Clinical Validation of A Novel Psoas Classification for Optimal L4 to L5 Access: OLIF versus LLIF

Study Design: Retrospective cohort study. Objective: To evaluate neurovascular complications of oblique lumbar interbody fusion (OLIF) and lateral lumbar interbody fusion (LLIF) at L4 to L5 stratified by psoas morphology using the Emami et al. classification system. Summary of Background Data: OLIF and LLIF are commonly used to treat L4 to L5 degenerative spondylolisthesis. The Emami et al. classification system categorizes the anterior-posterior position of the psoas muscle and may help guide surgical approaches to mitigate neurovascular risk. No prior study has correlated this classification system with neurovascular complication profiles. Methods: We retrospectively reviewed 350 patients who underwent single-level OLIF (n=178) or LLIF (n=172) at L4 to L5 for low-grade degenerative spondylolisthesis between 2012 and 2022. Preoperative axial T2-weighted MRI at the inferior endplate of L4 was used to classify psoas morphology: class A (>2 mm anterior to L4), class B (≤2 mm anterior or posterior to L4), and class C (>2 mm posterior to L4). Neurological and vascular complications were analyzed using multivariable logistic regression. Results: Neurological complications were more frequent in LLIF than OLIF (13.4% vs. 2.8%, P <0.001), particularly in class A (16.4% LLIF vs. 2.3% OLIF, P <0.001). Conversely, class C had fewer neurological injuries with LLIF (2.1%) than OLIF (8.9%) ( P =0.019). Vascular complications were more common in OLIF (5.1% vs. 1.2%, P =0.026), especially in class C (13.3% OLIF vs. 1.8% LLIF, P =0.011). Class A predicted neurological injury with LLIF (OR: 4.52, P <0.001), whereas class C predicted vascular injury with OLIF (OR: 5.29, P =0.003). Conclusions: Psoas morphology independently predicts approach-specific neurovascular complication risk. Class A favors OLIF due to lower neurological risk; class C favors LLIF due to lower vascular risk. Class B presents an intermediate risk. Preoperative psoas categorization using the Emami et al. classification system may optimize approach selection and improve patient safety.