Interpolation Flaps in Mohs Surgery: Techniques, Planning, and Common Pitfalls

Dr Anna Bar shares practical insights on mastering interpolation flaps in Mohs reconstruction, including how to select the right flap based on defect size and location, avoid common technical mistakes, and optimize outcomes. Learn key planning strategies, pearls for preserving blood supply, and tips to improve both functional and cosmetic results while managing patient expectations.

Transcript

Dr Bar: Hi everyone. My name is Dr Anna Bar. I'm a professor of dermatology at Oregon Health and Science University and director of the Mohs fellowship.

The Dermatologist: Interpolation flaps can feel intimidating—what’s the key mindset shift that makes them more approachable?

Dr Bar: The way I think about it, you are doing an interpolation flap today so that you don't have to do a scar revision later. And I always think it's harder to revise surgeries or fix things that didn't go well rather than doing the right repair at the right time in the first place. Well, for example, if you have a deep enlarged defect on the ala, most things that you would do could potentially cause alar contraction, and that's very hard to fix. But if you do an interpolated flap and make that ala the right size with a good blood supply, you won't have to do a revision later. Or if there's a large portion of the ear missing, doing the ear postauricular interpolation flap will prevent a revision down the road.

The Dermatologist: How do you decide when an interpolation flap is the right choice over other reconstructive options?

Dr Bar: It's usually two factors. It's the size of the defect and the location of the defect. So most of our interpolation flaps are the cheek and the nose. And of course there are other areas sometimes on the eyelid and medial canthus area as well, but it's usually a large defect. It's usually compromising the cosmetic unit or compromising the function of the cosmetic unit. If it's on the nose, it could be that the graft would not have enough blood supply. And if the graft contracted, then the graft would cause contraction or basically dysfunction of part of the nose. Or another repair with just a local flap couldn't be done because there's not enough donor tissue on the nose. So it's basically the size and location of the defect.

The Dermatologist: What are the most important factors in selecting between forehead, cheek-to-nose, tunneled, or postauricular flap variations?

Dr Bar: Well, between forehead and cheek-to-nose, that's always a tough selection. And I think generally cheek-to-nose is a little bit easier for the patient to tolerate because they can cover more of the repair with a mask and go out in public. So if I can choose a cheek-to-nose, I usually do that. Sometimes I can't choose that flap because either there's too much internal nasal loss and I have to use that folded over portion of the forehead or it's in the wrong location. It's harder to get this cheek-to-nose flap to the nasal tip or the dorsum of the nose, although it could be done. It's usually easier to get a forehead flap to go to those locations to make new mucosa or to do the top of the nose. But there are other factors such as hair. For example, in a man, they may have a lot of terminal beard hair here (cheeks) that you don't want to transpose to the nose, and then maybe you can go to the forehead there.

The length of the patient's forehead can make a difference. If they have enough donor site here to close a defect, or do they have enough cheek laxity here to close a defect? So those are some of the factors. Additionally, blood thinners. These flaps can have some postoperative bleeding. And if they're on several blood thinners, you may want to select a flap that has a smaller pedicle. So in my hands, a cheek-to-nose has a slightly smaller area that's left open. So potentially a smaller risk of bleeding for that patient overnight. As far as the tunneled flaps go, many times, not always, many times the tunneled flap defects are close or closer where you can leave some tissue connected and then take the defect that's deepithelialized donor site through the tunnel. But usually you don't want to stretch those too far. So those should technically be a little bit closer, whereas the other flaps, you can stretch probably a little bit more. But if the patient can't come back, the tunnel is a great option.
The postauricular flap would be for large ear defects. So through and through defects of the ear, you can fold that postericular flap on itself that can kind of recreate a helical rim. So that's one of the handiest flaps. And I do tend to graft almost everything that I can on the ear because it does fairly well, but if it's too big, too deep all the way through, then the postauricular flap is kind of the one that you're left with.

The Dermatologist: What’s the most critical step in planning that determines success before you even make an incision?

Dr Bar: Well, I like to obviously measure twice or measure five times and cut once. So I make a template of the defect, exact template. Then I transpose it to the donor site, and I measure by making a mark on a Q-tip. But because there is a little bit of pivotal restraint, so whenever you rotate or pivot something into place, the flap shortens just a little bit. So I make sure to account for a couple of millimeters in pivotal restraint to make sure that the flap has plenty of length to shift and cover the defect without putting undue excess tension on the pedicle or kinking the pedicle, because that could compromise the blood supply. So I would say make the template of the defect first.

The Dermatologist: In your experience, what technical mistake most often compromises flap outcomes?

