Does pre-operative psoas muscle cross-sectional area predict post-operative outcomes following lumbar spine fusion surgery?-a systematic review and narrative synthesis.

BACKGROUND: Low back pain remains a leading cause of disability globally, with surgical interventions commonly utilised as treatment. Rates of lumbar fusion surgery continue to rise, but poor outcomes remain common. Preoperative psoas muscle morphology has been proposed as a potential predictor of postoperative success. Evidence supporting a link between psoas morphology and outcomes post-fusion surgery is growing yet remains unclear. This systematic review and narrative synthesis aimed to evaluate whether pre-operative psoas muscle morphology predicts clinical and radiographic outcomes following lumbar fusion surgery. METHODS: Five databases (MEDLINE, PubMed, CINAHL, Web of Science, and SportDiscus) were searched from inception to June 2025. Included studies assessed pre-operative psoas morphology [cross-sectional area (CSA), volume] and reported post-operative clinical (pain, disability, quality of life) or radiographic outcomes (fusion rates, alignment, complications). Risk of bias was assessed using Quality in Prognostic Studies (QUIPS), and certainty of evidence evaluated using Grading of Recommendations, Assessment, Development, and Evaluations (GRADE). RESULTS: Sixteen retrospective cohort studies encompassing 2,250 patients were included. Overall, data indicates that psoas morphology is not a consistent predictor of outcomes following fusion procedures. A limited number of low-quality studies, however, suggest that the surgical approach may influence this relationship. Posterior fusion techniques showed some association where a larger psoas was linked to early improvements in pain and function, while lateral and anterior approaches demonstrated no relationship. CONCLUSIONS: Based on very low to low certainty evidence, routine pre-operative psoas assessment is not currently justified for predicting lumbar fusion outcomes. Higher-quality prospective studies are needed to clarify the prognostic value of psoas morphology in spine surgery.