Anterior to Psoas Corridor, Does It Really Exist?! An MRI Analysis

Technical Summary: Anterior to Psoas Corridor, Does It Really Exist?! An MRI Analysis

Problem Statement
Oblique Lumbar Interbody Fusion (OLIF) offers a minimally invasive anterolateral approach to the lumbar spine, utilizing a corridor between the great vessels and the psoas muscle to avoid direct manipulation of the lumbar plexus. However, the feasibility of this approach is constrained by the anatomical width of this corridor, which varies significantly by spinal level and patient anatomy. A critical limitation is that approximately 25% of patients may have a corridor obstructed by vascular structures or the psoas muscle, particularly at the L4–L5 level due to vascular bifurcation and psoas elevation. Furthermore, the "psoas maneuver" (retraction of the muscle to widen the corridor) carries risks of neurological injury, persistent pain, and motor dysfunction. While preoperative imaging is standard, there is a need to quantify how patient positioning (supine vs. surgical lateral decubitus) affects corridor dimensions and to determine the realistic limits for cage insertion without muscle retraction.

Methodology
This cross-sectional, comparative imaging study analyzed 25 adults (aged 18–70) with no history of anterior/lateral spinal surgery, significant scoliosis, or transitional vertebrae. Participants underwent axial T2-weighted MRI scans of the lumbar spine (L1–S1) in two distinct positions:

1. Dorsal Decubitus (DD): Supine position with hips and knees slightly flexed.
2. Right-Lateral Decubitus (RLD): Surgical position with hips and knees slightly flexed.

The primary metric was the "OLIF corridor width," defined as the linear distance from the lateral border of the inferior vena cava (or right iliac vein) to the medial border of the psoas muscle. Measurements were taken at levels L1–L2 through L4–L5. Statistical analysis employed permutation Welch t-tests to compare DD and RLD widths and linear mixed models to analyze level-to-level variations. The study also cross-tabulated corridor adequacy against standard cage widths (8 mm to 26 mm) to assess feasibility without psoas retraction.

Key Results

• Positional Impact: The RLD position produced significantly wider corridors than the DD position at L1–L2 (16.7 ± 5.7 mm vs. 13.2 ± 4.5 mm), L2–L3 (15.0 ± 5.6 mm vs. 13.2 ± 4.8 mm), and L4–L5 (11.9 ± 6.2 mm vs. 9.7 ± 5.7 mm). No statistically significant difference was observed at L3–L4.
• Caudal Trend: Within the RLD position, corridor width progressively declined from L1–L2 (16.7 mm) to L4–L5 (11.9 mm).
• Cage Feasibility: Without psoas retraction, the majority of corridors could accommodate cages up to 10 mm at all levels. However, the ability to accept an 18 mm cage dropped significantly at lower levels:
  • L1–L2: 52% of corridors were adequate.
  • L2–L3: 32% of corridors were adequate.
  • L3–L4: 12% of corridors were adequate.
  • L4–L5: Only 16% of corridors were adequate.
• Large Implants: Virtually no corridors (0% at L1–L2, L3–L4, and L4–L5) could accommodate a 26 mm cage without psoas mobilization.

Significance and Claims
The study concludes that the "anterior to psoas" corridor is not a uniform anatomical space but a variable window that narrows progressively from L1 to L5. The authors assert that even with optimal surgical positioning (RLD), fewer than half of the L4–L5 corridors can safely accept an 18 mm cage without psoas mobilization.

The paper emphasizes that preoperative MRI performed in the surgical position (lateral decubitus) is essential for accurate planning. Relying solely on supine imaging may overestimate the available working space. Consequently, the authors argue that surgeons must tailor cage width selection based on these specific measurements and be prepared to utilize psoas release or alternative approaches (such as LLIF) when the natural corridor is insufficient for the required implant size, thereby balancing the need for stability against the risk of neurological injury.

Limitations Noted by Authors
The authors acknowledge that the study did not assess the corridor with active psoas retraction, as this is a dynamic intraoperative event. Additionally, the study was exploratory without a formal sample size calculation, meaning the findings should be interpreted as hypothesis-generating signals rather than definitive confirmatory evidence.