Multilevel tandem spondylolisthesis associated with a reduced “safe zone” for a transpsoas lateral lumbar interbody fusion at L4–5

OBJECTIVE The aim of this study was to investigate the effect of degenerative spondylolisthesis (DS) on psoas anatomy and the L4–5 safe zone during lateral lumbar interbody fusion (LLIF). METHODS In this retrospective, single-institution analysis, patients managed for low-back pain between 2016 and 2021 were identified. Inclusion criteria were adequate lumbar MR images and radiographs. Exclusion criteria were spine trauma, infection, metastases, transitional anatomy, or prior surgery. There were three age and sex propensity-matched cohorts: 1) controls without DS, 2) patients with single-level DS (SLDS), and 3) patients with multilevel, tandem DS (TDS). Axial T2-weighted MRI was used to measure the apical (ventral) and central positions of the psoas relative to the posterior tangent line at the L4–5 disc. Lumbar lordosis (LL), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), and PI-LL mismatch were measured on lumbar radiographs. The primary outcomes were apical and central psoas positions at L4–5, which were calculated using stepwise multivariate linear regression including demographics, spinopelvic parameters, and degree of DS. Secondary outcomes were associations between single and multilevel DS and spinopelvic parameters, which were calculated using one-way ANOVA with Bonferroni correction for between-group comparisons. RESULTS A total of 230 patients (92 without DS, 92 with SLDS, and 46 with TDS) were included. The mean age was 68.0 ± 8.9 years, and 185 patients (80.4%) were female. The mean BMI was 31.0 ± 7.1, and the mean age-adjusted Charlson Comorbidity Index (aCCI) was 4.2 ± 1.8. Age, BMI, sex, and aCCI were similar between the groups. Each increased grade of DS (no DS to SLDS to TDS) was associated with significantly increased PI (p < 0.05 for all relationships). PT, PI-LL mismatch, center psoas, and apical position were all significantly greater in the TDS group than in the no-DS and SLDS groups (p < 0.05). DS severity was independently associated with 2.4-mm (95% CI 1.1–3.8 mm) center and 2.6-mm (95% CI 1.2–3.9 mm) apical psoas anterior displacement per increased grade (increasing from no DS to SLDS to TDS). CONCLUSIONS TDS represents more severe sagittal malalignment (PI-LL mismatch), pelvic compensation (PT), and changes in the psoas major muscle compared with no DS, and SLDS and is a risk factor for lumbar plexus injury during L4–5 LLIF due to a smaller safe zone.