Fascia, vessels, and nerves of the lower limb

Transcript

Content Reviewers:

Viviana Popa, MD

The lower limb contains an intricate metro system of blood vessels, muscles, and nerves. These structures are wrapped up and organized into different compartments by superficial and deep fascia layers, and together, they form the multifunctional limbs we know as legs.

Let’s begin with fascia, which is... well, what is it anyway? You can think of fascia as a pair of thin stockings made of connective tissue that support and bind together different parts of the body, including the lower limbs.

Now, each lower limb actually has two pairs of stockings on top of each other: the superficial fascia - which sits right underneath our skin - and the deep fascia - which sits on top of the muscles, organizing them into compartments.

Ok, so the superficial fascia - also known as subcutaneous tissue - is located directly deep to the skin and consists of loose connective tissue, with variable amounts of fat. It contains arteries, veins, nerves, lymph vessels and lymph nodes.

Superiorly, the fascia is continuous with the subcutaneous tissue of the anterolateral abdominal wall and buttocks. Inferiorly, at the knee, the superficial fascia loses its fat and blends with the deep fascia but fat is again present distal to the knee, in the subcutaneous tissue of the leg.

Moving on, deep fascia is especially strong and elastic, allowing it to limit the outward expansion of contracting muscles, and act as a barrier to help compress the veins, which in turn, propel blood towards the heart.

The deep fascia can be divided into the deep fascia of the thigh, which is called fascia lata; and the deep fascia of the leg, or the crural fascia. First, the fascia lata, is very strong and flexible.

Superiorly, the fascia lata attaches to the inguinal ligament and pubic bone anteriorly, to the iliac crest laterally and posteriorly, and to the sacrum, coccyx, sacrum, sacrotuberous ligaments, and ischial tuberosity posteriorly.

It also attaches to the membranous layer of subcutaneous tissue of the inferior abdominal wall, also called Scarpa fascia, just inferior to the inguinal ligament.

Regarding its inferior limits, anteriorly and on both sides of the knee, fascia lata attaches to the subcutaneous bony prominences of the knee joint and is continuous with the deep fascia of the leg inferior to the knee. That was a mouthful, wasn’t it?

The fascia lata encloses the large thigh muscles, and is thickened laterally, by an excess of longitudinal fibers, to form the iliotibial tract.

The iliotibial tract, which extends from the iliac tubercle superiorly to the anterolateral tubercle of the tibia inferiorly, also known as Gerdy tubercle. It is the shared aponeurosis of the tensor fasciae latae and gluteus maximus muscle.

And in case you were wondering, an aponeurosis is a type of deep fascia consisting of a sheet of pearly-white fibrous connective tissue that helps attach sheet-like muscles needing a wide area of attachment.

The deep surface of the fascia lata forms three intermuscular septa - a fancy word for walls. These are the medial, lateral, and posterior intermuscular septa, which attach to the linea aspera of the femur.

Out of the three, the lateral intermuscular septum is the strongest and extends from the iliotibial tract to the lateral lip of the linea aspera and lateral supracondylar line of the femur.

Now, the septa and the fascia lata form the walls of three thigh muscle compartments—anterior, medial, and posterior. Each of these compartments contain and separate the muscles of the thigh based on function, and also house their own neurovascular structures associated with those muscles.

The anterior compartment lies between the lateral and medial intermuscular septa, and contains muscles that flex the hip joint and extend the knee joint.

The medial compartment lies between medial and posterior septa and contains muscles that adduct the thigh. And finally, the posterior compartment lies between posterior and lateral septa and contains muscles that extend the hip and flex the knee.

There’s also an oval gap in the fascia lata - the saphenous opening, which is located inferior to the medial part of the inguinal ligament in the anteromedial thigh.

The center of the opening is about 4 centimeters below and lateral to the pubic tubercle and is about 3.75cm in length and 2.5cm in breadth.

The saphenous opening is not actually an opening since it is usually covered by a curtain of fat and fibrous tissue called the cribriform fascia. “Cribriform” comes from Latin, and it means “sieve”, or “strainer”.

So the cribriform fascia is pierced by numerous structures such as the great saphenous vein before it drains into the femoral vein, the superficial epigastric and superficial external pudendal vessels, and a few lymph vessels connecting superficial and deep inguinal lymph nodes.

