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.
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:
- Superomedial border: Semimembranosus.
- Superolateral border: Biceps femoris.
- Inferomedial border: Medial head of the gastrocnemius.
- Inferolateral 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.
The popliteal fossa is the main conduit for neurovascular structures entering and leaving the leg. Its contents are (medial to lateral):
- 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 is 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.
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 (rheumatoid or osteoarthritis). Whilst it usually self-resolves, the cyst can rupture and produce symptoms similar to deep vein thrombosis.
An aneurysm is a dilation of an artery, which is greater than 50% of the normal diameter. The popliteal fascia (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. The major features of tibial nerve compression are:
- Weakened or absent plantar flexion
- Paraesthesia of the foot and posterolateral leg
An aneurysm of the popliteal artery can be detected by an obvious palpable pulsation in the popliteal fossa. An arterial bruit may be heard on auscultation.
Rarer causes of a popliteal mass include deep vein thrombosis, adventitial cyst of the popliteal artery and various neoplasms (such as rhabdomyosarcoma).