Complex Chimeric Flap Microsurgery

Extensive and 3-dimensionally complex defects require free flap reconstruction with multiple tissue types. This has traditionally been accomplished with 2 or more free flaps. Recent advancement in perforator anatomy and perforator flaps has enabled us to explore flap options with multiple tissue components based on a single “mother” source vessel: the chimeric flap. A chimeric flap may consist of various combinations of 2 or more fasciocutaneous, muscular/myocutaneous, or osseous flaps with freedom for each tissue component to be separated to cover a complex defect. The design of chimeric flaps based on several vascular systems will be discussed in this article.

Key points

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A chimeric flap consists of two or more tissue types that are linked by blood vessels such as perforators with great freedom for each component.
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A single chimeric flap may provide excellent reconstruction of a complex three-dimensional defect that would otherwise require two free flaps.
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The most versatile vascular axes for chimeric flaps include the lateral circumflex femoral system, the peroneal system, and the subscapular system.
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An anteromedial thigh (AMT) flap can be harvested in conjunction with an ALT flap or as a replacement when ALT perforators are absent.
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Two distinct groups of cutaneous perforators in the peroneal system may provide a chimeric fibular flap option with more than one skin island.

Abbreviations

3D	3-dimensional
ALT	anterolateral thigh
AMT	anteromedial thigh
ASIS	anterior superior iliac spine
DCIA	deep circumflex iliac artery
TDAP	thoracodorsal artery perforator
TFL	tensor fascia lata
UAP	ulnar artery perforator

Introduction

Microsurgical techniques have revolutionized reconstructive plastic surgery and the ability to restore form and function. Complex 3-dimensional (3D) defects can require multiple tissue components for anatomic restoration. For example, in the head and neck region, radical tumor resection may require replacing the intraoral lining, craniofacial skeleton, and external skin. While reconstruction of multiple defects can be achieved with multiple free flaps, this can be less ideal for several reasons, including paucity of recipient vessels, prolonged surgical time, and additional donor site morbidity. A flap with multiple tissue components linked together may serve this purpose and simplify reconstruction. Such a flap is traditionally referred to as a “compound” flap. The simplest compound flap may be referred to as a “composite” flap in which 2 or more tissue components are naturally adherent to each other. The traditional angiosome concept proposed by Taylor demonstrates that the body is separated into distinct vascular territories composed of muscle, overlying adipose tissue, and skin. The specific angiosomes define the anatomic boundaries for composite tissue that can be harvested as a flap. While these composite flaps contain various tissue components, all the elements depend on an interconnected network of blood vessels that are not separated and, therefore, are not very maneuverable. Typical examples of a composite flap may be the pectoralis major myocutaneous flap, latissimus dorsi myocutaneous flap, and the traditional radial forearm fasciocutaneous flap. Such composite flaps are less useful for a complex 3D defect.

Chimeric flap concept and clinical considerations

A more versatile compound flap would be one in which the tissue components are linked by perforators from a common source vessel. These tissue components can be separated, allowing much greater flexibility and freedom in the movement of each subunit to cover various aspects of the defect compared with a composite flap. Such a flap is referred as a “chimeric” flap. ^, In Greek mythology, the Chimera was a fire-breathing monster with a lion’s head, a goat’s body, and a serpent’s tail. Chimeric flaps have distinct tissue components that are isolated on their own vascular pedicle or perforator and are independent of any other physical interconnection except their link to the common source vessel. This independent mobility of each tissue type allows for single-stage reconstruction of very complex defects.

Although the definition of a chimeric flap is somewhat controversial, it is traditionally considered a flap composed of different tissue types such as skin, muscle, and bone. A loose terminology may include flaps with a single type of tissue but is separated with perforators, such as 2 independent skin islands based on separate perforators from one vascular pedicle ( Fig. 1 A–E ). Naturally linked tissue components based on a single source vessel are considered “intrinsic” chimeric flaps. In contrast, tissue components can also be linked with microsurgical techniques by anastomosing one flap to another and have been referred to as “fabricated” chimeric flaps. This was first described by Sanger and colleagues, in a sequential connection, or serial connection, or flow-through design. Such a design was later found to have higher rates of flap complications, and a “parallel” connection was preferred by connecting the second flap to a branch of the vascular pedicle of the first flap. ^,^,

A series of five anatomical diagrams shows different ways a chimeric flap can be used in reconstructive surgery. — Fig. 1

