Rod Geometry and Material Characteristics as Risk Factors for Rod Breakage After Adult Spinal Deformity Surgery: A Multicenter Cohort Study with Five-Year Follow-up.

Abstract

Background Context Rod breakage is a frequent and clinically important late complication after adult spine deformity (ASD) surgery, yet most research has focused on patient and alignment factors rather than implant geometry. Purpose This study aimed to investigate the rod-related characteristics independently associated with rod breakage following ASD surgery, with particular focus on quantitative measures of rod material, rod shape and its postoperative evolution. Design Retrospective analysis of prospectively collected, six-center data. Patient Sample ASD patients, who underwent long posterior fusion to the pelvis with available serial full-length lateral radiographs at 6 weeks, 6 months, 1 year, and 2 years were included; patients with multiple-rod constructs were excluded. Minimum two-year follow-up. Outcome Measures Rod breakage (defined radiographically), time to rod breakage, and discrimination performance of a multivariable predictive model based on patient and rod characteristics. Methods Rod curvature was quantified at L1-S1 and L4-S1 and the maximum positive increase in L1-S1 curvature between consecutive postoperative timepoints was derived. Time-to-breakage was analyzed using Kaplan-Meier and Cox proportional hazards models, with data-driven thresholds defining high-risk geometry. Model discrimination was assessed using the concordance index. Results Eighty-three patients (mean follow-up 60.0 ± 23.0 months) were analyzed; 30 developed rod breakage at a mean 41.7 ± 22.2 months. L4-S1 rod curvature ≥33° at 6 weeks was associated with higher risk of breakage (HR 2.25; 95% CI 1.06-4.77; p = 0.036), as was a maximum L1-S1 curvature increase≤4° (HR 3.33; 95% CI 1.25-8.84; p = 0.016). Titanium rods had lower risk compared with cobalt-chromium (HR for titanium 0.50; p = 0.02), and diameter >5.5 mm was associated with increased risk (HR 1.90; p = 0.02). A multivariable patient-level model incorporating rod geometry showed improved discrimination (C-index 0.74). Conclusions Specific rod geometry characteristics-particularly early lower lumbar curvature and limited postoperative curvature increase-were independently associated with rod breakage after ASD surgery. Integrating rod shape into risk models may support implant selection, surgical planning, and postoperative monitoring. Level of Evidence Level III, retrospective cohort study.