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 (ε1) and minimum (ε2) 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 P<0.05.

Results:

Normalized ROM was significantly greater with FV versus 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 ε1 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 ε2 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.