Morphological changes in the psoas major muscle and postoperative symptoms following extreme lateral interbody fusion: a retrospective study

Background: Extreme lateral interbody fusion (XLIF) is a widely used surgical approach for spinal disorders; however, its impact on the psoas major (PM) muscle and surrounding structures remains underexplored. The aim of this study is to evaluate the morphological changes in the PM muscle following XLIF and their association with postoperative symptoms. Materials and Methods: This retrospective study included 30 patients who underwent XLIF (2019–2022) across 46 intervertebral levels. Pre- and postoperative CT and MRI scans were used to measure PM muscle CSA at the surgical and adjacent levels. Fatty infiltration in PPM was qualitatively assessed. Clinical data were analyzed to examine correlations between morphological changes and postoperative symptoms. Results: Significant reductions in the PM muscle CSA were observed at the operated level and adjacent levels ( P < 0.001). Reduced CSA correlated with higher risks of post-operative symptoms both above and below the surgical site ( P = 0.049 and P = 0.037). Contralateral PM CSA increased without instrumentation ( P = 0.003) and decreased with instrumentation ( P = 0.033). Fatty infiltration in PPM showed a craniocaudal pattern and increased significantly ( P < 0.001 and P = 0.026). Conclusion: XLIF significantly alters PM muscle morphology and increases fatty infiltration in surrounding muscles, which are associated with postoperative symptoms. These findings emphasize the importance of addressing muscle preservation and recovery strategies to optimize surgical outcomes. Future research should explore interventions to mitigate these morphological changes.