Turkish Journal of Plastic Surgery

: 2021  |  Volume : 29  |  Issue : 5  |  Page : 67--71

Double keystone flap for large lower thigh defect

Amrita More, Dharini Dharini, Anoop Sivakumar 
 Department of Burns, Plastic and Maxillofacial Surgery, VMMC and Safdarjung Hospital, New Delhi, India

Correspondence Address:
Dr. Anoop Sivakumar
D2-204, Kaka Nagar, New Delhi - 110 003


Large lower thigh defects are a reconstructive challenge. Keystone design perforator island flap in the lower thigh is not routinely performed as the skin here is less extensible. We present a case of a large anterolateral lower thigh defect wherein we used double keystone flaps from the remaining circumference of the thigh and grafted the donor site in the posterior thigh. Circumferential incision/scarring in the limbs risk damage of superficial lymphatico-venous circulation and distal edema. However, our case demonstrates an uneventful recovery and excellent aesthetic and functional outcome.

How to cite this article:
More A, Dharini D, Sivakumar A. Double keystone flap for large lower thigh defect.Turk J Plast Surg 2021;29:67-71

How to cite this URL:
More A, Dharini D, Sivakumar A. Double keystone flap for large lower thigh defect. Turk J Plast Surg [serial online] 2021 [cited 2022 Aug 10 ];29:67-71
Available from: http://www.turkjplastsurg.org/text.asp?2021/29/5/67/311442

Full Text


Keystone design perforator island flaps (KDPIFs) owing to its unique design takes advantage of the biophysical properties of the skin to make reconstruction of large defects possible with local tissue and minimal associated morbidity. First described by Behan in 2003, its concept and design has evolved with time and several modifications have been described since, with respect to design of flap, extent of flap undermining and other aspects.[1],[2],[3],[4] Large defects entail the use of double keystone flaps or Behan's Type 3 KDPIF and this design has proven advantageous in areas where skin is less extensible like the back.[1]

Conventional flaps in reconstruction of lower extremity like fasciocutaneous and perforator flaps are fraught with risk of congestion and edema due to closure tension or pedicle compression. Keystone island flaps on the other hand rely on multiple unnamed vertical musculocutaneous and fasciocutaneous perforators from the flap bed and have been found to be less prone to venous congestion and pin-cushioning.[1] The skin of the posteromedial thigh is more lax than the anterolateral and so is the skin of upper two third of thigh compared to the lower third. Keystone flaps have been used widely for small to large defects in the thigh with very good outcomes.[1],[2],[3],[4] But many authors have advised caution in its use in lower thigh defects.[3] We demonstrate an excellent outcome in reconstruction of a large lower thigh soft tissue defect with bilateral keystone flaps.

 Case Report

A 63-year-old gentleman presented with soft tissue sarcoma of the lower anterolateral left thigh. Oncosurgical excision resulted in partial excision of the quadriceps and iliotibial tract and an elliptical defect of size 20 cm × 13 cm extending from mid-thigh to a few centimetres above the patella. The width of defect was approximately one third the circumference of the thigh [Figure 1]. Approximately 2 cm of the defect could be closed primarily on both upper and lower pole and the final defect measured 16 cm × 13 cm.{Figure 1}

Operative technique

Medial keystone flap was planned first. Medial edge of the defect formed the inner arc of the flap and the outer arc was drawn medially at a distance equal to the defect roughly parallel to the inner arc. From the upper pole of the defect a line perpendicular to the inner arc was drawn medially that intersected the outer arc; from the lower pole a more horizontal line was drawn to avoid encroaching the popliteal fossa that intersected the outer arc to form the flap outlines. Skin incision was made along the flap margin; blunt dissection was carried out till the deep fascia protecting any major neurovascular structures. The great saphenous vein was protected along with some soft tissue at the upper and lower flap incisions. Deep fascia was incised circumferentially. Subfascial undermining of flaps were performed adjacent to the defect margin till large perforators were encountered which were around the medial intermuscular septae. Similarly flap undermining was performed from the posterior flap margin till large perforators were encountered anteriorly. The intermuscular septae were divided at the upper and lower flap margins up till the femur taking care not to injury any large perforators, vessels or nerves in the area. Flap was mobilized but covered only half of the defect.

