The main complication of anal fissure is that it fails to heal, resulting in a chronic anal fissure. This can happen if the tear extends into the ring of muscle that holds your anus closed (internal sphincter muscle) or if the pain causes the internal sphincter muscle to go into spasm. This can increase the pressure within your anus and make it even harder to have a bowel movement. The more you strain, the less likely it is that the tear will heal. A chronic anal fissure may require surgery to help it heal.
Attribution: BUPA