An interpolation flap is a 2-stage tissue flap in which the base of the flap is not directly adjacent to the recipient defect. These flaps are used when insufficient tissue or mobility in nearby skin prevents the coverage of a surgical defect with primary closure or an adjacent flap. The use of an interpolation flap results in a bridge of tissue, or pedicle, between the flap base and the surgical defect. It must be removed in a second stage after establishing vascularity between the wound and the flap.
The interpolation flaps most commonly used in dermatologic surgery are:
The paramedian forehead flap is an axial flap based on the supratrochlear arterial blood supply, although some authors have reported that with the abundant anastomoses in the forehead, it can be performed as a random flap. The cheek interpolation flap is a random flap, but it receives an ample blood supply from the perforating branches of the angular, nasal, and superior labial arteries. The postauricular interpolation flap is also a random flap with a broad pedicle supplied by branches of the posterior auricular, superficial temporal, and occipital arteries.
Interpolation flaps are used when insufficient tissue or mobility in nearby skin prevents the coverage of a surgical defect with primary closure or an adjacent flap. The forehead flap is especially useful on larger defects that are deep or full-thickness, and located on the distal nose. Cheek interpolation flaps are useful on the distal third of the nose, especially laterally, in situations in which adjacent tissue transfer or full-thickness grafts are not practical. The postauricular interpolation flap is utilized in medium-to-large defects on the helix and adjacent antihelix, with or without the loss of small amounts of cartilage, in situations in which single-stage flaps are not adequate for coverage. 
The use of interpolation flaps is contraindicated in patients who are unwilling or unable to tolerate multiple-staged surgical procedures. Likewise, these procedures should be avoided in patients who cannot leave their surgical sites undisturbed, or special measures must be taken to protect the sites in these patients. Actively infected skin should not be covered with a flap or used to form a flap. When performing the paramedian forehead flap (PFF) on a patient with a forehead of low vertical height, a variation of the forehead flap or another repair method may be required to avoid transferring hair-bearing skin to the nose.
Smoking is a relative contraindication to the use of staged island pedicle flaps because it increases the risk of flap necrosis. However, procedures with these flaps can usually be performed safely if fat is not thinned excessively from the undersurface of the flap. In addition, avoiding the use of previously radiated skin or a previous surgical site is generally best, unless no better repair options are available. Interpolation flaps should be performed with great care in patients who are receiving anticoagulant therapy or in patients with bleeding disorders. Consultation with the physician who prescribed the medication is prudent before discontinuing any anticoagulant therapy. The author rarely discontinues warfarin or aspirin prior to surgery. Contacting consulting physicians is appropriate before operating on individuals with bleeding dyscrasias.
Equipment required for interpolation flaps is generally the same for other dermatologic surgery, with a few possible additions.
Interpolation flaps can generally be performed with one surgical assistant, primarily to help manage bleeding and cut suture.
We find that it is very helpful to show patients (and their families) illustrations when discussing interpolation flaps prior to surgery. Otherwise, it can be difficult for patients to understand what is about to happen in the surgery and in its aftermath. Serial postoperative photographs can help allay concerns and help patients understand that the final result will take time. It is also essential to explain that minor revisions ("fine tuning") may be indicated in the weeks or months after the initial procedure to obtain optimal results.
Forehead Flap (Paramedian Forehead Flap, PFF)
The PFF technique first was used in India as early as 700 BC. For many years, a broad midline flap was popular. In the 1960s Menick introduced the PFF, which had a much more narrow and mobile pedicle based on the supratrochlear artery. The PFF uses tissue from the forehead to repair larger, deeper defects on the distal nose (fig. 1a), and sometimes provide nasal lining. In more extensive defects, the forehead flap can be used in combination with other techniques such as microvascular flaps, cartilage or bone grafts, and mucosal flaps. The PFF is an axial flap based on the supratrochlear artery (fig. 1b). The supratrochlear artery exits the orbit 1.7-2.2 cm from the midline, passing deep to the orbicularis oculi muscle and ascending superficially to the corrugator supercilii muscle. It then passes medial to the eyebrow and through the frontalis muscle ascending superiorly in the subcutaneous tissue and 1.5 to 2 cm from the midline.
The distal nasal defect is examined prior to designing the PFF. A template of the final defect is made, and the location and course of the supratrochlear artery can be identified by using Doppler ultrasonography or anatomic landmarks. The supratrochlear artery runs toward the scalp within 3 mm of a line drawn up from the medial canthus. Other authors have demonstrated the safety of simply taking a pedicle from the glabellar midline to 1.2 cm lateral to the midline. If more than 50% of an anatomic subunit is involved in the surgical defect, it is often preferable to remove the remainder of the subunit to obtain a better aesthetic result. A template of the defect is made, and then tubed gauze or another flexible material is used to measure the needed length for the flap to rotate and reach the defect from the pedicle base. The tube gauze is rotated to the forehead and the inverted template pattern is marked at the uppermost aspect of the pedicle. From that area, a 1 to 1.5 cm pedicle is drawn down to the planned pedicle base. The flap is then mobilized with its base situated at the inferior aspect of the forehead and often around the orbital rim (fig.1c). The forehead donor site is closed primarily and if the wider portion of that defect will not close completely, it is preferable to allow it to heal secondarily rather than to apply a skin graft (fig.1d). The flap is then thinned distally and secured in the nasal defect with simple interrupted sutures. Within 1 to 3 weeks, usually about 2 weeks, the pedicle trunk is excised, and its base defect is best closed primarily. The incised portion of the nasal flap is then thinned and inset into the defect. Minor revision procedures may be indicated during the coming weeks, and it is best to discuss this with the patient prior to starting this flap.
