Projection of the iliocaval confluence onto the lumbar spine as a function of pelvic incidence

Anterior lumbar interbody fusion requires careful preoperative planning because of the proximity of major vascular structures, particularly the iliocaval confluence (ICC). Although the vertical position of the ICC is known to vary, its relationship with spinopelvic morphology remains poorly defined. This study aimed to analyse the projection of the ICC onto the lumbar spine and its association with pelvic incidence (PI).

This retrospective, single-center study included 116 patients evaluated for planned lumbar fusion who underwent EOS imaging and lumbar CT angiography. Spinopelvic parameters (pelvic incidence, pelvic tilt, sacral slope) were measured. Patients were divided into low PI (< 55°) and high PI (≥ 55°) groups based on the measured average PI (µ_PI = 54.72° ± 12.93°). The vertical projection of the ICC was assessed using two quantitative ratios R1 and R2 and a qualitative classification relative to lumbar vertebral and disc levels. Correlations between ICC projection and spinopelvic parameters were analysed.

High PI patients demonstrated a more cranial ICC position, with higher R1 (0.85 ± 0.34 vs. 0.58 ± 0.26) and R2 (1.16 ± 0.47 vs. 0.78 ± 0.37) than Low PI patients. The distribution of visual ICC location also differed significantly between groups (p < 0.001). PI correlated positively with R1 (r = 0.465, p < 0.001) and R2 (r = 0.484, p < 0.001). PI remained independently associated with R1 and R2 when adjusted for age, sex, and BMI.

PI is significantly associated with the vertical projection of the ICC. Lower PI is associated with a more caudal ICC position relative to the lumbosacral junction, which may increase vascular constraints during anterior access to L5-S1. PI may therefore serve as a useful preoperative morphologic marker to identify patients in whom dedicated vascular imaging is most relevant.

Retrospective, single-center observational cohort study, level of evidence IV.