8.1 Introduction

This review of the vascular anatomy of the lower extremity is intended to emphasize the details of the anatomy of greatest interest to the reconstructive surgeon. Since the lower extremity is a common site for trauma and for reconstruction, knowledge of the vascular architecture of the lower limb is critical before deciding on an appropriate flap donor site or the planning of recipient vessels. The status of the main arterial supply to the lower extremity as well as the general pattern of individual perforators will be helpful in this regard.

8.2 Vascular Territories and the Angiosome Concept

Taylor and Palmer introduced the angiosome concept, which has been a guide to our understanding of the vascular territories of the human body. ¹ The word angiosome is derived from the Greek word angeion, which means vessel, and somite, which means segment of the body. The body can be divided into a series of angiosomes (vascular territories) based on the anatomy of the individual source artery to that angiosome. Each angiosome is a composite block of tissue all supplied by one main named source artery, and may consist of bone, muscle, fascia, and skin as well as other tissues. The main source vessel within the given angiosome gives off multiple branches or perforators, which in turn will be the vascular supply to each of its tissues. Individual angiosomes are linked to adjacent angiosomes via reduced caliber choke anastomotic vessels or true anastomotic vessels, which are subject to numerous physiologic influences (e.g., a delay procedure). The specific anatomy of a given angiosome and presence (or absence) of individual perforators or branches will then affect the survival of tissues harvested. A larger source artery with a larger angiosome and larger perforators may supply larger flaps and vice versa. The smallest division of the angiosome is that of an individual perforator that supplies a discrete region of tissue or perforator angiosome. The size of the vascular territory supplied by an individual perforator is dependent on a variety of factors including the size of the perforator, region of the body, and intrinsic patient factors, which may influence blood flow.

8.3 Arterial Territories

The integument of the human body can be divided into about 60 vascular territories that are supplied by about 400 perforators. However, the arterial framework of the body is actually a huge continuous interlocking arcade of vessels through all layers of all tissues. Arteries tend to give a branch to each structure they pass through, which has led to a myriad of terms such as fasciocutaneous, septocutaneous, and musculocutaneous perforators. The vessels that pierce the deep fascia and supply the skin may be direct, such as would be axial or septocutaneous perforators, or indirect, as those perforators which pass through other structures first en route to the skin such as would be musculocutaneous perforators. Note that there is roughly a 2/3:1/3 ratio of musculocutaneous to septocutaneous perforators throughout the body. The important point is to be able to identify the pattern of distribution of individual perforators in order to capture a suitable skin vascular territory in a flap.

The lower extremity has 21 vascular territories with a mean of about 180 perforators (Table 8‑1). ^{2 ,​ 3} The location of these vascular territories in the lower extremity was summarized in a series of cadaver studies (Fig. 8‑1). As a rule, the named source vessels to these vascular territories are large consistent arteries with associated venae comitantes. These provide the vascular supply to numerous flaps in the body and also are available as recipient vessels for free microvascular tissue transfers. The individual cutaneous perforators to skin in the lower extremity are highly variable in terms of size and exact position, but tend to cluster around the main source vessels often with a longitudinal orientation, which allows some anticipation of their location.

8.4 Venous Drainage

The venous anatomy of the body has received relatively less attention, but is certainly equally important for successful reconstructive surgery. It is well known that perforator flap failure is often caused by venous insufficiency. The integument of the body is drained by a superficial and a deep venous network, which are interdependent and interconnected. The deep venous architecture generally mirrors that of the arterial tree and in most regions is duplicated. The superficial venous network is quite different and consists of long named venous channels that connect to the deep system via venae communicantes. Adjacent venous territories of the skin are interconnected via veins without valves, which are called oscillating or bidirectional veins (Fig. 8‑2).

8.5 Neurovascular Territories

The cutaneous nerves of the lower extremity are longitudinally arranged. Each cutaneous nerve is associated with an arterial supply that has a variety of orientations. For example, a series of small arterial branches can join the course of the nerve in a chain-linked pattern, or a single artery could run the full length of the nerve. In general, the cutaneous nerves have a well-vascularized longitudinal axis with indirect branches to the integument, which provides an opportunity to harvest local or regional neurocutaneous flaps (Fig. 8‑3). ⁴

8.6 Vascular Anatomical Principles Applied to Flap Design

For decades, surgeons relied on dogma such as the length-to-width ratio and trial and error to try to predict flap survival. The so-called random flaps were commonly utilized to reconstruct defects with unpredictable results. Fasciocutaneous and musculocutaneous flaps gradually replaced random flaps, as these had a more specific source of circulation that allowed more consistent and reliable results. The more recent enthusiasm for perforator flaps has focused the spotlight on detailed descriptions of the anatomy of individual cutaneous perforators throughout the body, and particularly the lower extremity so that flap survival has been improved.

