Psoas Atrophy Following Lateral Lumbar Interbody Fusion: A Retrospective Case Series

Background: The direct muscular trauma caused by the trans-psoas approach of a lateral lumbar interbody fusion (LLIF) is a potential reason for its high rates of postoperative psoas weakness. Purpose: We sought to compare preoperative and postoperative psoas muscle parameters to evaluate iatrogenic psoas atrophy following LLIF. Methods: We conducted a retrospective case series including patients who underwent primary LLIF at a single tertiary academic center between 2012 and 2024. Exclusion criteria included history of spinal deformity, spine surgery, neuromuscular disorders, or LLIF indicated for infection or malignancy. Participants had both preoperative and postoperative magnetic resonance imaging (MRI) scans; the latter were obtained at least 3 months postoperatively. Psoas muscle atrophy was assessed using MRI measurements of cross-sectional area (CSA), anteroposterior (AP) and mediolateral diameters, and Goutallier classification for fatty infiltration. Paired and Welch’s t -tests compared postoperative to preoperative and contralateral values. Subgroup analysis compared patients with and without postoperative hip flexor weakness. Results: Forty patients met inclusion criteria (mean follow-up: 25 months). The ipsilateral psoas demonstrated significant reduction in CSA (−117 vs −22.7 mm 2 ) and AP diameter (−2.43 vs 0.34 mm) compared with the contralateral side. Six patients (15%) had persistent hip flexor weakness. On multivariate analysis, male sex and body mass index (BMI) were risk factors for psoas atrophy. Diabetes and length of stay predicted postoperative hip flexor weakness. Conclusions: This retrospective study identified evidence of long-term ipsilateral psoas atrophy following LLIF, potentially contributing to postoperative hip flexor weakness. Male sex, BMI, diabetes, and length of hospital stay were identified as predictors of psoas atrophy or weakness, highlighting the importance of postoperative rehabilitation focusing on psoas preservation. Future studies are warranted to further evaluate potential risk factors for postoperative hip flexor weakness following LLIF. Level of Evidence: Level IV: retrospective case series.