L5 Is a Reliable Lower Instrumented Vertebra in Selected Adult Spinal Deformity Patients

Abstract

Study Design Retrospective analysis of a multicenter prospective adult spinal deformity (ASD) database. Objective In ASD surgery, fusion to the sacropelvis improves stability but increases morbidity. Stopping at L5 preserves motion but predisposes to distal junctional failure (DJF). This study aims to identify risk factors for DJF in ASD patients undergoing posterior fusion with lowest instrumented vertebra (LIV) at L5. Methods Patients undergoing posterior fusion with LIV-L5 and minimum two-years follow-up were included. Demographic data, radiographic parameters, and patient-reported outcome measures were collected. The primary endpoint was surgical extension to the pelvis (EP) for DJF. Patients were divided into EP and NoEP. Survival analysis and multivariate regression were used to identify predictors of DJF and postoperative malalignment. Results Eighty-one patients met inclusion criteria, with a mean follow-up of 4.50 years. Fifteen patients (18.5%) required EP due to DJF. The EP group demonstrated significantly worse preoperative global alignment, including higher sagittal vertical axis (SVA) and global tilt. Postoperative SVA was a strong predictor of DJF: patients with postoperative SVA >45 mm had a 5-year survival free from EP of 48.6%, compared with 95.1% in well-aligned patients (p<0.001). Multivariate analysis identified older age and higher preoperative SVA as independent risk factors for postoperative malalignment. Conclusions Choosing L5 as LIV is safe in well-aligned ASD patients, with approximately 80% remaining free from pelvic extension at five years. Inadequate postoperative sagittal correction, significantly increases the risk of DJF. Older patients with preoperative malalignment may benefit from primary fusion to the sacrum.