Postoperative Muscle Loss and Preoperative Sarcopenia Predict Survival and Morbidity After Surgical Stabilization for Spinal Metastases: A Longitudinal CT-Morphometric Analysis of 285 Patients

BACKGROUND CONTEXT: Sarcopenia is associated with adverse outcomes in oncologic populations, yet the postoperative trajectory of skeletal muscle following spine surgery for metastatic disease remains poorly characterized. PURPOSE: To quantify postoperative changes in CT-based body composition after surgical stabilization for spinal metastases and to evaluate the prognostic impact of preoperative sarcopenia and postoperative muscle decline. STUDY DESIGN/SETTING: Retrospective cohort study at a tertiary spine center (2016-2024). PATIENT SAMPLE: 285 adults undergoing surgical stabilization for spinal metastases with preoperative CT (tCT1) and routine follow-up CT at approximately 9 months (tCT2). OUTCOME MEASURES: Overall survival, length of stay, postoperative complications, ECOG performance status, and CT-based morphometric parameters at L3 (SMI, PSMI, PMI, VAT, SMD). METHODS: CT morphometry was performed using standardized protocols. Preoperative sarcopenia was defined using validated sex-specific SMI thresholds. ROC analysis identified ≥15% postoperative SMI decline as the optimal survival-related cutoff. Multivariable Cox models adjusted for clinical and oncologic covariates. A non-oncologic fracture cohort undergoing lumbar stabilization was included to differentiate surgery-related muscle loss from cancer-related systemic catabolism. RESULTS: Skeletal muscle mass and quality declined substantially after surgery, particularly following lumbar stabilization. Minimally invasive procedures were associated with significantly less muscle loss than open approaches (SMI decline -24% vs. -40%, p<0.0001). Compared with fracture controls, metastatic patients demonstrated markedly greater losses in muscle indices and a distinct pattern of visceral fat depletion. An SMI decline ≥15% independently predicted reduced survival (median 284 vs. 730 days; HR 4.78) and poorer functional status. Preoperative sarcopenia, present in one-third of patients, also independently predicted mortality (HR 4.01), prolonged hospitalization, and higher complication rates. CONCLUSIONS: Postoperative skeletal muscle loss after stabilization for spinal metastases is frequent and clinically meaningful. Both preoperative sarcopenia and postoperative SMI decline independently predict survival, supporting routine morphometric assessment and the adoption of muscle-sparing surgical and rehabilitation strategies.