Popliteal Fossa

Popliteal Fossa

The popliteal fossa, also called the knee pit, is a shallow, diamond-shaped depression located behind the knee joint. It is similar to the cubital fossa found in front of the elbow. The popliteal fossa becomes more visible when the knee is slightly bent.

Boundaries

The popliteal fossa is bounded by the following structures:

• Superomedial: Semitendinosus and semimembranosus muscles
• Superolateral: Biceps femoris muscle
• Inferomedial: Medial head of the gastrocnemius muscle
• Inferolateral: Lateral head of the gastrocnemius muscle and the plantaris muscle

Roof (posterior or superficial wall)

The roof of the popliteal fossa is formed by the following layers:

1. Skin
2. Superficial fascia, which contains:
   • The small saphenous vein
   • Three cutaneous nerves:
     • Branches of the terminal part of the posterior cutaneous nerve of the thigh
     • Posterior division of the medial cutaneous nerve of the thigh
     • Peroneal (sural communicating) nerve
3. Deep fascia, which forms a strong covering and protects the structures inside the popliteal fossa.

Floor (anterior or deep wall)

The floor of the popliteal fossa is formed, from above downward, by the following structures:

1. Popliteal surface of the femur
2. Capsule of the knee joint
3. Oblique popliteal ligament
4. Fascia covering the popliteus muscle

Contents of Popliteal Fossa

The popliteal fossa contains important blood vessels, nerves, lymph nodes, and other structures.

Blood Vessels

• Popliteal artery and its branches
• Popliteal vein and its tributaries

Nerves

• Tibial nerve and its branches
• Common peroneal nerve and its branches

Lymphatic Structures

• Popliteal lymph nodes
• Pad of fat within the popliteal fossa

Other Structures

• Terminal part of the posterior cutaneous nerve of the thigh
• Descending genicular branch of the obturator nerve
• Terminal part of the small saphenous vein

Key description

The popliteal vein and the tibial nerve cross superficially behind the popliteal artery as they pass through the popliteal fossa. Their positions change from the upper part to the lower part of the fossa.

• Upper part of the popliteal fossa (from medial to lateral):
  • Popliteal artery
  • Popliteal vein
  • Tibial nerve
• Middle part of the popliteal fossa (from superficial to deep):
  • Tibial nerve
  • Popliteal vein
  • Popliteal artery
• Lower part of the popliteal fossa (from medial to lateral):
  • Tibial nerve
  • Popliteal vein
  • Popliteal artery

Common Peroneal Nerve

The common peroneal nerve passes obliquely across the popliteal fossa from the upper angle to the lateral angle. It runs along the medial border of the biceps femoris muscle.

Popliteal Artery

The popliteal artery is the main artery of the popliteal fossa.

Beginning

The popliteal artery is the continuation of the femoral artery. It begins at the adductor hiatus, an opening in the adductor magnus muscle.

Course

The artery starts at the junction of the middle one-third and lower one-third of the thigh. It passes downward and slightly laterally through the popliteal fossa to reach the lower border of the popliteus muscle.

Termination

At the lower border of the popliteus muscle, the popliteal artery divides into its two terminal branches:

• Anterior tibial artery
• Posterior tibial artery

Relations

Anterior (Deep)

The popliteal artery lies on the floor of the popliteal fossa. From above downward, it is related to:

1. Popliteal surface of the femur
2. Posterior aspect of the knee joint
3. Fascia covering the popliteus muscle

Posterior (Superficial)

The artery is covered by:

• Skin
• Superficial fascia
• Popliteal fascia

In the middle part of the popliteal fossa, the popliteal vein and tibial nerve lie superficial to the artery.

Medial

• Upper part: Semimembranosus muscle
• Lower part: Tibial nerve, popliteal vein, and medial head of the gastrocnemius muscle

Lateral

• Upper part: Biceps femoris muscle, popliteal vein, and tibial nerve
• Lower part: Plantaris muscle and lateral head of the gastrocnemius muscle

Branches

The popliteal artery gives off the following branches:

1. Cutaneous branches
2. Muscular branches
3. Five genicular (articular) branches:
   • Superior medial genicular artery
   • Superior lateral genicular artery
   • Inferior medial genicular artery
   • Inferior lateral genicular artery
   • Middle genicular artery

Genicular Arteries

• The middle genicular artery pierces the oblique popliteal ligament and supplies the cruciate ligaments and synovial membrane of the knee joint.
• The superior medial and superior lateral genicular arteries pass around the corresponding femoral condyles deep to the hamstring muscles.
• The inferior medial and inferior lateral genicular arteries pass around the corresponding tibial condyles deep to the collateral ligaments.
• These genicular arteries join with each other to form the genicular anastomosis around the knee joint.

Popliteal Vein

The popliteal vein is formed at the lower border of the popliteus muscle by the union of the venae comitantes of the anterior and posterior tibial arteries. The vein ascends through the popliteal fossa and crosses superficially over the popliteal artery along with the tibial nerve.

Termination

The popliteal vein passes through the adductor hiatus, where it continues as the femoral vein.

