1216 Exploring Cage Subsidence Within the Lumbar Spine in LLIF Patients: A Machine Learning Segmentation Analysis

INTRODUCTION: Interbody subsidence is a prevalent complication post-lateral lumbar interbody fusion(LLIF) among patients with low bone mineral density(BMD). While existing metrics evaluate BMD, none offer the granularity to assess individual vertebrae. Utilizing a machine learning(ML)-based automated computed tomography(CT) segmentation model, we can now achieve precise volumetric assessment of paraspinal musculature as well as Hounsfield(HU) distribution throughout the spine and individual vertebrae, serving as a proxy for BMD. METHODS: A retrospective analysis of 34 LLIF patients without posterior fusion (2014-2022) at a single institution was conducted. DICOM files were segmented using an open-source ML-program and analyzed using Python 3.9.6. Post-operative implant subsidence, graded according to Marchi criteria by two separate spine surgeons, was evaluated. Parameters included, but not limited to, average upper and lower vertebra(UV/LV HU) and endplates(EP-HU) HUs adjacent to cage, average trabecular bone HUs adjacent to cage(Tb-HU), average vertebral volume from L1-L5(cm3), and average lumbar paraspinal muscle volume(cm3). RESULTS: Twelve patients (35%) received PEEK implants, the remaining 22 received titanium implants. While all vertebral density parameters exhibited lower HU values in the subsidence cohort (n=6; Age 70.3±6.95; BMI 31.0±6.35; Surgical Levels=8) compared to the non-subsidence cohort (n=28; Age 70.3±6.95; BMI 31.0±6.35;Surgical Levels=52), only endplate HUs was significantly correlated to subsidence (EP 444.2±110.44vs510.6±116.03, p=0.036). Volumetric parameters also demonstrated lower cm3 values in the subsidence group versus the non-subsidence group (Lumbar Vertebral Volume 38.4±10.30vs40.7±11.22,p=0.644; Paraspinal Volume 335.4±79.87vs388.1±110.62,p=0.364). CONCLUSIONS: Automated ML segmentation identified lower endplate HU as significantly correlated to cage subsidence following LLIF. Future pre-operative HU analysis may help avoid this common LLIF complication.