If the fistula is superficial it can be 'laid open'. This is called a fistulotomy. The fistula is opened up so that it can heal from inside out. Sometimes it is necessary to leave a thread of e. g. suture material in the fistula tract. This is called a seton and may need to remain in place for some time. A dressing is usually worn over the fistula until it has fully healed.
Attribution: BUPA