Paraspinal Muscle Quality and Functional Outcome After Lumbar Spinal Stenosis Surgery

BACKGROUND: Outcomes after surgery for lumbar spinal stenosis (LSS) vary substantially. Paraspinal muscle quality may reflect functional reserve, but its prognostic value in LSS surgery remains incompletely defined. PURPOSE: To assess whether preoperative multifidus muscle fatty infiltration and psoas muscle size are associated with functional outcomes after LSS surgery. STUDY DESIGN: Retrospective cohort study. METHODS: Patients undergoing surgery for LSS were identified from the Danish National Spine Registry. Preoperative MRI was used to grade multifidus fatty infiltration using the Goutallier classification system and to measure psoas muscle index. The primary outcome was Oswestry Disability Index (ODI) at two-year follow-up. Secondary outcomes included change in ODI from baseline (ΔODI) and achievement of a minimal clinically important difference (MCID; ΔODI ≥12). RESULTS: A total of 344 patients were included. Psoas muscle index was not associated with ODI at two years (P=0.163), ΔODI (P=0.134), or MCID responder status (P=0.211). In contrast, increasing Goutallier grade was associated with worse ODI at two years and less improvement in ΔODI (both P=0.028), and reduced odds of achieving MCID (OR 0.73 per grade; P=0.026). CONCLUSION: Preoperative multifidus muscle fatty infiltration is associated with worse long-term functional outcome and a lower likelihood of clinically meaningful improvement after surgery for lumbar spinal stenosis in a dose-response manner, whereas psoas muscle index is not predictive of outcome. Assessment of paraspinal muscle quality on routine MRI may aid in preoperative risk stratification and patient counseling.