The Mediating Role of Segmental Lordosis Changes in the Relationship Between Surgical Approach and the Incidence of Adjacent Segment Disease in Patients With Degenerative Spinal Disorders: A Retrospective Cohort Study.

Abstract

Minimally invasive lateral lumbar interbody fusion (MIS-LLIF) and transforaminal lumbar interbody fusion (TLIF) are widely used for lumbar degenerative disease. However, their comparative risk for adjacent segment disease (ASD) remains controversial. Changes in segmental lordosis (∆SL) may play a mediating role in ASD development, but this pathway has not been rigorously quantified. This study aimed to compare the risk of ASD following MIS-LLIF versus TLIF and to evaluate whether ∆SL mediates this relationship. An exploratory retrospective cohort study of patients who underwent single-level lumbar fusion at a single institution. We reviewed 143 patients who underwent single-level MIS-LLIF or TLIF between January 2017 and December 2022. The primary outcome measure was the incidence of radiographically confirmed ASD with a minimum 2-year follow-up. The mediating variable was the change in segmental lordosis (∆SL). Baseline demographics, surgical parameters, and radiographic outcomes were collected. The mediating effect of ∆SL was assessed using Baron and Kenny with Sobel testing, adjusting for covariates. A counterfactual-based mediation analysis with bootstrap confidence intervals (1,000 samples) was also performed to validate the findings. The overall incidence of ASD was 16.1% (23.2% in MIS-LLIF vs. 9.5% in TLIF; P=0.045). MIS-LLIF resulted in less segmental lordosis improvement (∆SL) than TLIF (1.50° vs. 2.60°; P<0.001). Initially, MIS-LLIF was associated with higher ASD odds (OR 2.78, 95% CI: 1.12-7.45; P =0.027). Mediation analysis (α = 0.10) identified ∆SL as a mediator, accounting for 64.6% of the total effect (indirect effect: 0.083, 95% CI: 0.01-0.20, P=0.028). After adjusting for ∆SL, the surgical approach was no longer significantly associated with ASD (OR 1.78, 95% CI: 0.56-5.95; P=0.300), whereas ∆SL remained an independent protective factor (OR 0.53 per degree, 95% CI: 0.31-0.87; P=0.010). In this exploratory analysis, ∆SL statistically explained a substantial proportion of the association between surgical approach and ASD risk. Optimizing segmental lordosis restoration may be a critical and modifiable factor for mitigating ASD, warranting prospective validation.