There are potential drawbacks to using chimeric flaps. Anatomic variability in the distribution and number of perforators can restrict the design and use of certain tissue types. This degree of unpredictability requires a flexible mindset for the reconstructive surgeon who plans to utilize a chimeric flap configuration. The mobility of each component can be limited depending on the length of the branch or perforator of its components. This can lead to the shortening of the overall pedicle length or excessive tension during the inset creating a new set of challenges such as the need for vein graft. Ensuring that each component is not twisted or kinked is critical to avoid ischemia of individual tissue components. Additionally, while there is a theoretic benefit to harvesting multiple tissues from a single donor site, the potential for greater donor site morbidity compared with harvesting multiple flaps, each individually smaller than the single chimeric flap exists. Ultimately, the successful use of chimeric flaps requires a thorough understanding of the defect, well thought out planning, and the ability to adjust based on anatomic variability encountered during flap harvest.

Flaps based on the subscapular system are among the earliest flaps discovered in the 1980s. ^,^, Its vascular anatomy is well known to have the ability to carry various tissue components: the scapular and parascapular flaps, latissimus dorsi, serratus anterior, and the scapular bone. With the better understanding of perforator anatomy and the popularization of perforator flaps in the last 2 decades, chimeric flaps from other vascular systems have been described and popularized. The purpose of this article is to discuss various types of chimeric flaps based on their vascular axis and operative techniques for successful flap harvest.

Chimeric flap systems

The Lateral Circumflex Femoral System

The lateral circumflex femoral vessels originate from the profunda femoris and have 3 main branches: ascending, transverse, and descending. These branches nourish a rich vasculature network providing perfusion to the tensor fascia lata (TFL), rectus femoris, and vastus muscles, as well as the overlying thigh skin. The transverse and descending branches, in particular, serve as the basis for the workhorse thigh-based flaps, the anterolateral thigh (ALT), anteromedial thigh (AMT), and the TFL flaps. The ALT flap has become one of the most popular soft tissue flaps in the last 2 decades. This flap incorporates the anterior and lateral thigh skin on a longitudinal axis from the anterior superior iliac spine (ASIS) to the superolateral patella. There are typically 1 to 3 cutaneous perforators just lateral to this line, with the most consistent perforator at the midpoint (perforator B) and 2 other perforators found approximately 5 cm proximal (A) and distal (C) to the perforator B. ^, Our studies have shown that in the majority of cases (96%), the ALT flap perforators originate from the descending branch. However, the perforators can also come from the transverse branch (2%). These perforators travel either through the intermuscular space (septum) between the rectus femoris and the vastus lateralis muscles (septocutaneous perforator) or through the vastus lateralis muscle (musculocutaneous perforator). In either case, these perforators enter the skin lateral to the septum. Approximately half of the patients have 2 perforators, 26% have a single perforator, and 24% have all 3 perforators. The descending branch is also the main blood supply to the vastus lateralis muscle. Therefore, a chimeric flap based on the descending branch can carry one ( Fig. 2 A–D ) or more ( Fig. 3 A–D ) independent skin paddles as well as the vastus lateralis muscle.

Four surgical photographs show the process of a thigh flap being harvested and used for facial reconstruction. — Fig. 2

Four surgical photos show tissue taken from the thigh and shaped into two skin flaps for repairing both inside the mouth and the outer cheek. — Fig. 3

In our initial study, we found that, in 2% of the cases, the ALT flap perforator originated neither from the descending branch nor from the transverse branch. Instead, it pierced the rectus femoris muscle to reach the skin medial to the septum. We later found that this perforator actually came from the rectus femoris branch that originated from the proximal part of the descending branch, traveled medially underneath the rectus femoris muscle, then descended along the medial edge of the rectus femoris muscle while sending out perforators to the skin either through the rectus femoris muscle or the septum between the rectus femoris and sartorius muscle ( Fig. 4 A, B ). These perforators are, in fact, the primary blood supply to the AMT flap. ^, Our study also showed that ALT flap perforators were absent in 4.3% of the thighs. Further perforator mapping revealed a reciprocal relationship between the ALT and AMT flap perforators. When there are no ALT perforators, there is a high probability of having AMT perforators. While ALT perforators are present in 96% of cases, AMT perforators are only present in 51% of the cases. However, in nearly half of the cases, there are both ALT and AMT perforators, especially when there is only one ALT perforator present, making it possible to create a chimeric ALT-AMT flap ( Fig. 5 A–C ).