Tributaries

The popliteal vein receives blood from the following tributaries:

1. Small saphenous vein
2. Veins accompanying the branches of the popliteal artery
3. Venae comitantes of the anterior tibial artery
4. Venae comitantes of the posterior tibial artery

Tibial Nerve

The tibial nerve is the main nerve of the posterior compartment of the leg.

Root Value

The tibial nerve is formed from the ventral primary rami of L4, L5, S1, S2, and S3 spinal nerves.

Course

The tibial nerve is the larger terminal branch of the sciatic nerve. It begins in the popliteal fossa and descends vertically from the upper angle to the lower angle of the fossa. As it passes through the popliteal fossa, the tibial nerve crosses superficially over the popliteal vessels from the lateral side to the medial side. It then enters the posterior compartment of the leg, where it continues its course.

Branches

Muscular Branches

The tibial nerve supplies the following muscles:

1. Medial head of the gastrocnemius
2. Lateral head of the gastrocnemius
3. Plantaris
4. Soleus
5. Popliteus

The nerve to the popliteus winds around the lower border of the muscle and enters its deep (anterior) surface.

Cutaneous Branch

The sural nerve is the cutaneous branch of the tibial nerve. It supplies the skin over the lower half of the back of the leg and the lateral border of the foot up to the tip of the little toe.

Articular Branches

The tibial nerve gives three articular branches to the knee joint:

1. Superior medial genicular nerve
2. Middle genicular nerve
3. Inferior medial genicular nerve

The middle genicular nerve accompanies the middle genicular artery through the oblique popliteal ligament to supply the interior of the knee joint.

Key description

• Most muscular branches of the tibial nerve arise from its lateral side. The only exception is the nerve to the medial head of the gastrocnemius, which arises from the medial side.
• During surgical procedures, the medial side of the tibial nerve is considered safer, while the lateral side should be handled carefully because it gives off most of the muscular branches.
• The sural nerve leaves the popliteal fossa by piercing the deep fascia at its lower angle before descending into the leg.

Common Peroneal Nerve

The common peroneal nerve is the smaller terminal branch of the sciatic nerve.

Root Value

The common peroneal nerve is formed by the dorsal divisions of the ventral primary rami of L4, L5, S1, and S2 spinal nerves.

Course

The common peroneal nerve begins at the upper angle of the popliteal fossa beneath the long head of the biceps femoris muscle. It descends downward and laterally along the medial border of the biceps femoris to reach the lateral angle of the popliteal fossa. The nerve then winds around the neck of the fibula, pierces the peroneus longus muscle, and divides into its two terminal branches:

• Superficial peroneal nerve
• Deep peroneal nerve

Branches

Cutaneous Branches

• Lateral cutaneous nerve of the calf (lateral sural cutaneous nerve): Supplies the skin over the upper part of the lateral side of the leg.
• Sural communicating nerve: Arises near the head of the fibula and passes superficially over the lateral head of the gastrocnemius to join the sural nerve.

Articular Branches

• Superior lateral genicular nerve
• Inferior lateral genicular nerve
• Recurrent genicular nerve

The superior and inferior lateral genicular nerves accompany the arteries of the same name. The recurrent genicular nerve arises just before the common peroneal nerve divides. It ascends in front of the knee joint and supplies the tibialis anterior muscle and the knee joint. Thecommon peroneal nerve does not give any muscular branches in the popliteal fossa.

Popliteal Lymph Nodes

The popliteal lymph nodes are usually 6 to 7 in number and are embedded in the fat of the popliteal fossa.

Location

The popliteal lymph nodes are located in the following positions:

• One node lies at the junction of the small saphenous vein and the popliteal vein.
• A few nodes lie deep to the popliteal artery.
• The remaining nodes are found on either side of the popliteal artery.

Areas Drained

The popliteal lymph nodes receive lymph from:

• The back and lateral side of the calf
• The sole of the foot
• The knee joint

The lymph from these nodes drains into the superficial and deep inguinal lymph nodes.

Clinical Correlation:

Popliteal lymphadenopathy refers to enlargement of the popliteal lymph nodes. It may occur due to local infections, abscesses, or melanoma involving the lower limb.

Anastomosis Around Knee Joint

The anastomosis around the knee joint is a network of arteries that maintains a continuous blood supply to the knee and leg, especially during flexion of the knee when some blood vessels may become compressed.

Arteries Contributing to the Anastomosis

The arterial network around the knee is formed by the following vessels:

1. Genicular arteries
   • Descending genicular branch of the femoral artery
   • Superior medial genicular artery
   • Superior lateral genicular artery
   • Inferior medial genicular artery
   • Inferior lateral genicular artery
   • Middle genicular artery
2. Descending branch of the lateral circumflex femoral artery: This branch contributes to the arterial network on the lateral side of the knee.
3. Anterior tibial recurrent artery: This artery ascends from the anterior tibial artery to participate in the anastomosis.
4. Posterior tibial recurrent artery: This recurrent branch also contributes to the arterial network around the knee.
5. Circumflex fibular artery: The circumflex fibular artery, a branch of the posterior tibial artery, also takes part in the anastomosis around the knee.

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