148 Impact of Lateral Plate and Screw Fixation on Long Term Radiographic Subsidence in Lateral Lumbar Interbody Fusion

INTRODUCTION: LLIF can be performed in a stand-alone manner, with lateral plate and screw fixation, or with supplemental posterior instrumented fusion. In vitro biomechanical data suggests lateral plate fixation limits segmental motion. 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, cage material, 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 stepwise multivariate logistic regression was performed. 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. On univariate analysis, age, sex, smoking status, operative level bone density, and BMI were not significantly associated with radiographic subsidence on a per level basis (p>0.05). Multivariable logistic regression revealed 3D printed titanium cages were significantly associated with a reduced risk of subsidence (OR 0.40, p=0.0112). Lateral plate fixation was strongly associated (OR 0.484, p=.102) with reduced subsidence. 4 cases (3.5%) of grade two (severe) subsidence occurred in stand-alone LLIF patients. One delayed vertebral body fracture occurred in a lateral plate patient. CONCLUSIONS: LLIF with lateral plate and screw fixation is strongly associated with a lower rate of radiographic subsidence at long term follow up when compared with stand-alone LLIF. Our results confirm the superiority of 3D printed porous titanium cages over PEEK cages. Lateral plate and screw fixation appears to protect against severe (grade two or worse) radiographic subsidence.