Magnetic Resonance Imaging Characteristics and Age-Related Changes in the Psoas Muscle: Analysis of 164 Patients with Back Pain and Balanced Lumbar Sagittal Alignment

OBJECTIVE: The psous muscle (PS), 1 of the para-vertebral core muscles, is associated with sarcopenia. It also has clinical relevance in lateral-access spinal surgery (LASS) as a determinant structure affecting the operative window. We aimed to identify age-related patterns of PS degeneration, and we propose that our results be used to evaluate the operative window in LASS. METHODS: We included 164 participants with back pain, no leg symptoms or claudication, and normal lumbar lordosis and sagittal balance. We evaluated the cross-sectional morphology of the PS on magnetic resonance imaging, specifically assessing the anterior to posterior (AP)/medial to lateral (ML) ratio and the cross-sectional area (CSA). We assessed the locational relationship of the PS and the intervertebral disc using the anterior margin gap (AMG; the distance between the anterior margins of the PS and the intervertebral disc) and the center gap, and compared all measurements by surgical level, sex, and age group. RESULTS: At the L2-3 to L4-5 levels, the PS showed a decreased AP/ML ratio, increased CSA, ventral retraction of the anterior margin without center shift, and decreased operative window length. The degeneration patterns were decreased ML width and CSA and dorsal retraction of the anterior margin. Youth, male sex, and lower lumbar level were associated with higher AMGs, indicating an increased need for the transpsoas approach in LASS. CONCLUSIONS: In patients without sagittal imbalance, the PS showed significant imaging characteristics. Our detailed data may aid the identification of degeneration patterns and specific preoperative planning regarding the operative window for LASS.