Anal fissure is a common anal canal and anal disease, which is a small ulcer formed after the skin layer of the anal canal below the dentate line is cracked. It is oriented parallel to the longitudinal axis of the anal canal, is about 0.5-1.0 cm long, poke-shaped or oval, and often causes severe perianal pain. The majority of fissures are located in the posterior midline of the anal canal, but can also be in the anterior midline. If fissures occur laterally, inflammatory bowel diseases (such as tuberculosis, ulcerative colitis and Crohn’s disease) or tumors should be considered. The etiology of anal fissures is unknown and may be related to a variety of factors. Mechanical trauma during defecation caused by chronic constipation and dry stools is the direct cause of most anal fissures. There are several theories about the pathogenesis of anal fissures: 1. Anatomical defects theory: Anal fissures mostly occur in the posterior part of the anus, mainly because the superficial layer of the external sphincter forms a “Y” shaped weak zone in the front and rear of the anal canal, and the puborectalis muscle strengthens both sides of the canal. Therefore, the posterior and anterior portions of the anal canal are not as strong as the two sides. The anal canal forms an angle with the posterior rectal wall after going down. When defecating, the pressure of dry stool on the weak area behind the anal canal is the greatest, and because of the poor blood flow and less elasticity in the midline after the anal canal, the back of the anal canal is easily injured and torn. This is the reason why the location of anal fissure fissures often occur in the rear. 2, infection theory: the upper end of the anal canal at the dentate line of special structures such as anal sinus, anal flap, anal papilla, anal gland, etc., easily damaged by the dry hard fecal mass discharge, so that local infection and inflammation, or lead to anal gland obstruction pus, ulcerated to form anal fissure, due to local inflammatory stimulation, sphincter spasm, blood supply is insufficient, the wound does not heal for a long time, forming a chronic ulcer. 3, sphincter spasm theory: the lower part of the external anal sphincter skin due to pathological factors, fibrosis and loss of soft characteristics, in the lower part of the shallow “Y”-shaped weak area of the external anal sphincter formed inelastic transverse gate, when the dry hard fecal mass through the anal canal, this weak area is prone to rupture and fissure, the fissure does not heal over time to form a chronic ulcer. 4, trauma theory – anal slit theory: most scholars believe that the dry hard fecal mass through the anal canal, easy to cause anal canal tear, especially the anal congenital dysplasia, more vulnerable to injury.