Supplementary material from "Flow simulations of rectal evacuation: towards a quantitative evaluation from video defecography"

Mechanistic understanding of anorectal (patho)physiology is missing to improve the medical care of patients suffering from defecation disorders. Our objective is to show that complex fluid dynamics modelling of video defecography may open new perspectives in the diagnosis of defecation disorders. Based on standard X-ray video defecographies, we developed a bi-dimensional patient-specific simulation of the expulsion of soft materials, the faeces, by the rectum. The model quantified velocity, pressure and stress fields during the defecation of a neostool with soft stool-like rheology for patients showing normal and pathological defecatory function. In normal defecation, the proximal–distal pressure gradient resulted from both the anorectal junction which formed a converging channel and the anal canal. The flow of the neostool through these anatomical parts was dominated by its shear-thinning viscous properties, rather than its yield stress. Consequently, the evacuation flow rate was significantly affected by variations in pressure applied by the rectum, and much less by the geometry of the anorectal junction. Lastly, we simulated impaired defecations in the absence of obvious obstructive phenomena. Comparison with normal defecation allowed us to discuss critical elements which should lead to effective medical management.