Cage Overhang Ratio and Increased Radiographic Subsidence in Stand-Alone Lateral Lumbar Interbody Fusion.

Abstract

Lateral lumbar interbody fusion (LLIF) is increasingly utilized as a minimally invasive means of stand-alone arthrodesis. This study aims to evaluate the effect of endplate overhang on long-term radiographic implant subsidence in patients following stand-alone LLIF. Patients who underwent LLIF between 2014 and 2022 at two quaternary referral centers were reviewed. Age, sex, BMI, smoking status, number of operative levels, cage material, average operative level bone density (Hounsfield units), cage length, width, and endplate length were collected. The overhang ratio was calculated as the total cage area divided by the area in contact with the endplate normalized to 10. Post-operative implant subsidence at long-term follow-up was graded according to Marchi criteria by two separate spine surgeons. Univariate and multivariable firth regression were used in Python version 3.10.18, P<0.05 was considered to be significant. 144 patients (218 levels) met the inclusion criteria. In univariate regression, PEEK cage material (p=0.033), overhang (p=0.041), overhang area (p=0.028), and overhang ratio (p=0.033) were significantly associated with greater odds of subsidence, while lateral plate fixation was associated with decreased odds (p=0.013). Age (OR: 1.047, p=0.044), PEEK cage material (OR: 3.674, p=0.007), lateral plate fixation (OR: 0.271, p=0.007) and overhang ratio (OR: 1.943, p=0.007) were associated with increased odds of subsidence on multivariable regression. LLIF cages with increased overhang ratio were associated with a higher rate of radiographic subsidence at long-term follow-up. Subsidence is reduced by choosing a cage that most closely matches the endplate length, minimizing the overhang ratio.