How Does the Lordosis Apex, Lordosis Arcs, and Inflection Point According to Roussouly Predict Outcomes After Adult Spinal Deformity Surgery?

Study Design. Retrospective cohort study Objectives. In adult spinal deformity (ASD) surgery patients, we sought to:1)report preoperative/postoperative lordosis apex, number-of-vertebrae in lower/upper lordosis arc, and inflection point, and 2)determine their impact on postoperative outcomes. Summary of Backgrounds Data. Impact of lordosis apex, arcs, and inflection point on postoperative outcomes remains unclear. Methods. ASD patients (2009–2021) with ≥5-level fusion, sagittal/coronal deformity, and ≥2-year follow-up was analyzed. Primary exposures were pre/postoperative lordosis apex, vertebrae in upper/lower arcs, and inflection point. Outcomes included mechanical complications, reoperations, patient-reported outcome measures, and postoperative alignment. Multivariable regression controlled for age, body mass index (BMI), and comorbidities. Results. Among 202 patients (mean age:64.4±16.7 y,77.2% females): Lordosis Apex: Most common preoperative apex was L5(32.7%), followed by L4(20.3%). Postoperatively, 125(61.9%) had an apex change—89(71%) cranially-directed and 36(29%) caudally-directed. Cranially shifts led to 6.3±14.1° decrease in L4-S1 lordosis, caudal change showed 3.7±13.9° increase( P =0.002). Lordosis Arcs: Mean vertebrae in lower and upper lordotic arcs were 1.4±1.0 and 2.6±1.1, which postoperatively increased by 0.2±0.8 and 0.5±1.5( P =0.043), respectively. Greater increase in upper-arc vertebrae correlated with higher 2-year numeric rating scale (NRS)-back pain (ρ=0.020, P =0.030;β=0.40, 95%CI:0.03-0.78, P =0.036). Inflection Point: Preoperatively, 86(42.6%) patients had a T12/L1 inflection point, of which 72(83.7%) remained at T12/L1 postoperatively. Of 116(57.4%) patients with inflection point above/below T12/L1, 59(50.9%) transitioned to T12/L1 postoperatively. Preoperative inflection point above/below T12/L1 was linked to more spinopelvic complications (38.8% vs. 22.1%, P =0.012;OR=0.49, 95%CI:0.25-0.94, P =0.033). Postoperative T12/L1 inflection was associated with higher radiographic proximal junctional kyphosis (PJK) (56.0% vs. 40.8%, P =0.041;OR=1.96, 95%CI=1.03-3.72, P =0.040). Conclusion. After ASD surgery, most patients showed a cranial lordotic apex shift, with greater increase in upper than lower arc vertebrae—highlighting the difficulty of restoring lordosis caudally. Cranial apex shift was associated with smaller L4–S1 lordosis and greater 2-year back pain, while a preoperative inflection point outside T12/L1 increased the risk of spinopelvic complications. Incorporation of Roussouly principles may help spine surgeons improve outcomes and mitigate complications.