Inflammatory bowel disease (Crohn's and ulcerative colitis) or previous surgery are common causes. Anal fissure: A tear in the lining of the. IBD Surgery: Perianal abscess and fistula Default profile photo When there is tear in anus due to constipation or hard stools, I sometime. Most patients with rectal cancer undergo surgery about eight weeks after finishing chemotherapy and/or radiation therapy. About 20 percent of patients do not go.