effects of Facet Joint Violation After Single-Level Lumbar Fusion With Transpedicular Screw and Rod Instrumentation

Study Design.In vitro biomechanical study. Objective.This study aimed to investigate the biomechanical effects of facet joint violation (FV) on mobility and optically tracked intervertebral disc (IVD) surface strains at the upper level adjacent to L4-5 pedicle screw-rod fixation. Summary of Background Data.FV is a complication that can occur when placing lumbar pedicle screws; the reported incidence is as high as 50%. However, little is known about how FV affects superior adjacent-level spinal stability, and especially IVD strain, after lumbar fusion. Methods.Fourteen cadaveric L3-S1 specimens underwent L4-5 pedicle-rod fixation, 7 in the facet joint preservation (FP) group and 7 in the FV group. Specimens were tested multidirectionally under pure moment loading (7.5 Nm). Colored maps of maximum (& epsilon;1) and minimum (& epsilon;2) principal surface strain changes on the lateral L3-4 disc were generated, with the surface divided into 4 quarters anterior to posterior (Q1, Q2, Q3, and Q4, respectively) for subregional analyses. Range of motion (ROM) and IVD strain were normalized to intact upper adjacent-level and compared between the groups using analysis of variance. Statistical significance was set at PResults.Normalized ROM was significantly greater with FV vs. FP in flexion (11% greater; P=0.04), right lateral bending (16% greater; P=0.03), and right axial rotation (23% greater; P=0.04). Normalized L3-4 IVD & epsilon;1 during right lateral bending was greater on average for the FV group than the FP group: Q1, 18% greater; Q2, 12% greater; Q3, 40% greater (P<0.001); Q4, 9% greater. Normalized & epsilon;2 values during left axial rotation were greater in the FV group, the highest increase being 25% in Q3 (P=0.02). Conclusions.Facet joint violation during single-level pedicle screw-rod fixation was associated with increased superior adjacent level mobility and alteration of disc surface strains, with significant increases in selected regions and directions of loading.