The mechanics of the anal sphincter complex

The anal sphincter complex consists of circumferentially arranged muscle fibres, which surround a relatively thick anal lining. This apparatus was modelled mechanistically as two concentric homogeneous isotropic linear elastic cylinders. The inner cylinder (anal lining) was considered to be thick walled, while the outer (the circular muscle) was assumed to be thin walled. The model predicts that the anal sphincter tension varies linearly with luminal diameter. This prediction was confirmed experimentally under normal conditions as well as during external sphincter contraction and internal sphincter relaxation. Under conditions of negligible hoop stress in the anal lining the model also predicts that the intra-luminal pressure falls to zero before the luminal diameter reaches zero. Hence, an autoregulatory mechanism of anal cushion thickening, as the luminal pressure falls to zero, to produce anal closure was proposed. Deficiencies in this autoregulation mechanism may explain anal incontinence and the obstructed defaecation often found in subjects with haemorrhoids.