Anterior Lumbar Plexus Position on Preoperative Supine MRI Is Associated With Postoperative Motor Deficits After Lateral Lumbar Interbody Fusion at L4-5

Study Design. Retrospective propensity score-matched case-control study. Objective. To evaluate whether preoperative MRI-based psoas morphology, lumbar plexus position, iliac crest height, and psoas cross-sectional area (CSA) are associated with postoperative motor deficits (PMDs) following standalone lateral lumbar interbody fusion (SA-LLIF) at L4-L5. Summary of Background Data. PMDs remain a recognized complication of LLIF. Although anatomical characteristics have been implicated in neurological risk, the clinical value of classifying these features on preoperative MRI remains incompletely understood. Methods. Patients who developed PMDs after L4-L5 SA-LLIF were propensity score matched to patients without PMDs. Preoperative supine MRI was used to assess operative-side psoas morphology (Amaral classification), lumbar plexus position (Moro zones), iliac crest height, and absolute and height-adjusted psoas CSA. Conditional logistic regression analyses, including a multivariable model incorporating both psoas morphology and lumbar plexus position, were performed to evaluate their associations with PMDs. Results. A total of 172 matched patients were included. An anterior lumbar plexus position extending beyond the midpoint of the vertebral body on preoperative supine MRI (Moro Zones 1-2) remained independently associated with PMDs after adjustment for psoas morphology (OR 14.37, 95% CI 1.78-115.95; P =0.012). Although Type III psoas morphology according to the Amaral classification was associated with increased odds of PMDs in univariable analysis (OR 4.77, 95% CI 1.65-13.76; P =0.004), this association was no longer statistically significant after adjustment for lumbar plexus position (OR 2.54, 95% CI 0.90-7.18; P =0.078). Iliac crest height, psoas CSA, and height-adjusted psoas CSA were not associated with PMDs. Conclusions. Psoas morphology and lumbar plexus position on preoperative supine MRI were associated with PMDs following L4-L5 SA-LLIF. An anterior lumbar plexus position remained independently associated with PMDs after adjustment for psoas morphology. These findings support preoperative MRI-based anatomical assessment for neurological risk stratification and patient counseling.