In addition to the deep fascia of the thigh, we have the deep fascia of the leg called the crural fascia which is continuous superiorly with the fascia lata.

This deep fascia attaches to the anterior and medial border of the tibia, where it is continuous with the bone’s outer layer - the periosteum. Posteriorly, behind the knee, it forms the popliteal fascia, which covers the popliteal fossa.

Proximally, the fascia is thick and forms part of the proximal attachments of the underlying muscles. And distally, the deep fascia of the leg forms thickened horizontal bands of tissue, both superior and anterior to the ankle joint, called the extensor retinaculum.

The crural fascia also has an anterior intermuscular septum and a posterior intermuscular septum. These two septa extend inward from the deep crural fascia surface and attach to the anterior and posterior borders of the fibula, respectively.

The septa, together with interosseous membrane, which is a membrane that unites the fibula and tibia, divide the leg into three compartments. There is a lateral compartment, separated by the anterior and posterior septa, which contains the evertors of the foot.

The anterior compartment is separated by the anterior septum and interosseous membrane, which contains muscles that invert the foot and extend the ankle joint. By the way, a fancy word you might want to remember for extension of the ankle joint is “dorsiflexion”.

And finally there is the posterior compartment which is separated by the posterior septum and interosseous membrane and contains muscles that invert the foot and flex the ankle joint. Another fancy term for flexion of the ankle joint is “plantar flexion”.

The posterior compartment is further subdivided by a transverse intermuscular septum creating a deep and superficial portion of the posterior compartment. And that’s a wrap - get it? Because fascia acts as a wrap for… nevermind!

Let’s instead take a break and try to recognize some of the most important features of the fascia of the lower limb! Let’s now focus on the veins of the lower limb. These can be divided into superficial veins and deep veins.

The superficial veins are located in the superficial fascia or subcutaneous tissue, and, therefore, they run independent of major arteries. So, unlike other major veins of the body that are named according to the arteries they travel with, these are named on their own.

The deep veins of the thigh are located underneath the deep fascia and accompany the major arteries of the lower limb. Both superficial and deep veins have valves, however, these are more numerous in deep veins because they transport most of the blood of the lower limbs.

You can think of valves as elastic flaps of endothelium with cup-like cusps that fill from above. When they are full, the valve cusps block the lumen of the vein, preventing reflux of the blood distally, as blood tends to flow downward, due to gravity.

The valves preventing backflow, combined with the compression exerted by muscle contraction, forms the musculovenous pump, which overcomes the force of gravity to return the blood the heart.

Ok so, first, the two main superficial veins are the great and small saphenous veins, which lie in the superficial fascia. The great (or long) saphenous vein is formed at the dorsum of the foot by the dorsal vein of the great toe and the dorsal venous arch of the foot.

Now, the great saphenous vein is ambitious and aims high: from the dorsum of the foot, it passes on the anterior part of the medial malleolus of the tibia at the ankle, and scends almost vertically from the medial to the posterior part of the tibia.

It then reaches the popliteal region, where it’s situated on the posterior part of the medial condyle of the femur. At the superior border of the popliteal region, it becomes anterior again, where it traverses the thigh anterosuperiorly to the saphenous opening, where it finally empties into the femoral vein. A long and winding road, indeed!

Now, as it climbs up the leg and thigh, the great saphenous vein receives numerous tributaries or branches, while also communicating in several locations with the small saphenous vein through communicating veins.

Higher up, tributaries from the medial and posterior parts of the thighs frequently unite to form the accessory saphenous vein. When present, this vein becomes the main communication between the great and small saphenous veins.

And finally, near its termination, the great saphenous vein also receives the superficial circumflex iliac, superficial epigastric, and external pudendal veins.

Moving on, the small saphenous vein arises on the lateral side of the foot from the union of the dorsal vein of the little toe with the dorsal venous arch.

The small saphenous vein ascends posterior to the lateral malleolus, it passes along the lateral border of the calcaneal tendon, and then it ascends roughly in the middle of the leg before piercing the deep fascia.

From here on, it ascends between the heads of the gastrocnemius muscle, until it reaches the popliteal fossa, or the space behind your knee, to empty into the popliteal vein.

