At a glance
An advancement flap is a surgical technique used to close the internal opening of an anal fistula. Instead of cutting through tissue to expose the tract — as in a fistulotomy — the surgeon creates a flap of healthy tissue and moves it to seal the opening from the inside.
This approach is particularly valuable for complex fistulas where cutting through the sphincter would risk changes to bowel control. The flap preserves the muscle while closing the source of the problem.
This guide focuses on the mechanism — how the procedure actually works and why. For recovery information, see our guides on advancement flap surgery and the experience accounts from people who have been through it.
The problem the flap solves
An anal fistula is a tunnel connecting the inside of the anal canal to the skin outside. It persists because the internal opening — where the tunnel begins inside the canal — keeps re-infecting the tract.
For simple fistulas that do not involve much sphincter muscle, a fistulotomy (laying the tract open) is often effective. But when the fistula passes through a significant portion of the sphincter, cutting it open would mean cutting through the muscle. This creates a real risk of weakening bowel control.
The advancement flap offers a different approach. Rather than cutting the sphincter, it seals the internal opening with healthy tissue, removing the source of ongoing infection while leaving the muscle intact.
How the procedure works
The basic steps of an advancement flap for fistula repair are:
Identifying the tract. The surgeon maps the full path of the fistula — the internal opening, the external opening, and the route between them. This may have been done beforehand with an MRI.
Cleaning the tract. The fistula tunnel is carefully cleaned out (curetted) to remove infection and granulation tissue. This gives the repair the best chance of success.
Creating the flap. A flap of tissue is raised from the rectal wall. This typically includes the inner lining (mucosa), the underlying connective tissue (submucosa), and sometimes a thin layer of muscle. The flap is made larger than the opening it needs to cover.
Closing the internal opening. The muscle layer at the internal opening is closed with stitches, sealing the entrance to the tract.
Advancing the flap. The flap is moved — advanced — to cover the closed internal opening. It is stitched into place, creating a healthy tissue barrier over the site where the fistula began.
The external opening. The external opening is typically left open to allow any remaining infection to drain. Over time, with the internal source sealed, the tract gradually closes from the inside out.
The entire procedure is performed under general or regional anaesthesia and typically takes 45 minutes to over an hour.
Why it preserves sphincter function
The key distinction from fistulotomy is what happens to the sphincter muscle. In a fistulotomy, the tissue overlying the tract — including any sphincter muscle in its path — is divided to lay the tunnel open. For simple fistulas involving minimal muscle, this is straightforward and safe.
For complex fistulas, the tract may pass through 30% or more of the sphincter. Dividing that much muscle creates a meaningful risk of incontinence.
The advancement flap avoids this entirely. The sphincter is not cut. The repair works from the inside — covering the opening rather than exposing the tunnel. The muscle stays intact.
This is why advancement flaps are particularly important for:
- Complex or high fistulas — those involving a significant portion of the sphincter
- Recurrent fistulas — where previous procedures may have already affected some sphincter tissue
- Fistulas in people with Crohn’s disease — where tissue integrity may already be compromised
- Anterior fistulas in women — where the sphincter is naturally thinner and less able to tolerate division
Types of flap used for fistulas
Several flap variations are used. The choice depends on the fistula’s location and the surgeon’s assessment.
- Mucosal advancement flap — the most commonly described technique for fistulas. The flap consists of the inner lining of the rectum and the tissue just beneath it. It is advanced downward to cover the internal opening.
- Full-thickness rectal wall flap — includes the full thickness of the rectal wall. Used when more substantial tissue coverage is needed.
- Dermal advancement flap — in some cases, a skin-based flap from the perianal area is used to cover the external aspect of the repair.
Your surgeon will explain which technique they recommend based on your specific anatomy and fistula characteristics.
What the flap needs to succeed
Understanding why the recovery is strict helps with compliance. The flap is essentially a transplanted piece of tissue in a new location. For it to succeed, it needs:
- Blood supply to establish. The flap brings its own blood vessels, but these need to grow connections to the tissue beneath. Disruption during this critical period can cause partial or complete separation.
- Freedom from infection. The tract needs to be clean and the flap needs to heal without new infection taking hold. This is why the external opening is left open for drainage.
- Minimal mechanical stress. Straining, hard stools, and excessive physical activity can pull on the repair. Stool management during recovery is not optional — it is central to the procedure’s success.
- Time. The healing process takes weeks. People who rush the recovery sometimes describe setbacks that could have been avoided with patience.
When the flap does not work
Not every advancement flap succeeds. Recurrence rates vary in the medical literature, and your surgeon can discuss the outcomes they see in their practice.
When a flap fails, it usually means the internal opening has reopened — either because the flap separated, infection recurred, or the tract did not fully close. This does not mean treatment is over. Options after a failed flap include:
- A repeat flap procedure
- A seton placement to manage the tract
- Other techniques your surgeon may recommend
- A period of healing before attempting further repair
People who have been through a failed flap describe frustration, but also the knowledge that further options exist. A failed first attempt does not mean the fistula cannot be closed.
Advancement flap vs other fistula treatments
Understanding where the flap fits among fistula treatments helps frame the conversation with your surgeon.
- Fistulotomy — simpler, faster recovery, high success rate, but involves cutting through any sphincter muscle in the tract’s path. Best suited to simple fistulas with minimal sphincter involvement.
- Seton placement — a thread placed through the tract, either to drain it or gradually divide it. Can be used as a temporary measure before a flap, or as a long-term management tool.
- Advancement flap — seals the internal opening without cutting the sphincter. More involved, longer recovery, but preserves muscle function. Best suited to complex fistulas.
- LIFT procedure — ties off the fistula tract between the sphincter muscles. Another sphincter-preserving option that your surgeon may discuss.
- Fibrin glue and collagen plug — less invasive options that seal or fill the tract. Lower success rates but minimal recovery. Sometimes tried before considering a flap.
No single procedure is best for every fistula. The right approach depends on the tract’s path, its complexity, your anatomy, and your surgeon’s experience.
Talking to your surgeon
If advancement flap surgery is being discussed for your fistula, some questions that others have found helpful:
- How much of my sphincter does the fistula involve?
- Why is a flap recommended over other approaches for my specific situation?
- What type of flap will you use?
- What are the success rates you typically see?
- What happens if the flap does not work?
- How strict does the recovery need to be, and for how long?
- When can I expect to know whether the repair has been successful?
Write your questions down. It is easy to forget in the moment, especially when you are anxious.
If you experience increasing pain, fever, pus or foul-smelling discharge, or any new symptoms after surgery, contact your surgical team promptly.