Sphincter-induced dynamic obstruction in a synthetic intersphincteric anal fistula model: a proof-of-concept fluid-structure interaction study.

Introduction: Perianal abscess often progresses to anal fistula despite timely drainage, and recurrence remains common after sphincter-preserving surgery. Although inadequate drainage is a key factor, existing explanations focus mainly on anatomy, while the role of anal sphincter activity is unclear. This study examined whether sphincter contraction impairs intraluminal flow and contributes to drainage failure. Methods: A subject-specific fluid-structure interaction model was constructed from high-resolution pelvic MRI. A three-dimensional finite element model of the anal sphincter complex with a synthetic intersphincteric fistula tract was reconstructed. Time-dependent physiological loading simulated sphincter contraction, and bidirectional analyses were performed. Intraluminal velocity fields were quantified using an image-based inverse mapping approach to characterize low-velocity retention zones. Results: Sphincter contraction induced extensive low-velocity regions despite anatomical patency, with a progressive increase during mid-loading. Connectivity analysis revealed an aggregation-retention-fragmentation cycle, and multi-threshold sensitivity analysis supported the robustness of low-velocity region identification. Conclusion: In this healthy-volunteer-derived, synthetic intersphincteric fistula model, sphincter contraction generated low-velocity retention zones despite preserved anatomical patency. These findings support a plausible biomechanical hypothesis for ineffective fistula drainage, but should be interpreted as qualitative proof-of-concept evidence pending validation in patient-specific native fistula models and clinical studies.