Interposition flaps are a suitable surgical technique to treat broad contractures that cover larger areas.

General principles

Broad contractures that cover larger areas (e.g. are not a single-strand) are more extensive and often require a different approach to the archetypal Z-plasty.

If no slack is available in the surrounding skin, a skin graft might be the best option. But if there is slack next to the broad scar, an interposition flap might be a better option to provide vascularized elastic skin.

An interposition flap is like a Z-plasty with a 90-degree release incision in the pathological (scar) tissue, followed by an inlay of a triangular flap of (preferably healthy) tissue. While the Z-plasty angles are asymmetrical to facilitate this, interposition flaps do not necessarily need to be triangular – you can shape them to suit the local situation.

As contractures are characterized by problematic amounts of skin tension, incisions made in the affected skin in the direction of the contracture have a tendency to shrink, while the incision in the healthy skin has the ability to stretch. To account for these effects, it is often wise to release the contracture and dissect the interposition flap in a stepwise fashion.

Doing this gives you the opportunity to assess the need and availability of tissue intra-operatively. It also allows you to adjust the length of the incisions to meet the needs of the individual contracture at hand.

Procedure

This guidance covers the interposition flaps procedure from preparation, including how to mark the flap design, to the evaluation of results.

Here you can find guidance for a relatively simple interposition flap. As you gain experience, you will see more possibilities and solutions. You can then start varying, combining, and fine-tuning your technique to create the optimal treatment for each contracture.

Preparing for interposition flaps surgery

Marking the flap design

Before marking the flap, it is essential to determine the line of maximum tension (the direction of the contracture). You can do this by extending the affected joint or body part, then determining the direction and amount of slack using the pinch test.

After assessing the affected area, mark the contracture release incision perpendicular to the direction of the contracture. To be effective, the incision should run all the way down the scar tissue causing the contracture into the healthy skin. You can then design the interposition flap. The interposition flap itself should be on the healthy, non-contracted skin with the most available slack.

The width of the contracture defines the length of the flap, while both the amount of slack available and the amount of tissue needed to lengthen the contracture define the base of the flap, and thereby the angle of the triangle.

To ensure the blood supply to the tip of the flap, the length should not normally be longer than two to three times the length of the base of the flap.

With this design, the healthy tissue will be interposed into the defect created by the release of the contracture, with tension-free donor site closure.

Anesthesia

Depending on the location of the contracture, the estimated size and extent of the surgery, and patient characteristics, you can perform an interposition flap under local (WALANT), regional, or general anesthesia.

When using local anesthetic, anesthetize first, then disinfect and drape.

Even if the procedure is performed under general or regional anesthesia, infiltrate the operative site with local anesthetic and adrenaline/epinephrine. This will greatly reduce the bleeding during the operation, make the dissection easier, and provide postoperative pain relief. Allow 30 minutes for the adrenaline/epinephrine to have the optimal effect and use a long-lasting local anesthetic agent for optimal postoperative pain relief.

Positioning

The positioning of the patient depends on the location of the burn contracture.

Sterile field

Make sure that the entire site is disinfected and that the sterile drapes do not restrict movement, so that the affected body part can move freely during the procedure. This can help improve exposure and allows you to assess the effect of the contracture release immediately.

Surgery

1. Start with the releasing incision down all the fibrotic tissue to the healthy skin, creating maximum release.
2. Determine whether the rest of the design (the interposition flap) is still adequate. Does the defect created by the release match the flap? Is it still possible to close the donor site (in other words: the base of the flap)? If so, continue with the next incision; if not, adjust the design.
3. When you have made all the incisions, dissect the transposition flap so that you can transpose the flap easily. In a well-designed transposition flap, the flap will almost fall into place by itself. Make sure that the dissected skin is the same thickness all over, or slightly thicker at the base. The dissection should be within the subcutaneous or suprafascial tissues, depending on the location of the contracture.
4. Check for active bleeding and coagulate if necessary. Be cautious when coagulating on the skin flap, as this could damage the vascularization.
5. When suturing the transposition flap, start by closing the donor site, because this is where the tension will be. You can then suture the interposition flap in place. If the local skin has a thick layer of subcutis, you can use subcutaneous sutures to minimize tension on the skin.
6. Ideally, use monofilament sutures for the skin layer, especially in small children or difficult anatomical locations, as they avoid the need for removal.
7. Depending on the size and location, you can place a small drain.

This example shows a patient with a neck burn contracture. It is an an example of a interposition flap described by a pioneer in developing local flaps, professor Limberg (1894–1974).

Dressing and fixation

In most cases, you can apply a simple dressing with dry gauzes as a first layer. If the flaps tend to move away from the underlying tissue layer (this is called ‘tenting’), you can apply an elastic bandage or even a tie-over dressing.

Joints that have been affected by a contracture tend to return to their contracted position, and you must avoid this. We recommend fixation of the affected joint in a position opposing the contracture until the wounds have healed. You can achieve this with a Plaster of Paris slab splint. K-wires are a very useful alternative in the hands, fingers, and toes, where plaster may be less effective, especially in young children.

Postoperative care

After surgery, change bandages after 3-5 days, or earlier if there is a risk of infection. Remove sutures after 2-3 weeks if they are non-absorbable.

If you used K-wires, remove them after 3-4 weeks due to the risk of infection. You can continue splinting for a longer period, until there is no further risk of the contracture recurring. If the affected joint is not fixed for long enough in the position opposing the contracture, there is a high risk of the contracture recurring.

Physiotherapy

The patient should start active movement and exercises as soon as wound healing permits to maintain the ROM they gained. The availability of physical therapy makes a big difference. If physical therapy is not available, emphasize the importance of exercises to the patient.

You can continue splinting between exercises and during the night. Splinting during the night is sometimes continued for months, even after the wounds are fully healed and the affected limb has returned to normal use. Splinting works well when the skin quality is good, but it is less effective in stretching remaining scar tissue. In these cases, additional surgery to add extra tissue where needed might be more effective, followed again by appropriate positioning after the additional procedure.

Evaluation of results

We recommend reevaluating the patient until the scars have healed, ideally at least after six weeks, three months, six months, and one year. Measure and document the ROM preoperatively, and monitor the development of ROM postoperatively. Explain to the patient that the ROM can continue to improve for months, provided they do their exercises. Make the final evaluation measurement one year after the procedure.

Management of complications

As burn scars are very fibrotic and therefore often poorly vascularized, complications in wound healing are common. Wound breakdown, necrosis, and infection are the most common complications. This is particularly true when wounds are closed under skin tension. If a local flap does not provide enough tissue to close the wound without tension, we advise adding a skin graft or accepting that the joint cannot be fully extended (yet).

You can start antibiotic treatment when indicated for infection. Treat small areas of necrosis and minor wound breakdown conservatively by cleaning the wound and applying antibacterial dressings. The small defects will heal by secondary intention and the end result will often still be adequate in terms of contracture release, so no further treatment will be required.

Surgically debride larger breakdowns with extended necrosis. If the defects are substantial (for instance a complete triangle of the Z-plasty has become necrotic), treat the area with a skin graft as soon as the wound bed allows. Without this secondary surgery, these cases will very likely end with a recurrence of their contracture.