Surgical Intensity and Survival After Surgery for Extradural Spinal Metastases: Mechanical Instability, Tumor Biology, and Systemic Reserve

Background/Objectives: Surgery for extradural spinal metastases must balance neurologic and mechanical goals against limited survival. We evaluated whether a predefined three-tier surgical-intensity classification aligned with established measures of operative burden and whether intensity was associated with 90-day mortality or overall survival. Methods: This retrospective cohort included 141 adults with extradural spinal metastases operated on between June 2020 and December 2025. Surgical intensity was classified as low, moderate, or high. SMII was calculated for all patients as a secondary convergent construct measure. Firth penalized logistic regression and Cox regression adjusted for performance status, albumin, and tumor biology. Results: There were 37 low-, 39 moderate-, and 65 high-intensity procedures. Median SMII increased from 6 [5–11] to 14 [11.5–18.5] and 23 [18–28] across the three groups (p < 0.001; Spearman ρ = 0.711). High-intensity surgery was concentrated among unstable SINS lesions and was associated with longer operative time, transfusion, and hospitalization. Ninety-day mortality was 24.3%, 12.8%, and 23.1% for low-, moderate-, and high-intensity procedures, respectively. High intensity was not statistically associated with 90-day mortality (adjusted OR 0.81, 95% CI 0.28–2.36) or overall survival (adjusted HR 1.08, 95% CI 0.65–1.80). ECOG ≥ 3 and aggressive solid/unknown tumor biology were associated with worse outcomes. Conclusions: The intensity classification showed strong convergent validity with SMII and perioperative burden. Within this surgically selected cohort, intensity was not independently associated with early mortality or overall survival, although clinically meaningful differences cannot be excluded. Operative magnitude should be interpreted alongside mechanical need, systemic reserve, and tumor biology.