Two medical photos show the anatomy of the anteromedial thigh flap with the main vessel running beneath the rectus femoris muscle and tiny branches reaching the overlying skin. — Fig. 4

Three surgical photos show a combined anterolateral thigh (A L T) and anteromedial thigh (A M T) flap being prepared for a surgical procedure. — Fig. 5

Since half of the patients do not have AMT perforators, surgeons usually start with the ALT flap design and make the medial incision first to explore ALT perforators. When no ALT perforators are found, or only one ALT perforator is present, but the reconstruction requires 2 skin paddles, AMT perforators can be explored through the same incision, proceeding medially over the rectus femoris muscle. Because the rectus femoris branch supplies the rectus femoris muscle and the descending branch supplies the vastus lateralis muscle, the chimeric option can include both ALT and AMT skin paddles as well as 1 or 2 muscle components, making the thigh one of the most versatile options for chimeric flap harvest ( Fig. 6 A, B ).

Two medical photos show a harvested surgical flap consisting of skin and muscle. — Fig. 6

The thigh-based chimeric flaps can also include a component of bone, either the iliac crest or a portion of the femur. The ascending branch of the lateral circumflex femoris vessels typically runs deep to the rectus femoris and medial to the TFL muscle. It supplies branches to the adjacent muscles as it ascends toward the ASIS and ultimately runs on the periosteum of the lateral aspect of the iliac crest. This branch can be used to harvest a chimeric flap consisting of the split lateral iliac crest as a bony component, with the other branches of the circumflex system supplying the soft tissue of the thigh. The length of the ascending branch of the lateral femoral circumflex system is approximately 10 cm, allowing for significant mobility of the bony component. The femur can also be harvested as a chimeric bony component with the traditional ALT flap. The descending branch of the lateral femoral circumflex vessels traditionally gives off deep branches to the vastus intermedius muscle. A cuff of vastus intermedius and the associated periosteum can be isolated based on this penetrating branch to harvest a vascularized piece of lateral corticocancellous femur. While the risk of fracture in the weight-bearing femur is a concern, biomechanical studies have shown that as much as 40% of the femoral circumference can be removed without significantly weakening the bone. Although the bone-containing thigh-based chimeric flaps are technically possible, they never gained popularity because (1) surgical dissection of these deep tissue components and vessels is more difficult with potentially higher donor site morbidities, and (2) other bone flaps may offer better quality of bone and the possibility of multiple osteotomies.

The Peroneal System

The peroneal system provides the blood supply to the fibular bone, which remains the workhorse osseous flap to this day. The peroneal artery originates from the popliteal artery via the tibioperoneal trunk and descends in the lower part of the leg between the flexor hallucis longus and tibialis posterior muscles. During this course, it sends several branches to the soleus muscle as well as numerous perforators which typically run in the posterior crural septum to supply the overlying lateral leg skin. Historically, the viability of the fibular flap skin paddle was an issue. ^,^,^, With a better understanding of the perforator anatomy, the peroneal system has become one of the most versatile chimeric flap options. Our study demonstrated that there are 2 distinct groups of cutaneous perforators. The distal group, consisting of up to 3 perforators (A, B, and C), is generally located over the third quarter of the fibula, with perforator A near the midpoint, perforator B near the two-thirds point, and perforator C near the three-quarter point. These distal perforators are commonly used for skin paddle design. In addition, a distinct proximal perforator (P) is consistently located at approximately one-third of the length of the line from the fibula head to the lateral malleolus, about 13 cm from the fibular head in adults. Because of the independence of the perforator P, a second skin paddle can be designed to create a multiple skin paddle chimeric flap. A thorough understanding of the perforator P anatomy is therefore important.

While most of the distal perforators are septocutaneous, perforator P is usually (90%) musculocutaneous, piercing the soleus muscle. The origin of perforator P can be variable. Based on the origin and course, we have classified the anatomy of the P perforator into 4 types. In type I (59%), the P perforator originates from the peroneal artery and pierces the soleus muscle to reach the skin. In type II (6%), the P perforator originates from the peroneal artery and travels in the posterior septum to reach the skin. In type III (32%), the perforator P originates from the posterior tibial artery and travels through the soleus muscle. In type IV (9%), the P perforator receives dual blood supply from both the peroneal and posterior tibial arteries via intramuscular connections within the soleus muscle. Therefore, perforator P can be used to create a second skin paddle based on the peroneal pedicle in approximately two-thirds of the patients. This is particularly useful in mandibular reconstruction when there is a need for both intraoral and extraoral resurfacing ( Fig. 7 A–F ).