A second lateral keystone flap was planned next of width equal to the original defect width and was marked and raised similar to the medial flap [Figure 2]. The outer arcs of the medial and lateral flaps intersected in the posterior thigh. Flap was undermined adjacent to defect and posteriorly till larger perforators were encountered. Longitudinally the central thirds of both the flaps remained attached to the bed. Once the lateral flap was mobilized the medial flap was able to advance further anteriorly to cover almost two thirds of the defect. The lateral flap comparatively advanced only about 4–5 cm. The two flaps were advanced into the defect and sutured to each other. The upper outer corners of the flap were closed with V to Y closure and the exposed posterior muscle compartment of thigh was skin grafted. No drains were placed and a splint was applied with knee in extension [Figure 3].{Figure 2}{Figure 3}


Operative time was 90 min. Wound healing was fast and painless. First dressing was done on day 5 and splint was removed on day 10 once graft take was acceptable [Figure 4]; patient was allowed to ambulate and discharged on day 10. Once he started ambulating, the flap as well as the limb developed moderate edema and patient was advised leg elevation at rest and compression garment [Figure 5].{Figure 4}{Figure 5}

Histopathology revealed undifferentiated pleomorphic sarcoma and patient underwent a full course of radiotherapy. At 6 months follow up, flap edema had settled completely, the limbs appeared symmetrical and there was only minimal edema restricted to the ankle and foot; grafted area in the posterior thigh had contracted significantly; there was no tightness/contracture in the popliteal fossa; he was able to walk comfortably and did not use compression garment. Although the sensations over the flap were not objectively assessed since operation, patient reported some improvement in sensations in the flap region [Figure 6] and [Video 1].{Figure 6}


Keystone design island perforator flaps are an ingenious design, making closure of large defects possible with local tissue and minimal donor site morbidity. Initially one of the principles of these flaps included no undermining of flap and hence the blood supply was thought to be dual-through longitudinal subcutaneous connections that would be preserved at the flap margins and through vertical perforators from the flap bed but subsequent modifications have established that perforators alone are adequate for flap vascularity.[2],[5] Contrary to nonislanded flaps, there is a postoperative hyperemia seen in keystone flaps probably due to a sympathectomy like effect and a preferential vascular supply exists to the flap periphery.[6] Incidence of vascular compromise and edema in keystone island flaps is lesser compared to fasciocutaneous and perforator flaps.[1],[7]

Behan classified keystone flaps into four types. In Type 1 keystone flaps, the deep fascia is left intact. In Type 2 keystone the deep fascia is released. Type 2 is further divided into 2a and 2b. In Type 2a the donor site is closed primarily whereas in Type 2b donor site is covered with split thickness skin graft. In Type 3, double keystone defects are used to cover the defect. In Type 4, keystone flaps are raised with undermining upto 50% of the flap subfascially.

Moncrieff et al. described Type 1 flap where skin bridge is intact and fascia is released along the greater arc of the flap. Type 2 and Type 3 are depicted in the [Figure 7].{Figure 7}

Behan added the omega variant of keystone flap to reduce the tension at the centre of the original defect. Similar modification was suggested by Rao and Raine.

Bhat modified keystone flap as a modification of Type 2a flap with V incision avoided on the side with deficient skin. Behan described the Yin yang flaps which utilize two disparate KDPIFs to reconstruct a single large defect. Saint-Cyr described the design axis of flap parallel and centred over major perforator clusters.[1],[2],[3],[4],[5],[8]

Owing to the dependent nature of lower limb, any trauma or surgery is usually associated with prolonged edema. Therefore, circumferential dissection in the lower limb is not advisable. To our surprise our patient showed excellent recovery and very minimal edema despite circumferential dissection and radiotherapy. Literature has multiple reports of large keystone flaps of the thigh encompassing more than half the thigh circumference, entire length of thigh and flaps with significant undermining.[9],[10] In all such cases the major medial superficial lymphatic system is likely to have been partially if not completely disrupted. However there is no mention of associated prolonged lymphedema any more than expected with any other lower limb surgery.

