1079 Preoperative Inflection Point According to the Roussouly Classification Is a Predictor of Outcomes Following Adult Spinal Deformity Surgery

INTRODUCTION: According to the Roussouly classification, the spine’s inflection point is where lordosis transitions to kyphosis. METHODS: A single-institution, retrospective cohort study was performed for patients undergoing ASD surgery from 2009-21. Inclusion criteria were: =5-level fusion, sagittal/coronal deformity, and 2-year follow-up. The primary independent variable was the inflection point. Postoperative outcomes were overall mechanical complications, reoperations, proximal junction kyphosis, pseudarthrosis, rod fracture, spinopelvic complications, and patient-reported outcomes measures (PROMs) at 2-years. Univariate logistic regression was performed. RESULTS: Of 202 patients undergoing ASD surgery, 79 (39.1%) patients had preoperative inflection point at the classically-described thoracolumbar junction at T12 or L1, whereas 60.9% had an inflection point at the following levels: L2 (48.0%), L3 (17.9%), T11 (13.8%), L4 (12.2%), T10 (4.1%), T9 (3.3%), and T8 (0.8%). Postoperatively, 75/79(83.4%) which started at T12 or L1 remained unchanged, while 4 (5.1%) changed to be above T12. Conversely, of the 123 (60.1%) patients with inflection point either above T12 or below L1, 61(49.5%) transitioned to a T12 or L1 postoperatively. Patients with non-T12/L1 inflection points had a greater change in inflection points postoperatively (1.2 ± 1.8 vs. 0.3 ± 0.9 levels, p < 0.001). Patients with a preoperative inflection point above or below T12 or L1 had more spinopelvic complications than those with an inflection point at T12 or L1 (39.0% vs. 24.1%, p = 0.027). CONCLUSIONS: In patients undergoing ASD surgery, 61% had a preoperative inflection point above the classically-described T12 or L1 levels. Having a preoperative inflection point above or below T12 or L1 was associated with an increased risk of spinopelvic complications but not reoperation or PROMs.