Frailty-Muscle Phenotypes Predict Outcomes After Lumbar Fusion in Adults Aged ≥75 Years: A Retrospective Cohort Study

Objective: To evaluate whether combining clinical frailty with magnetic resonance imaging (MRI)-derived posterior paraspinal muscle degeneration identifies perioperative risk phenotypes in adults aged ≥75 years undergoing lumbar fusion. Methods: We retrospectively studied patients aged ≥75 years undergoing lumbar fusion with preoperative lumbar MRI. Frailty was assessed using the Fried phenotype (frail: score ≥3). Posterior paraspinal muscle degeneration across L1-S1 was quantified using automated segmentation and a composite posterior frailty index (PFI); severe degeneration was defined as the upper quartile of PFI. Patients were classified into 4 frailty×muscle phenotypes. Primary outcomes were any in-hospital complication and prolonged length of stay (LOS ≥16 days). Results: Among 248 patients, phenotypes A-D (A, nonfrail/nonsevere; B, frail/nonsevere; C, nonfrail/severe; D, frail/severe) comprised 132, 54, 20, and 42 patients, respectively. Any in-hospital complication occurred in 18.2% of phenotype A compared with 50.0%-57.1% in phenotypes B-D (p<0.001). Prolonged LOS (≥16 days; cohort 75th percentile) occurred in 0.8% of phenotype A versus 38.9% (B), 35.0% (C), and 78.6% (D) (p<0.001), corresponding to absolute risk increases of +34.2 to +77.8 percentage points. After adjustment, higher-risk phenotypes remained independently associated with increased odds of any complication and prolonged LOS; however, the prolonged-LOS odds estimates were imprecise due to sparse events in the reference group. Phenotype was not independently associated with 90-day readmission. Pain improvement (ΔVAS [visual analogue scale]) was attenuated in phenotypes B and D, while differences in ΔODI (Oswestry Disability Index) were not statistically significant. Conclusion: Integrating frailty and MRI-based posterior paraspinal degeneration provides actionable stratification of complication and prolonged LOS risk after lumbar fusion in older adults.