Ok, now, bear in mind that the superficial and deep vein systems communicate with each other, and blood is continuously diverted from these superficial veins to the deep veins via the perforating veins.

Perforating veins are called that because they perforate the deep fascia of muscles to connect the superficial veins to the deep veins, and they contain valves which only allow blood to drain from superficial to deep.

These perforating veins are also important to the mechanism of the musculovenous pump because as they pass through the deep fascia, they are compressed during muscular contraction.

This prevents blood from flowing from the deep to superficial veins, and directs venous blood to the heart. Now let’s look at the deep veins of the lower limb, which lie in the fascial compartments formed by the deep fascia.

They accompany all the major arteries of the lower limb and their branches. Additionally, deep veins usually come in pairs that flank the artery they accompany, meaning that each major artery of the lower limb is paired with two deep veins .

Now, the veins and the artery they follow are wrapped up within a vascular sheath. As the muscular arteries undergo pulsatile contractions, they compress the deep veins against the vascular sheath, propeling blood towards the heart.

Ok, so the main deep venous structures of the foot begin from the medial and lateral plantar veins, which arise from the deep plantar venous arch on the plantar aspect of the foot.

The medial and lateral plantar veins travel proximally to the posterior side of the medial malleolus, where they unite to form the posterior tibial vein. They also often contribute to the formation of the fibular vein which ascends posterior to the fibula.

Additionally, the anterior tibial vein travels through the anterior leg arising from perforating veins. Travelling proximally up the knee, these three veins, the anterior tibial, posterior tibial, and fibular veins unite to form the popliteal vein.

The popliteal vein travels upward through the popliteal region until it reaches the adductor hiatus. where it enters the thigh and becomes the femoral vein.

The femoral vein accompanies the femoral artery throughout its upward trajectory in the thigh, until it reaches the inguinal ligament and leaves the thigh, at which point it is known as the external iliac vein.

Moving on, veins that accompany the perforating arteries branching from the profunda femoris artery drain blood from the thigh muscles and terminate in the profunda femoris vein, or deep vein of the thigh, which is another major venous structure that empties into the terminal portion of the femoral vein at the inferior-most portion of the ischial tuberosity.

Overall, deep veins are more variable and they anastomose much more frequently than the arteries they accompany. See? All that hard work was not in vein - because we’ve actually reached the end of this section.

Time for a break and for you to recognize some high-yield landmarks. Ok, so, moving on, the lower limb also has both superficial and deep lymphatic vessels.

The superficial lymphatic vessels are like a network of lines that converge and accompany the saphenous veins and their tributaries.

Now, the lymphatic vessels that accompany the great saphenous vein originate on the dorsal surface of the foot and end in the superficial inguinal lymph nodes.

These lymph nodes are separated into two groups, the horizontal group which is about 2 centimeters below the inguinal ligament, and the vertical group, which is around the proximal end of the great saphenous vein.

Most lymph from these nodes passes directly to the external iliac lymph nodes, which are located along the external iliac vein, but some lymph from the superficial nodes also passes to the deep inguinal lymph nodes.

On the other hand, the lymphatic vessels accompanying the small saphenous vein arise from the lateral surface of the foot and enter the popliteal lymph nodes, located in the fat of the popliteal fossa surrounding the popliteal vein.

The deep lymphatic vessels of the lower limb follows the corresponding main artery and enter the popliteal lymph nodes. Most lymph from the popliteal lymph nodes ascends through deep lymphatic vessels to the deep inguinal lymph nodes.

Lymph from the deep inguinal nodes passes to the external and common iliac lymph nodes and then enters the lumbar lymphatic trunks. So now let’s talk about the peripheral nerves, which can be divided into cutaneous, motor, and mixed nerves.

Here we’re going to focus on the cutaneous innervation of the lower limb, which is derived mostly from the lumbar plexus, formed by the anterior rami of the L1 to L4 spinal nerves, and the sacral plexus, formed by the anterior rami of the L4 to S4 spinal nerves.

The cutaneous nerves are located in the subcutaneous tissue and they supply sensory innervation to the skin of the lower limb. Ok, so let’s start with the cutaneous innervation of the thigh.