Position-associated intraluminal flow disturbance in idealized straight anal fistula tracts under sphincter loading: a clinically motivated proof-of-concept fluid–structure interaction study

Background Anal fistula management remains clinically challenging because durable eradication of fistulous sepsis must be balanced against preservation of sphincter function. Although current surgical concepts emphasize fistula anatomy, internal-opening management, and drainage of septic tracts, the potential contribution of sphincter-induced biomechanical loading to intraluminal flow organization remains insufficiently explored. This clinically motivated proof-of-concept study investigated whether tract position within the sphincter complex is associated with different flow-disturbance patterns under controlled idealized conditions. Methods A female anal sphincter complex was reconstructed from pelvic MRI using Mimics and 3-Matic. Two idealized straight cylindrical synthetic fistula tracts with an identical diameter of 3 mm were embedded at anterior and lateral positions within the same anatomical model. Bidirectional fluid–structure interaction (FSI) simulations were performed using ANSYS Workbench. Standardized pseudocolor velocity maps from 0.01 to 0.06 s were processed using lumen region-of-interest R O I definition, color-to-velocity mapping, normalized velocity-index calculation, low-velocity segmentation, connected-component analysis, axial segmentation, and temporal reorganization analysis. Results Both configurations showed very low Reynolds numbers, consistent with viscosity-dominated laminar flow. Compared with the lateral tract, the anterior tract showed a lower mean velocity index 0.431 vs. 0.547, lower image-derived mean velocity 3.08 × 10 −3 vs. 3.91 × 10 −3 m/s, and higher low-velocity fraction 0.613 vs. 0.419 using the primary threshold of 0.4× v max . The anterior tract also showed a larger normalized maximum connected low-velocity area 0.593 vs. 0.328 and a more broadly distributed axial low-velocity burden. The anterior configuration demonstrated earlier and more sustained low-velocity organization, whereas the lateral configuration showed relatively preserved early flow followed by greater late-phase expansion and temporal reorganization. Conclusion In this idealized single-anatomy model, tract position was associated with distinct intraluminal flow-disturbance descriptors under sphincter loading. These findings do not establish a clinical mechanism of recurrence or predict failure of sphincter-preserving procedures. Rather, they provide preliminary biomechanical evidence supporting future patient-specific studies that incorporate realistic tract geometries, male and female anatomies, posterior configurations, biological variability, and clinical validation.