A Proof-of-Concept Finite Element Analysis of Biomechanical Changes Following Graded Anal Sphincter Injuries: Implications for Fistula Surgery

Objective The extent of anal sphincter complex (ASC) injury strongly influences postoperative continence after anal fistula surgery, yet the biomechanical consequences of different injury extents remain incompletely defined. This proof-of-concept study developed a subject-specific three-dimensional finite element model (FEM) of the male ASC to explore the quantitative biomechanical consequences of graded sphincter injuries and to identify potential mechanistic trends relevant to functional preservation. Methods A high-resolution pelvic MRI from a healthy male volunteer was segmented to reconstruct the internal anal sphincter (IAS), external anal sphincter (EAS), and puborectalis muscle (PRM). Five progressive injury scenarios were simulated, ranging from distal one-third EAS division to complete EAS+PRM disruption, using literature-derived material properties and anorectal manometry–inspired pressure loads. Probe surface stress and inner anal-wall displacement were computed, and normalized probe-stress retention relative to the intact model was used as a surrogate measure of preserved biomechanical capacity. Results Normalized probe-stress retention declined with injury severity, from 72.9% for distal one-third EAS injury to 59.3% for complete EAS+PRM loss. Distal one-third EAS injury showed the smallest reduction in surrogate biomechanical capacity among all scenarios. Injuries involving distal two-thirds EAS damage, complete EAS disruption, and complete EAS plus partial PRM injury showed similar normalized probe-stress retention values of 64.2%, 63.8%, and 63.6%, respectively, suggesting an apparent biomechanical plateau under the assumptions of this model. Complete EAS+PRM loss showed the lowest retention. Conclusions This proof-of-concept FEM identifies preliminary biomechanical trends across graded ASC injury scenarios. Distal one-third EAS involvement showed relatively preserved surrogate biomechanical capacity, consistent with the current guideline rationale for classifying limited EAS involvement as lower risk in selected patients. Injuries beyond this range showed an apparent plateau, whereas complete EAS+PRM loss showed the lowest stress retention. These findings should be interpreted as hypothesis-generating biomechanical trends derived from a single-subject model rather than validated clinical thresholds. This study provides a preliminary quantitative framework for exploring associations between modeled sphincter injury extent and surrogate biomechanical function, supporting future patient-specific validation.