Interpolation Flaps

Interpolation flaps (also called importation flaps) are distinguished from other local flaps by a critical feature: the flap base is separated from the defect by intervening normal tissue. The pedicle must therefore bridge over or tunnel under this intervening tissue to reach the recipient site. Because the pedicle is initially exposed or tethered, a second surgical stage is required to divide the pedicle after the flap has developed sufficient neovascularization from the wound bed (typically 2 to 3 weeks). Interpolation flaps are indicated when local tissue is insufficient in quality or quantity to reconstruct the defect with a single-stage flap, which is most commonly the case for large nasal defects, complex lip defects, and auricular defects with cartilage loss. The two-stage nature is a disadvantage for patients and surgeons, but the superior cosmetic and functional outcomes achievable with these flaps often justify the additional procedure.

A critical distinction among interpolation flaps is their blood supply pattern. Flaps are classified as axial (based on a named artery running along the long axis of the flap) or random (blood supply derived from the subdermal plexus without a named axial vessel). This distinction has major implications for flap design, pedicle width, and safe flap dimensions. Among the common interpolation flaps, only the paramedian forehead flap is truly axial, based on the supratrochlear artery. All other interpolation flaps (melolabial, retroauricular, Abbe) are random pattern flaps and are therefore more constrained in their length-to-width ratio.

The paramedian forehead flap is the gold standard for reconstruction of large nasal defects (greater than 1.5 to 2 cm) and defects involving multiple nasal subunits. It is an axial flap based on the supratrochlear artery, which emerges from the orbit at the medial brow and courses cephalad through the corrugator and frontalis muscles. The flap is designed vertically on the forehead, centered over the supratrochlear artery, with a pedicle width of 1.2 to 1.5 cm. The distal portion of the flap is designed as a template that precisely matches the nasal defect dimensions. The flap length is determined by measuring from the defect to the hairline along the pedicle course; the forehead provides a remarkable amount of tissue (up to 10 cm of length). At stage 1, the flap is elevated from distal to proximal, initially in the subcutaneous plane above the frontalis muscle, then transitioning to the submuscular plane at the level of the brow to protect the supratrochlear artery. The flap is rotated 180 degrees on its pedicle and inset into the nasal defect. The forehead donor site is closed primarily from distal to proximal, often leaving a small open area at the base that heals by secondary intention. At stage 2 (3 weeks later), the pedicle is divided, the proximal pedicle is returned to the glabella, and the inset portion is refined.

The melolabial interpolation flap recruits cheek tissue from the melolabial fold region and transposes it over the intervening alar tissue to reconstruct defects of the nasal ala, alar rim, and lateral nasal tip. Unlike the single-stage nasolabial transposition flap (which crosses the melolabial fold directly), the interpolation variant has its base superior to the defect, and the pedicle passes over the intact alar skin to reach the recipient site. This is a random pattern flap dependent on the rich subdermal plexus of the medial cheek. The flap provides excellent color and texture match for nasal skin and is particularly useful for alar rim defects where the tissue must be folded to create both an external and internal surface. Pedicle division is performed at 3 weeks. A known limitation is the potential to blunt or efface the alar crease, which must be recreated with deep tacking sutures during flap inset.

The retroauricular interpolation flap uses skin from the postauricular sulcus and mastoid region to reconstruct large defects of the helical rim, antihelix, and conchal bowl that involve cartilage loss. The flap is designed on the posterior surface of the ear and/or the mastoid skin, with the pedicle based superiorly or inferiorly depending on the defect location. At stage 1, the flap is elevated, a cartilage graft is placed if needed for structural support, and the flap is draped over the helical rim to cover the anterior defect while the pedicle bridges the posterior ear surface. The ear is temporarily tethered to the mastoid region by the pedicle. At stage 2 (3 weeks later), the pedicle is divided, and the flap is inset and contoured. The postauricular donor site is closed primarily or allowed to heal by secondary intention. This is a random pattern flap reliant on the subdermal plexus.

The Abbe flap (lip switch flap) transfers a full-thickness segment of one lip to reconstruct a defect of the opposite lip. The flap is based on the labial artery, which provides axial blood supply. The flap is designed on the lip opposite the defect, with a width equal to approximately 50% of the defect width (because the donor lip also loses tissue). The flap is incised through the full thickness of the lip (skin, orbicularis oris, mucosa), rotated 180 degrees on its labial artery pedicle, and inset into the recipient lip defect. The pedicle bridges the oral commissure or philtrum, temporarily tethering the lips together. Pedicle division is performed at 2 to 3 weeks. The Abbe flap is the only method that can restore orbicularis oris muscle continuity across a lip defect, making it functionally superior to other lip reconstructive options for moderate-sized defects (one-third to two-thirds of lip width). It is most commonly used for upper lip defects, with the flap harvested from the lower lip.

The reverse nasolabial flap (Spear flap) is a modification of the nasolabial interpolation flap that folds upon itself to simultaneously recreate both the external skin surface and the internal nasal lining of the alar rim in a single stage. The flap is designed in the melolabial fold with a length sufficient to fold over the alar rim. The distal portion becomes the internal lining and the proximal portion becomes the external cover. This eliminates the need for a second pedicle division procedure. The technique is particularly useful for full-thickness alar rim defects where both surfaces must be reconstructed and the defect is not so large as to require a paramedian forehead flap.

Pedicle division is the second stage of all interpolation flaps (except the single-stage Spear flap). The standard timing is 3 weeks after stage 1, which allows adequate neovascularization from the wound bed to sustain the flap after the pedicle is severed. At pedicle division, the pedicle is transected, the proximal stump is returned to its donor site (forehead, cheek, or lip), and the distal flap is refined as needed. Refinement at this stage may include flap thinning, debulking of subcutaneous tissue, and contouring to match the surrounding skin surface. Some surgeons prefer to delay flap thinning to a third stage (6 to 8 weeks after pedicle division) to minimize the risk of vascular compromise. The decision depends on the flap vascularity, patient factors, and surgeon experience.