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 BACKGROUND DATA: The impact of lordosis apex, arcs, and inflection point on postoperative outcomes remains unclear. MATERIALS AND METHODS: ASD patients (2009-2021) with ≥5-level fusion, sagittal/coronal deformity, and ≥2-year follow-up were 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 yr, 77.2% females): Lordosis apex: the 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 two-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 an 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 a 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 two-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.