1229 Number of Fused Levels is Inversely Associated With Long Term Radiographic Subsidence in Lateral Lumbar Interbody Fusion Without Posterior Instrumentation

INTRODUCTION: Multilevel lateral lumbar interbody fusion (LLIF) without posterior instrumentation is a minimally invasive option for various spinal pathologies. Cage subsidence, however, has been reported in up to 30% of patients. METHODS: A review of patients who underwent LLIF between 2014 and 2022 at a single quaternary referral center was conducted. All patients had postoperative CT scans at one year. Patients with posterior instrumented fusion were excluded. Age, sex, BMI, smoking status, number of operative levels, cage width, cage material, lateral plate fixation, and operative level bone density (Hounsfield units) were collected. Post-operative implant subsidence at long term follow-up was graded according to Marchi criteria by two separate spine surgeons. A multivariate logistic regression with respect to cage subsidence on a per level basis was performed utilizing the aforementioned covariates. RESULTS: 144 patients (211 surgical levels) met inclusion criteria. LLIF with lateral plate and screw fixation was performed in 65 patients (45%) and 79 patients (55%) underwent standalone LLIF. Average radiographic follow-up was 24.1 months. 12 levels in patients with multilevel LLIF had long term radiographic subsidence versus 15 single levels (12/115, 10% vs 15/96 15%). After multivariate analysis, porous titanium cages (vs PEEK, OR 0.41, 95% CI 0.18 to .96, p=.039) and number of fused levels (OR 0.60, 95% CI 0.35 to 0.97, p=.048) were associated with a reduced likelihood of long-term radiographic subsidence on a per level basis. CONCLUSIONS: At long-term follow up, the number of LLIF levels is inversely associated with radiographic subsidence on a per level basis. In multilevel LLIF cases, we recommend porous titanium cages to further reduce the risk of long-term radiographic subsidence at each level.