The superficial lymphatic system of the lower limb is extensive and runs along the great and short saphenous veins and drains into inguinal and popliteal nodes respectively. The deep lymphatics are fewer in number and accompany the deep arteries of the leg and drain mainly in the popliteal nodes. The possible explanations for minimal edema in our case could be that the short saphenous vein, deep lymphatics in the leg and the draining popliteal nodes were undisturbed; the great saphenous vein and soft tissue accompanying it were preserved that may have protected some superficial lymphatic continuity. Also since radiotherapy involved anterolateral thigh, the popliteal fossa and the main superficial lymphatic channels that lie medially were safeguarded. Another theory is an increase in angiogenesis and probably lymph angiogenesis in the keystone flaps.[1] This could be a reason why the edema that had appeared in the early postoperative period resolved with time.

Other local reconstructive options such as medial fasciocutaneous flap would have been inadequate and we deferred a perforator propeller flaps as the major anterolateral thigh flap perforators were damaged during resection and did not feel confident to raise such a large flap based on a single medial thigh perforator. Our design was a modification of Behan Type 2a (circumferential deep fascia release with skin graft of donor site) and Type 3 (double keystone) KDPIF and to our knowledge the first description of use of circumferential incisions in the limb for keystone flaps.[1] Undermining the flap increases the extent of coverage by further advancement or rotation and few described modifications require undermining of up to 90% of flap area.[4] We also considered other design modifications with more aggressive undermining and rotation advancement of flap.[1],[3],[11] However, the large size of the defect, proximity of knee joint and tight lower thigh skin restricted our options, and we believe that we achieved superior aesthesis with our design. The only other viable option to restore good tissue cover for this defect was a free tissue transfer.

The reasons for good outcome in our case were use of large flaps, inclusion of multiple perforators, circumferential division of fascia and use of skin graft posteriorly that reduced tension across the flaps. In addition to the clinical advantages like rapid and painless recovery, low morbidity, good aesthesis, tolerance to radiotherapy, this flap is easy to perform, has a lesser learning curve, obviates microsurgical dissection and saves operative time. The most important thing is knowledge and preservation of the perforator hotspots.[12] The contraindications for keystone flap include previous surgery and degloving-avulsion trauma in the adjacent skin. In previously irradiated skin, larger flaps are safer. Young patients, Fitzpatrick skin Types 4 and 5 and areas with tight skin like palms and soles have less favourable viscoelastic properties for keystone flaps.[3]

In our opinion, other areas like the leg, trunk and upper limb and patients with deep vein thrombosis may not be suitable candidates for a circumferential design. More cases need to be done to explore the safety and applicability of this design. A study of the lymphatic system could help to elucidate if superficial lymphatics regenerate across the keystone flaps with time.

Flap design for a defect would depend on the site and size of defect, extensibility and concentration of perforator clusters in the adjacent skin. Flap design for a defect would depend on the site and size of defect, extensibility and concentration of perforator clusters in the adjacent skin. Initially flaps are planned as Type 1 and if more advancement is required then lateral fascia is incised, thus making it Type 2. Further advancement is provided by undermining the flap if defect is large the bilateral flaps may be raised.

Keystone flaps are avoided in areas with degloving injury or previous surgery as there may be underlying injury to perforators. In areas with previous radiation or suspected injury, larger flaps including more perforators are safer than smaller.


The design of keystone flap should be carefully chosen as per the defect size and location. We successfully executed a double KDPIF in the lower thigh by using circumferential incisions and skin grafts of donor site for a large defect. However, further studies on circumferential keystone flaps in the limb and study of lymphatic regeneration postsurgery need to be done to prove its safety.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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