The Effect of L5-S1 Fusion on Global Spine Motion: A Range of Motion Analysis Comparing 2-level TDR Versus Hybrid at L4-S1 in 235 Patients.

Background context: Lumbar total disc replacement (TDR) is a treatment option with 30 years of experience and extensive publications on clinical results. However, there is sparse literature on mid- and long-term mobility or the difference between L4-S1 two-level TDR and TDR/ALIF hybrid constructs with anterior lumbar interbody fusion (ALIF) at L5-S1 and TDR at L4-L5.

Purpose: The purpose of this study was to measure and compare key mobility parameters in flexion-extension for both groups. These included motion at L4-L5, participation of pelvis mobility, global lumbar motion, and the effectiveness of overall lumbar flexion-extension. In addition, we looked for potential compensation above and below L5-S1 fusion in the hybrid group versus two-level TDR group.

Study design/setting: Retrospective clinical study.

Patient sample: We analyzed 235 patients who had surgery between 2003-2013; 170 patients received 2-level TDR (TDR group) and 65 received L4-L5 TDR and L5-S1 ALIF (Hybrid group). The average follow-up was 124 months for TDR group and 97 months for the hybrid group. Baseline demographics and patient-reported preoperative clinical parameters were equivalent in both groups.

Outcome measures: Clinical measures included the following: Oswestry Disability Index (ODI), Visual Analog Scale (VAS) back and leg pain, Satisfaction Index Scores and time of patient return to work after surgery. Complication, reoperation, and revision rates, and perioperative data points were also assessed. Radiographic evaluation included measurement of the following: pelvic parameters (Incidence, Pelvic Tilt, Sacral Slope), L4-L5 and L5-S1 flexion-extension range of motion (ROM), pelvic motion as measured by sacral slope in flexion-extension, and flexion-extension L1 ROM (newly described in the body of manuscript as "L1 Race") to show the effect the lumbopelvic complex has on global motion.

Methods: The radiographic evaluation was performed on pre- and postoperative lateral and dynamic flexion-extension X-rays at the latest follow-up (minimum of 24 months follow-up).

Results: When L5-S1 is fused, there is no compensation from pelvic motion to overcome the loss of mobility. TDR group shows a pelvi-femoral ROM (defined as sacral slope in extension minus sacral slope in flexion) gain of 16.77°, vs a gain of only 6.11° in the Hybrid group. L5-S1 fusion also reduces L4-L5 TDR mobility in the Hybrid group compared to the 2-level TDR group and decreases flexion compared to baseline. There is a mean reduction in lumbar (L1-S1) ROM of 1.53° in Hybrid group versus 20.02° gain in TDR group. L1 Race also reflects the superiority of 2-level TDR vs hybrid with a gain of 32.58° in TDR vs 4.68° in Hybrid, demonstrating that reduced global motion is principally due to the loss of L5-S1 influence on motion above and below. ODI, VAS back and leg pain, and satisfaction index scores were equivalent between groups. Return to work was statistically earlier for the 2-level TDR group both in terms of delay in return to work and the percentage who return.

Conclusion: The absolute motion and relative gain of 2-level TDR shows its functional superiority over Hybrid constructs in all measured parameters. This comparison between 2-level TDR and Hybrid also demonstrates a lack of compensation through lumbar mobility and pelvic motion when L5-S1 is fused. Two new ROM parameters introduced here-Pelvic motion and L1 Race quantify pelvic participation in mobility and the functional effectiveness of motion preservation. In this first long-term comparison of mobility between 2-level TDR vs L4-S1 Hybrid, 2-level TDR demonstrates overall superiority. It could be argued that 2-level TDR should be considered as first surgical option in case of 2-level degenerative disease.