The Popliteal Fossa

The popliteal fossa is a diamond shaped area found on the posterior side of the knee. It is the main path in which structures move from the thigh to the leg.

In any anatomical area such as this, it is important to look at the borders, contents, and any clinical relevance.


Borders

The popliteal fossa is diamond shaped, with four borders. These borders are formed by the muscles in the posterior compartment of the leg and thigh:

  • Superiomedial border: Semimembranosus.
  • Superiolateral border: Biceps femoris.
  • Inferiomedial border: Medial head of the gastrocnemius.
  • Inferiolateral border: Lateral head of the gastrocnemius and plantaris.

The popliteal fossa also has a floor and a roof. The floor of the popliteal fossa is formed by  the posterior surface of the knee joint capsule, and by the posterior surface of the femur. The roof is made of up two layers; popliteal fascia and skin. The popliteal fascia is continuous with the fascia lata of the leg.

Fig 1.0 - The Muscular borders of popliteal fossa

Fig 1.0 – The Muscular borders of popliteal fossa


Contents

The popliteal fossa is the main conduit for neurovascular structures entering and leaving the leg. Its contents are (medial to lateral): 

Fig 1.1 - The contents of the popliteal fossa.

Fig 1.1 – The contents of the popliteal fossa.

  • Popliteal artery
  • Popliteal vein
  • Tibial nerve
  • Common fibular nerve

The tibial and common fibular nerves are the most superficial of the contents of the popliteal fossa. They are both branches of the sciatic nerve. The common fibular nerve follows the biceps femoris tendon, running along the lateral margin of the popliteal fossa.

The small saphenous vein pierces the popliteal fascia of the popliteal fossa to enter the diamond, and empty into the popliteal vein.

In the popliteal fossa, the deepest structure in the popliteal artery. It is a continuation of the femoral artery, and travels into the leg to supply it with blood.

Clinical Relevance: Swelling in the Popliteal Fossa

The appearance of a mass in the popliteal fossa has many differential diagnoses. The two major causes are baker’s cyst and aneurysm of the popliteal artery.

Baker’s Cyst

Fig 1.2 - MRI Scan of a Baker's cyst.

Fig 1.2 – MRI Scan of a Baker’s cyst.

A baker’s cyst (or popliteal cyst) refers to the inflammation and swelling of the semimembranosus bursa - a fluid filled sac found in the knee joint. The usually arise in conjunction with arthritis of the knee (rhuematoid or osteoarthritis). Whilst it usually self-resolves, the cyst can rupture and produce symptoms similar to deep vein thrombosis.

Popliteal Aneurysm

An aneurysm is a dilation of an artery, which is greater than 50% of the normal diameter. The popliteal fascial layer (the roof of the popliteal fossa) is tough and non extensible, and so an aneurysm of the popliteal artery has consequences for the other contents of the popliteal fossa.

The tibial nerve is particularly susceptible to compression from the popliteal artery. Damage to this nerve will present as leg anesthesia, or loss of leg motor function.

An aneurysm of the popliteal artery can be detected by an obvious palpable pulsation in the  popliteal fossa, with abnormal arterial sounds. 

Other Causes

Rarer causes of a popliteal mass include deep vein thrombosis, adventitial cyst of the popliteal artery and various neoplasms (such as rhabdomyosarcoma).

Quiz

Which of these are the superiomedial border of the popliteal fossa?
Medial head of gastrocnemius
Lateral head of gastrocnemius
Semitendinosus
Semimembranosus

Quiz

Along the tendon of which muscle does the common fibular nerve run?
Semimembranosus
Semitendinosus
Biceps femoris
Plantaris

Quiz

Which of the following is the deepest structure in the popliteal fossa?
Tibial nerve
Common fibular nerve
Popliteal vein
Popliteal artery

Quiz

Which of the following structures become inflamed to form a baker's cyst?
Popliteal artery
Semitendinosus bursa
Semimembranosus bursa
Popliteal vein

Quiz

Which of the following nerves is most likely to become compressed by a popliteal aneurysm?
Sciatic
Tibial
Deep fibular
Common fibular

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