Biomechanical Formation Mechanism of the “Avascular Plane” in the Pelvis and Retroperitoneum: Redefining Surgical Cleavage Planes Created by Fetal Poisson Compression and Shear Stress

Background: The "avascular plane," which surgeons navigate during total mesorectal excision (TME) and robot-assisted radical prostatectomy (RARP), has conventionally been explained as a static boundary, such as an "embryological plane" where different developmental compartments adjoin, or a "fusion fascia" where tissues contact during embryogenesis. However, these classical models harbor fundamental contradictions; they cannot causally explain clinical facts such as why blood vessels cannot invade despite the mesenchyme sharing the same mesodermal origin, why only nerves remain, the continuity between the lateral ligament and the neurovascular bundle (NVB), and the pronounced bilateral asymmetry of the prehypogastric nerve fascia. Methods: This study is a theoretical-clinical hybrid investigation that integrates anatomy, embryology, materials science, mechanical modeling, and clinical perspectives. By extracting the temporal gap between the lamination timing of fetal pelvic connective tissue (fibrous period: the period between 13 and 20 weeks of gestation when connective tissue becomes fibrous) and the timing of nerve wiring establishment, we constructed a fetal mechanical field model combining the poroelastic transition of the extracellular matrix (ECM), the anomalous Poisson effect (orthogonal compression), and shear stress caused by organ movement. Furthermore, we theoretically verified the density, lamination, bilateral asymmetry, and neurovascular condensation patterns of cleavage planes by evaluating the consistency between this mechanical model and numerous intraoperative observations during TME and RARP. Results: Due to the phase transition of the ECM and the stiffening of the truncal skeleton between 13 and 20 gestational weeks, multiaxial tension accumulates in the mesenchymal tissue of the retroperitoneum and pelvis, causing simultaneous lamination via anomalous Poisson compression. Shear stress associated with the cyclic volumetric changes of organs concentrates at the lamination boundaries, physically destabilizing the anchoring of capillaries, thereby perpetuating an avascular state. Conversely, nerves establish their wiring prior to lamination and remain safely in the gliding plane because they withstand shear stress due to the robust perineurium. The three-layered fascia of the presacral space is formed as a mechanically inevitable structure through tension shadowing and passive lamination. While the lateral ligament holds a clinical function in fixing the rectum, it is reasonable to interpret that it is essentially a shear refuge formed between the rectal proper fascia and the prehypogastric nerve fascia, acting as the proximal root of the NVB. On the left side, strong tension is applied due to the mechanical anchor effect of the sigmoid mesocolon, forming a thin, sharp membrane. In contrast, tension on the right side is weak, often resulting in an indistinct membranous structure. Conclusion: The avascular plane is not a static embryological boundary but a "biomechanical fault line" carved continuously by the tension field, Poisson compression, and shear stress generated during fetal life, which physically inhibit the anchoring of angiogenesis. The act of a surgeon selecting a safe cleavage plane and preserving nerves is nothing less than intraoperatively tracing the mechanical structures formed during fetal development. This study provides a novel anatomical paradigm that dramatically enhances the safety of pelvic surgery.