An interpolation flap is a 2-stage tissue flap in which the base of the flap is not immediately adjacent to the recipient site. These flaps are used when insufficient tissue or mobility in nearby skin prevents coverage of a surgical defect with primary closure or an adjacent flap. Interpolation flaps are similar to transposition flaps in that the flap is lifted over an area of normal skin to reach the defect. While the base of a transposition flap is adjacent to the defect, the base of the interpolation flap is located a distance away from the area to be repaired. This arrangement results in a bridge of tissue, or pedicle, between the flap base and the surgical defect that typically must be removed in a second stage after vascularity is established between the wound and the flap.
The following 3 interpolation flaps are used most commonly in dermatologic surgery:
Cheek interpolation flap
The forehead flap uses tissue from the forehead to repair the nose. The cheek interpolation flap takes advantage of the loose skin of the cheek to repair small but somewhat deep defects in the nose, and the postauricular flap uses skin from the posterior part of the ear and the retroauricular aspect of the scalp to repair defects in the helix and anterior surface of the ear.
History of the Procedure
The forehead flap is believed to have been used in India as early as 700 BCE. Antonio Bronca of Italy performed the procedure in the 15th century. The first reports of the midline forehead flap in the English-language literature appeared in 1793. Since the 1960s, many advances have been made, including Menick's use of the paramedian forehead flap, which is based on a narrow vascular pedicle supplied by the supratrochlear artery.  This modification allows easier closure of the forehead defect, as well as greater flap mobility, and it is currently the most commonly used forehead interpolation flap.
The cheek interpolation flap is believed to have been used since 600 BC, when it was first used in India. In contrast, the 2-stage postauricular helical flap is a relatively recent technique, which Lewin described in 1950. 
The forehead flap is used to repair more extensive defects on the nasal tip and ala for which simpler techniques cannot provide adequate coverage. It is sometimes used to provide nasal lining as well. In very extensive defects, the forehead can be used in combination with other techniques such as microvascular free flaps, cartilage or bone grafts, and mucosal flaps to achieve satisfactory function and appearance. [3, 4]
Select small-to-medium, deeper defects on the nasal ala and nasal tip can be repaired by using the cheek interpolation flap.
The postauricular helical 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. 
The described interpolation flaps are most commonly used to repair surgical defects resulting from tumor excision, but they can also be used to repair traumatic wounds.
The paramedian forehead flap is an axial flap based on the supratrochlear artery. Cadaver studies show that the supratrochlear artery exits the orbit 1.7-2.2 cm from the midline, passing deep to the orbicularis oculi muscle and ascending superficial to the corrugator supercilii muscle. It then passes medial to the eyebrow and through the frontalis muscle ascending superiorly in the subcutaneous tissue, 1.5-2 cm from the midline. The angular and supraorbital arteries also contribute to the vascularity of this flap through a rich periorbital plexus of anastomoses. 
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 scalp has a rich vascular supply from branches of the posterior auricular, superficial temporal, and occipital arteries. Therefore, the postauricular helical flap, which is a random flap, is rarely affected by vascular necrosis.
Also see Anatomy in Cutaneous Surgery.
The use of pedicle 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 never be covered with a flap or used to form a flap. With a forehead with a low vertical height, a variation of the forehead flap or another repair method may be required.
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 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 prior skin surgery. Similarly, the author only stops aspirin usage initiated by the patient and not when prescribed by a physician. Contacting consulting physicians is appropriate before operating on individuals with bleeding dyscrasias.
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