The keystone flap is a random pattern multiperforator-based island advancement flap.
Perforator-based island flaps derive their blood supply from 1 or multiple vascular perforating branches that emanate from within the central unelevated base of the flap.
The keystone flap was originally described as 4 types (types I–IV) and numerous subsequent modifications to original keystone flap design have been reported.
Keystone flap design, application, and elevation are extremely straightforward, presuming that the surgeon possesses a strong foundational knowledge of perforator island flap physiology.
Keystone flaps have applications for cutaneous reconstruction within the head and neck as well as virtually any other anatomic subsite within the body.
When considering reconstruction of cutaneous defects within the head and neck, there often are myriad surgical options available for a given location and size of defect. It remains incumbent on the surgeon to utilize their experience, knowledge base, and skillset to select most appropriate method of reconstruction for a given patient. A fundamental axiom of reconstructive surgery is that local tissue adjacent a surgical defect almost invariably provides the best reconstructive match in terms of tissue quality, thickness, consistency, and color. In this regard, the use of locoregional flaps not only has the potential to reduce operative complexity but also frequently provides more esthetic outcomes compared with other more complicated forms of reconstruction. Maxillofacial surgeons must, therefore, cultivate a broad and diverse knowledge of locoregional soft tissue flaps within their own surgical armamentarium.
The keystone flap is a random pattern multiperforator-based island flap that was originally conceived by Behan in 2003 for repair of cutaneous defects resulting from skin cancer excisions. Although it represents a lesser known local tissue flap within the realm of maxillofacial surgery, it nevertheless holds excellent utility for reconstruction of cutaneous defects within the head and neck. The name keystone flap is derived from its semblance to the wedge-shaped keystone of the Roman arch ( Fig. 1 ). The keystone was the critical stone in the Roman arch’s construction that was responsible for locking the adjacent stones of the vault into place and redirecting the load of the arch laterally and downward into the adjacent pillars and foundations. Behan’s original description of the keystone flap included a classification system that organized flap design into 4 distinct categories (types I–IV), with further subdivision of the type II flap into type IIA and type IIB, depending on whether skin grafting was required to reconstruct the donor site ( Fig. 2 ). Although this classification system loosely correlates with the overall size and relative reconstructive complexity of the surgical defect, from a practical standpoint, having a thorough understanding of the anatomic and physiologic principles of the most basic (type I) keystone flap is all that is needed to understand the basis for the remaining flap subtypes.
Basic island perforator flap anatomy and physiology
As discussed previously, Behan’s original description of the keystone flap was that of a multiperforator-based random pattern island advancement flap. Due to the fact that this type of flap is islanded through a circumferential incision that fully interrupts the subdermal plexus from the surrounding tissue, the flap must, therefore, derive its blood supply from one or multiple perforating vessels extending through a broad subcutaneous bed lying directly beneath the flap. This broad subcutaneous bed also must, by necessity, remain adherent to the deeper fascia and underlying muscular bed so as not to disrupt the septocutaneous and musculocutaneous perforators that are emanating from these deeper layers to nourish the overlying skin island ( Fig. 3 ). For the basic keystone flap design, these underlying perforators are never skeletonized or identified during flap mobilization, which is what enables this flap to be elevated safely and quickly—presuming strict adherence to sound surgical/anatomic principles. To ensure that an adequate density of perforators is captured within the flap, Behan has frequently recommended the use of a dermatomal roadmap for keystone flap design. , In this manner, the planned excision of the primary lesion is oriented such that the long axis of the fusiform excision parallels the regional dermatomal distribution. This excision design and the resulting keystone flap orientation increase the probability of centering the flap within an area of concentrated musculocutaneous and septocutaneous perforators. This methodology assures adequate flap viability without the need for directly identifying underlying perforators with Doppler localization or other methods. By convention, dermatomal segments run longitudinally within the upper and lower extremities and transversely within the chest, abdomen, and pelvis ( Fig. 4 ). , Due to the rich vascularity within the tissues of the head and neck, strict adherence to this dermatomal orientation of keystone flap design generally is less important compared with reconstruction of defects involving the truncal or appendicular regions of the body. The intrinsic vascularity of the head and neck, however, does not absolve the surgeon from encompassing a sound knowledge of anatomy and fundamental understanding of flap physiology when keystone flaps are used for head and neck reconstruction.
Keystone flap classification and design
As discussed previously, Behan’s original description of the keystone flap subdivided its design into 4 distinct types (I–IV) with further subdivision of the type II flap. The basic premise of keystone flap design and execution is understood most readily with the type I flap, because the remaining flap subtypes can be easily extrapolated from the type I design. The following is a basic description of the 4 originally described types of keystone flaps.
