Lateral lumbar interbody fusion (LLIF) has transformed minimally invasive spine surgery by establishing a transpsoas lateral corridor to the disc space. However, the spatial relationship between the kidney and the psoas muscle (PM) at upper lumbar levels remains incompletely characterized. This study aims to provide a comprehensive radioanatomical map of the renal-psoas interface and establish a quantitative surgical safety algorithm for the upper lumbar levels (L2–L4).

A retrospective evaluation of 100 abdominal computed tomography angiography (CTA) scans (200 sides). Morphometric measurements included the minimum kidney-PM distance at the L2–L3 and L3–L4 disc levels, the percentage of PM overlap by the kidney, and renal vascular parameters (artery origin, length, and presence of variants). Statistical analysis utilized percentile analysis, and Receiver Operating Characteristic (ROC) curves were employed to assess if renal artery (RA) length could predict high-risk anatomy.

The kidney was related to the PM in 94% of sides at L2–L3 and 47.5% at L3–L4. The 5th percentile safety threshold for the kidney-PM distance at both levels was 0.0 mm, indicating direct parenchymal contact in a significant portion of the population. Females exhibited significantly shorter kidney-PM distances (3.26 mm vs. 7.33 mm at L3–L4, p = 0.006) and higher rates of 100% psoas overlap (61.1% vs. 38.2%). Cases with 100% overlap demonstrated the closest kidney-PM distances (3.76 ± 0.5 mm). ROC analysis showed that RA length was a poor predictor of high-risk anatomy (AUC = 0.44).

A three-type classification system according to our measurements was proposed for the determination of surgical risk. High-risk renal anatomy (type 3) was the majority (65.5%) in the lateral surgical corridor, particularly among female patients. Because vascular morphometry does not reliably predict kidney position, individualized cross-sectional imaging is essential to mitigate the risk of occult renal injury during the lateral approach.