In addition to knowing the anatomy of cutaneous perforators, an awareness of the physiologic effects on the vascular supply of tissues is important. Cormack and Lamberty described this as the anatomic, dynamic, and potential flap anatomy (Fig. 8‑4). A flap based on a single perforator will completely survive if designed to include only its perforator angiosome, or anatomic territory. A flap that includes that same perforator and an adjacent perforator should also completely survive via flow-through choke or true anastomoses, except when a compromising physiologic condition exists, which is the dynamic territory. Capture of a potential territory by inclusion of a second adjacent perforator territory is possible with a delay procedure or equivalent maneuver (Fig. 8‑5).

The variable nature of perforator distribution, size, and behavior throughout the body is particularly evident in the lower limb. In many regions of the lower extremity, perforator territories are small, and consequently, reliable flaps are smaller. In addition, physiologic challenges to the dynamic or potential surviving territory of a flap are commonplace, including vascular compromise due to diabetes or peripheral vascular disease, dependent edema, and trauma. Thus, it is not surprising that historically a 1:1 length-to-width ratio for a cutaneous flap in the lower extremity was the rule.

8.7 Main Source Vessels of the Lower Extremity

8.7.1 Arterial System

The blood supply to the lower extremity is predominantly a continuation of the external iliac, which proceeds all the way to the foot with name changes as it passes defined landmarks. After the external iliac enters the thigh deep to the inguinal ligament, it is referred to as the common femoral artery (FA). This continues as the superficial FA, popliteal artery (PA), tibioperoneal trunk, posterior tibial artery (PTA), and, ultimately, terminates as the medial and lateral plantar arteries (Table 8‑2). The other well-known arteries of the lower limb are branches arising from this main trunk (Fig. 8‑6).

Table 8.2 Arteries of the lower extremity and their anatomic location

	Main vessel (boundaries)	Primary branches (boundaries)	Secondary branches (boundaries)
Distal—Proximal	Common iliac	>Internal iliac	>Superior gluteal Exits pelvis through greater sciatic foramen, superior to the piriformis Superficial branch between gluteus maximus and medius, deep branch between gluteus medius and minimus >Inferior gluteal Exits pelvis through greater sciatic foramen inferior to piriformis Deep to gluteus maximus
	Sacroiliac joint
	External iliac Medial border of the psoas major
	Inguinal ligament
	Femoral artery Within femoral sheath in femoral triangle bordered by sartorius, adductor longus and inguinal ligament	>Profunda femoris Proximal: adductor longus and brevis Distal: adductor longus and magnus
	Sartorius
	Superficial femoral artery Hunter’s (adductor) canal: vastus medialis, sartorius, adductor longus, and magnus	>Descending genicular Adductor canal Saphenous branch between the sartorius and gracilis, accompanied by saphenous nerve
	Adductor hiatus
	Popliteal artery In popliteal fossa, midline, deep to all muscles	>Anterior tibial artery Between two heads of tibialis posterior, then lies on anterior surface of the interosseous membrane, deep to tibialis anterior >Sural arteries Deep to heads of the gastrocnemius at the level of the femoral condyles	>Dorsalis pedis Anterior to medial malleolus, lateral to the tendon of extensor hallucis longus tendon, overlying medial cuneiform
	Distal edge of popliteus
	Tibial peroneal trunk Midline, deep to soleus	>Peroneal Lies of the posterior surface of the interosseous membrane between FHL and tibialis posterior
	Posterior tibial Proximal: in interval between Achilles and flexor digitorum longus Distal: in tarsal tunnel posterior to medial malleolus
	Medial malleolus
	Medial plantar Proximal: deep to abductor hallucis Distal: lateral to abductor hallucis	>Lateral plantar Between superficial and middle layers of muscle; deep to abductor hallucis, flexor digitorum brevis, and abductor digiti minimi; superficial to quadratus plantae
Abbreviation: FHL, flexor hallucis longus.

The internal iliac artery also has a small contribution to the lower extremity via the gluteal vessels (superior and inferior), which are its terminal branches. The gluteal arteries supply the buttocks and form an anastomoses in the posterior thigh with the medial and lateral circumflex femoral vessels.

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