Decompression Procedures Adjacent to Fused Segments in the Lumbar Spine: A Biomechanical Study

Abstract

Introduction The gold standard treatment for degenerative lumbar listhesis is decompression with fusion. According to a SPORT trail, 35% of patients with this condition present with additional symptomatic stenosis most commonly on the adjacent segment. However, the most appropriate surgical technique to treat the stenosis adjacent to the instrumented segment remains controversial. Extended fusion with open decompression could lead to adjacent segment disease; simple decompression without fusion might lead to clinically relevant instability at the index level. To date, there is no study evaluating the biomechanical impact of decompression procedures in this clinical scenario. The presented work is the first to analyze the mechanical effect of minimally invasive (MI) and open decompression procedures adjacent to instrumented segments in the lumbar spine. Material and Methods Tested conditions: Seven cadaveric lumbar spines were tested. The entire lumbar spine was assessed intact (baseline) and for the following conditions: (1) Instrumented fusion at L4/L5. (2) MI decompression through a tubular retractor at L3/L4 with a unilateral approach for bilateral internal decompression and complete removal of the flavum ligament. (3) Additional bilateral partial facetectomy at L3/L4 (20% of the facet joints). (4) Laminectomy at L3/L4. Mechanical testing: Specimens were carefully cleaned of muscular tissue and potted at T12 and S2–S4. Nonconstraining, nondestructive pure moment (torque) loading (7.5 Nm) was applied to each specimen through a system of cables and pulleys in conjunction with a standard servohydraulic test system. Loads were applied in 1.25 Nm increments until the maximum load was reached. Each condition was tested for flexion extension, lateral bending, left and right axial rotation, and compression. The range of motion (ROM) of each segment was assessed stereophotogrammetrically by capturing the three-dimensional displacement of infrared-emitting markers which were rigidly attached to each vertebra from L1–S1. ROM was measured in degrees and used to quantify spinal stability. One-way repeated measures analysis of variance (ANOVA) followed by the Fisher least significant difference test were used to compare ROM. Results All data are shown as normalized ratio. Compared with intact L3/L4 after L4/L5 fusion, MIS decompression significantly increased the L3/L4 ROM by 13% ( p = 0.034) for flexion extension (20% more than baseline) and 23% for axial rotation ( p = 0.003) (26% more than baseline). Partial facetectomy further increased the ROM by 15% for axial rotation ( p = 0.032) (46% more than baseline) but not for flexion extension. Laminectomy further increased the ROM for flexion extension by 12% ( p = 0.05) (38% more than baseline) and for axial rotation by 17% ( p = 0.023) (58% more than baseline). There were no significant changes for lateral bending or compression tests for any tested condition. Looking at the overall motion of the lumbar spine, L4/L5 fusion decreased flexion extension by 20% (< 0.001) and axial rotation by 19% ( p < 0.001). With each decompressive procedure, the overall motion returned to almost normal in a stepwise manner reaching 95% of baseline after laminectomy. Conclusion Mechanical alterations of MI decompression adjacent to fused segments are comparable to mechanical alterations of these procedures without adjacent fusion shown in other studies. Therefore, adjacent fusion does not significantly increase instability of MI decompression. The preservation of the facet joints is crucial for segmental stability. Laminectomy adjacent to fused segment causes an up to 58% increase in segmental ROM and is therefore not recommended in the presented clinical scenario.