Objective:

To examine whether sphincter contraction impairs intraluminal flow and contributes to drainage failure in anal fistulas.

Approach:

• Model Construction: A subject-specific fluid–structure interaction model was created from high-resolution pelvic MRI, incorporating a three-dimensional finite element model of the anal sphincter complex with a synthetic intersphincteric fistula tract.
• Simulation and Analysis: Time-dependent physiological loading simulated sphincter contraction, and bidirectional analyses quantified intraluminal velocity fields to identify low-velocity retention zones. Multi-threshold sensitivity analysis supported the robustness of low-velocity region identification.

Key Findings:

• Sphincter contraction induced extensive low-velocity regions despite anatomical patency.
• Low-velocity retention zones increased progressively during mid-loading.
• Connectivity analysis indicated an aggregation–retention–fragmentation cycle.

Interpretation:

The findings support a biomechanical hypothesis that sphincter contraction may lead to dynamic obstruction, impairing drainage in anal fistulas.

Limitations:

• The study is based on a synthetic model derived from a healthy volunteer, not patient-specific native fistula models.
• Results are qualitative proof-of-concept evidence and require validation in clinical studies.

Conclusion:

In this healthy-volunteer-derived, synthetic intersphincteric fistula model, sphincter contraction generated low-velocity retention zones despite preserved anatomical patency.