Classification of iliac vessels and selection of surgical approach and window in oblique lumbar interbody fusion at L5-S1 based on magnetic resonance imaging

Objective To investigate how the position of iliac vessels affects the choice of surgical approach (left or right oblique abdominal incision) and window (central or lateral) during L5-S1 oblique lumbar interbody fusion (OLIF), through imaging anatomy research. Methods CT and axial T2-weighted MRI at L5-S1 were used to evaluate the iliac vessels’ position and classify the central vascular window (CW). Measurements were taken for the central window (CW), distance from the midline of the L5-S1 disc to the vessel's medial surface (DV), and psoas to iliac vein distances (LPV/RPV). Classification was based on the location of the common iliac vein/internal iliac artery: The intervertebral space was divided evenly from anterior to posterior into four zones—I, II, III, and IV. Based on iliac vein positions, patients were classified as follows: Type A (both veins in Zone I, left I/right I), Type B (left I/right II), Type C (left II/right I), Type D (both veins in Zone II, left II/right II), and Other Type (veins in Zone III or IV). A comparative analysis of imaging findings was performed. Results 302 patients were included (124 Type A, 30 Type B, 62 Type C, 82 Type D), and the remaining 4 patients, whose iliac veins were located in Zone III or IV, were defined as Other Type. Gender differences were observed, with Type A predominantly female. Type A had the smallest CW, while Type D had the largest. Type D showed the largest DV and RDV compared to other types. LPV and RPV were statistically significant between groups, with Type A and B having greater LPV than C and D. Conclusions Classification based on iliac vessel positioning helps determine the optimal approach for OLIF: Left ATP-OLIF is preferred for Type A and B; Right ATP-OLIF or Left O-ALIF for Type C; Left O-ALIF for Type D. The classification simplifies approach selection, minimizing vascular and nerve damage.