Dr Bar: Okay. For paramedian forehead flap, what would compromise the flap outcome? Maybe sometimes patient selection. You can have some difficult outcomes if the patient is a smoker, if they're on a lot of blood thinners. There's a technical mistake you can make by making it too short to where it wouldn't reach. You could make a technical mistake of potentially kinking or torsioning the pedicle too much. So you want to make sure to really free it up down at its base, which is near the eyebrow, but could even go below into the medial campus if you need to get some more reach. It's a fairly robust flap. So most of the time it does survive. Of course, there are always risks, but this paramedian forehead flap has the most robust blood supply of any of the flaps that we do.

For the postauricular flap, a nice pearl is to widen the base of the postauricular flap to make sure that you get that really good survival. So instead of taking that flap back directly the size of the defect, make it slightly wider as you taper it back to the donor site and include some muscle in there because that's the blood supply of the flap.
For the nasolabial interpolation flap, the biggest mistake that I see is making the pedicle too narrow or in the wrong spot for a nasolabial interpolation. So basically the apical triangle is a very important landmark that you shouldn't not violate when you make your donor site for a nasolabial interpolation flap. Most of the blood supply of the flap is actually not right at the alar crease. Most of the blood supply of the flap is about a centimeter away with the angular artery in the cheek. So if you make your flap base there, the flap has a much more rich blood supply. And some people go down to the muscle here. I really don't, but I am in the deep fat at the blood supply of the nasolabial interpolation flap. So that ensures that the flap can have good survival. I think when people have trouble with survival, they're making the pedicle really, really too medial and too small. They have to just remember that this is a better blood supply for that flap.

The Dermatologist: What are your go-to pearls for ensuring both functional restoration and optimal cosmetic results?

Dr Bar: Well, function, of course, is first. If you're reconstructing the nasal ala and you see that it's not stable, that the ala is collapsing, consider putting cartilage in the defect either at the time or at another stage so you can harvest cartilage from the ear.That's pretty important in assessing the patient to see if their ala is going to be stable and not collapsing.

As far as optimal cosmetic results, these flaps do require some thinning. So knowing how much to thin at the first stage is important. With the forehead flap, you can thin it much more than with the nasolabial interpolation flap or the postauricular flap is already fairly thin with that donor site. But the forehead flap, when I have to fold it on itself, I pretty much thin all the fat off of it so that it will fold nicely. If it's a thicker, deeper defect here on the nasal dorsum or nasal tip, you can leave that fat there. You just have to replace what you removed with the Mohs surgery. So the thinning.

And then if you do a forehead flap, you can potentially make it a three-stage forehead flap. Although I don't often do that, you can do thinning of the forehead flap and then put it back down to ensure that you can get it thin, but not compromise the survival.

The other thing with these flaps is they're often edematous for months. So I'm not very quick to revise them because they do atrophy to significant levels almost to normal in most patients. If I need to help speed that along, I do intralesional cortisone and laser for the patients, but I'm not quick to go and make revisions because a lot of this is edema that will be short-lived.

The Dermatologist: How do you manage patient expectations, especially given the staged nature of many interpolation flaps?

Dr Bar: Well, I do show them pictures and most people I don't see in consult first. So when I meet them for their Mohs surgery, if I'm looking at a large defect, I often prep them by saying, "Hey, we are going to need a two-stage surgery. You might have to come back another time for a shorter surgery. It's not going to be like today, like a long, long, long day with the Mohs, but we will probably need a flap. We have to leave it attached to its blood supply, and then you come back in two weeks and I'm going to take away the blood supply and you'll be looking good." So I do talk to the patient and I show them pictures and I actually take down these flaps usually earlier than other people do. I usually take them down at two weeks, although I've taken them down at one week as well with very good results.

We published our results at OHSU of taking down interpolation flaps at one or two weeks, and we found that they did just as well as flaps that we left for three weeks for these patients. So we're trying to make their lives a little bit easier because we did not notice any adverse outcomes at taking them down a little bit early.

The Dermatologist: For surgeons looking to build confidence, what are some practical tips that can immediately improve their outcomes?

Dr Bar: Okay. I have learned so many practical tips to improve outcomes and I have learned those by coming to the Mohs meeting year after year and watching all of our amazing speakers talk about their pearls and their difficulties and their revisions. So I have learned a lot by coming to the Mohs meeting and just listening to other people's experience.

One of my practical tips that I have is TXA injection has really helped my practice. We published at OHSU a study of patients that had TXA injection during the time of their interpolated flap surgery, and it significantly reduced bleeding for that first night. And that is a real tough thing for patients. Sometimes it results in coming in the middle of the night or going to the ER, they're very scared. So what I do is I take one to three ccs of TXA and I inject it around the flap and the pedicle. It's not a lot of TXA, it's a small amount, but it's been in my hands really helpful for cutting down on that bleeding that they have the first night that is really distressing to them.