Cheek Interpolation Flap
First used in India in about 600 BC, the medial cheek skin is an excellent color and texture match for defects on the caudal third of the nose (fig. 2a). Based superiorly, the cheek interpolation flap can be based on a cutaneous (fig. 2b) or a subcutaneous pedicle (fig. 2c). The subcutaneous pedicle has greater mobility and vascular supply than the cutaneous pedicle. For small-to-medium alar defects, an inferiorly based flap can also be used to avoid transfer of facial hair. The cheek interpolation flap is a random flap, but it receives an ample blood supply from the perforating branches of the angular, nasal, and superior labial arteries. Small-to-medium, deeper defects on the nasal ala and nasal tip can be repaired by using the cheek interpolation flap. Foil or nonabsorbent surgical dressing is used to make a template of the surgical defect to be repaired. Gauze is rolled into a tube, and one end is held in place in the area of the planned flap base, in and lateral to the melolabial crease. While the proximal end is held in place, the remainder of the gauze tube is turned to overlap the wound without tension. The most distal portion covering the wound is marked, and the gauze is swung back into the melolabial crease to mark where the flap should reach. The template is placed on the cheek with its superior edge placed medially and touching the melolabial crease. The portion of the template representing the lateral wound edge furthest from the flap is placed most inferiorly (or distally) on the cheek, touching the mark previously placed on the cheek. A tapered incision is planned distal to this tissue and parallel to the melolabial crease to allow closure without a dog-ear defect. The flap is mobilized, thinned, trimmed and secured in the recipient wound (fig. 2d). The cheek donor site is sutured and the pedicle is wrapped in non-adherent dressing material. The flap is divided and its remainder is inset in about 3 weeks. The remainder of the donor defect is repaired at that time (fig. 2e).
Postauricular Interpolation Flap
First described by Lewin in 1950, the postauricular interpolation flap provides good coverage and cosmesis in medium-to-large defects on the helix and adjacent antihelix, with or without the loss of small amounts of cartilage (fig. 3a). Branches of the posterior auricular, superficial temporal, and occipital arteries supply the broad pedicle of this flap. Although it is a random flap, it is rarely affected by vascular necrosis. This flap uses skin from the posterior ear and retroauricular sulcus and retroauricular skin to repair defects of the middle third of the auricular helix and adjacent skin. Scars remain well hidden behind the ear. Reconstruction begins by making a template of the helical defect. The template is placed behind the ear with its leading (anterior) edge over the loose portion of the posterior ear and the planned flap is drawn with a surgical marker (fig. 3b). Lines are drawn toward the scalp, gradually tapering outward as they approach the hairline. Burow's triangles can be drawn lateral to the base of the pedicle, but the actual excision of the triangles is often not necessary. Incisions are made through the skin along the planned lines, and the flap is elevated. The surrounding tissue is undermined for a short distance to allow greater tissue mobility, decrease pincushioning, and allow better placement of subcutaneous sutures. The flap is draped over the defect to ensure that coverage of the anterior portion of the defect does not cause excessive tension, and the Burow's triangles can be excised if necessary. Electrocoagulation is used to obtain meticulous hemostasis before the sutures are placed, making every effort to avoid postoperative bleeding. A few interrupted, subcutaneous, absorbable sutures can be used to securely anchor the flap into place, and superficial sutures are placed for fine approximation (fig. 3c). Particularly on the posterior aspect of the ear, it is preferable to use absorbable sutures to avoid the need to manipulate the ear for suture removal. A few basting sutures can be placed through the flap into the cartilage to recreate the helical sulcus if desired. Division and inset are performed at about 3 weeks. The postauricular donor defect may be closed or allowed to heal secondarily.
Potential complications common to all skin surgery include infection, scarring, bleeding and disfigurement. In addition, pedicle necrosis is possible but infrequent.
Interpolation flaps provide a method of repairing surgical defects that otherwise would be difficult, if not impossible, to repair with other reconstructive techniques.
The surgical team should pay particular attention to control and prevention of postoperative bleeding, which is most commonly experienced in the first 24-28 hours after surgery. Salient points are outlined below.
The paramedian forehead flap is especially prone to postoperative bleeding, most commonly from the proximal flap pedicle in the glabella/brow area. This is best managed as follows:
Cheek interpolation flaps are not as prone to heavy bleeding, but do tend to ooze in the early postoperative period.
Postauricular transposition flaps, when they bleed, are particularly challenging due to being located in the tight quarters behind the ear, which is draped with a flap. Prevention is key to success.