After excision of the primary lesion in a fusiform fashion, a keystone flap is designed adjacent the surgical defect with a 1:1 ratio between the width of the defect and the width of the flap ( Fig. 5 ). Choosing the side of the defect with a greater intrinsic soft tissue laxity aids in mobilization of the flap during the later stages of closure. Note that the short straight limbs of the keystone flap extending off of the terminal points of the fusiform excision are made at 90° angles (see Fig. 5 ). Once the flap is designed, a skin incision is created circumferentially and deepened into the subcutaneous tissue. There should be minimal undermining beneath the central base of the flap, because this ensures capture of the greatest number of undisturbed perforators within the flap. In the process of undermining the tissues surrounding the keystone flap, care is taken to preserve any superficial veins or nerves coursing across or into the flap itself. The preservation of the superficial venous drainage system augments the venous outflow of the flap whereas preservation of cutaneous nerves allows for a sensate reconstruction. Once the circumferential tissue dissection is performed to the level of the underlying deep fascia, the dissection of the flap is complete. The hallmark of the type I keystone flap is that the entirety of the deep fascia remains intact circumferentially. Flap closure then commences with the terminal portions of the flap being closed in a V-Y fashion (see Fig. 5 ). This increases the tissue laxity across the central portion of the flap and thus enables primary closure of the ablative defect and the donor site simultaneously (see Fig. 5 ).
The type II keystone flap is designed in an identical fashion to the type I flap, with all steps proceeding as described previously. In circumstances in which additional flap mobilization is required to attain primary closure of the defect and donor site, however, the deep fascia is incised along the outer curvilinear margin of the flap ( Fig. 6 ). This additional fascial release enables further mobilization of the keystone flap into the defect while keeping the perforators located in the central portion of the flap intact. The type II keystone flap is further subclassified into types IIA and IIB based on whether or not a skin graft is required to reconstruct the secondary donor site defect (see Fig. 6 ). Typically, skin grafting of the donor site is necessary in circumstances in which excessive skin tension persists during closure despite typical flap mobilization and subsequent fascial release, as described previously. It bears mentioning at this juncture that an alternative method for eliminating the need for skin grafting involves increasing the width of the keystone flap so that it exceeds the typical 1:1 ratio with the ablative defect (2:1, 3:1, 4:1, and so forth) (see Fig. 18 ). This specific keystone flap modification is discussed later.
The type III keystone flap is essentially composed of 2 opposing keystone flaps designed around a central surgical defect ( Fig. 7 ). The type III keystone flap is particularly useful for reconstructing larger ablative defects or defects that are located in areas of the body with low intrinsic soft tissue laxity. Due to the natural laxity of tissues within the face and neck, however, this specific type of keystone flap configuration is not commonly used for head and neck reconstruction. Further impeding its use within the head and neck is the reconstructive principle of camouflaging incisions within relaxed skin tension lines and natural anatomic subunit boundaries of the face in order to minimize visible scarring. The difficulty in attempting to camouflage the incision lines of double opposing keystone flaps within the face can be appreciated, given the inherent geometric complexity of the flaps’ design.
The type IV keystone flap, as originally described by Behan, is a keystone flap with up to one-half to two-thirds of its subcutaneous base undermined in order to facilitate rotation and/or advancement into an adjacent surgical defect ( Fig. 8 ). , The vascular supply to the type IV flap originates from perforators arising within the unelevated portion of the flap, which by design are providing vascular supply to the elevated tip of the flap through both the subdermal plexus and suprafascial adipose tissue plexus. In the primary author’s opinion, the type IV keystone flap represents a stark departure from the standard flap design and elevation principles that define the hallmark types I to III keystone flaps. Although the successful execution of a type IV keystone flap is certainly founded on the same basic anatomic vascular principles as those of the standard keystone flap, it represents nothing more than a random pattern multiperforator-based pedicled island flap. Although Behan’s original description of the type IV flap still incorporated the overall keystone shape to the flaps design, this strict keystone shape no longer is requisite when significant undermining for the advancement/rotation is required to gain closure. In these circumstances, surgeons may just as easily deviate from the standard keystone configuration and instead choose an alternative flap design, such as fusiform, ovoid, or other geometric pattern, based on the requirements of the ablative defect. ,
There are no specific preoperative studies that are required prior to performance of keystone flap reconstruction. It remains a sound operative practice for the surgeon to have preemptively identified how the primary tumor will be resected and how the keystone flap will be designed prior to entering into the operating theater. This planning also should incorporate a well-established plan to accommodate any ablative defect enlargement required to obtain oncologically clear margins and modifications to flap design (increasing the flap to defect width ratio, progressive incision/release of deep fascia, adjunctive need for secondary skin grafting, and so forth) as well as the possibility of selecting an alternative method of local or regional flap reconstruction.
Relative to planning of keystone flap design, surgeons can rely on either the dermatomal roadmap, as previously described by Behan or, alternatively, a handheld Doppler localization of perforators adjacent to the surgical defect. Perforator localization via handheld Doppler frequently is unnecessary, however, for routine types I to III keystone flaps where there is no disruption to the underling broad subcutaneous base of the flap. This is especially true within the head and neck, where a rich vascular network with extensive collateralization exists almost universally. Furthermore, in areas where dominant axial septocutaneous or musculocutaneous perforators are not present, the handheld Doppler does not always return an audible signal, even though a sufficient density of small random perforators exist within the flap base that would adequately sustain the projected skin island. In this circumstance, a novice surgeon may be unnecessarily deterred if relying solely on Doppler corroboration of adequate flap vascularization—a fact that only underscores the importance of the surgeon possessing a commanding knowledge of the fundamental anatomic vascular principles of keystone flaps and perforator-based island flaps in general.