External landmarks and boundaries
The boundaries of the popliteal fossa are defined by conventional lines: the superior line runs circularly 6 cm above the base of the patella, and the inferior line is located at the level of the tibial tuberosity. The femoral epicondyles serve as the main bony landmarks.
The shape of the fossa depends on the position of the limb. If the leg is flexed at the knee joint, the muscle and tendon margins that form the diamond can be clearly palpated:
- On the medial side, the fossa is bounded by the semitendinosus and semimembranosus muscles (m. semitendinosus et semimembranosus).
- On the lateral side, the biceps femoris muscle (m. biceps femoris) is outlined.
- The inferior angle is formed by the muscle bellies (heads) of the gastrocnemius muscle (m. gastrocnemius), although it can be difficult to separate them upon palpation.
With full extension of the limb, the fossa flattens, and a longitudinal ridge corresponding to the accumulation of adipose tissue becomes visible in its center.
Layered structure and floor of the fossa
The structure of the region consists of several layers:
- Skin — thin and mobile.
- Subcutaneous tissue — contains branches of the posterior cutaneous nerve of the thigh (reaching the joint line) and the saphenous nerve (at the medial border). Superficial popliteal lymph nodes also lie here.
- Popliteal fascia (fascia poplitea) — a direct continuation of the fascia lata. It is incredibly dense, almost like an aponeurosis. Because of its rigidity, it is very difficult to palpate the pulse on the popliteal artery when the leg is extended.
The floor of the popliteal fossa is formed by deep bone and joint structures: the popliteal surface of the femur, the posterior part of the knee joint capsule (reinforced by the oblique popliteal ligament of Winslow), and the popliteus muscle (m. popliteus). The latter originates from the lateral epicondyle of the femur and attaches to the posteromedial surface of the tibia.
Topography of the neurovascular bundle
Large vessels enter the fossa through the exit opening of the adductor canal (canalis adductorius), bounded by the femur and the tendon of the adductor magnus. The elements of the bundle are located in the adipose tissue strictly in layers — from back to front (from superficial to deep) and from the lateral side to the medial.
- Tibial nerve (n. tibialis) lies the most superficially, directly beneath the popliteal fascia. It gives off muscular branches to the posterior compartment of the leg and the medial sural cutaneous nerve, which runs into the groove between the heads of the gastrocnemius muscle.
- Popliteal vein (v. poplitea) is located medially and deeper than the tibial nerve.
- Popliteal artery (a. poplitea) lies the deepest, closest to the bone and the capsule of the knee joint. In the fossa, it gives off muscular branches and 5 genicular arteries.
Separately, lateral to the tibial nerve, runs the common fibular nerve (n. fibularis communis). It closely adheres to the medial margin of the biceps femoris tendon and heads toward the neck of the fibula.
Pathways of purulent infection spread
Deep popliteal lymph nodes, which collect lymph from the entire leg (with outflow to the deep inguinal nodes), lie in the adipose tissue on either side of the popliteal artery. The cellular space of the fossa is not closed, which creates pathways for the migration of purulent collections:
- Upward (along the course of the sciatic nerve) — infection can penetrate into the posterior fascial compartment of the thigh.
- Upward and anteriorly (through the adductor canal along the vessels) — pus can ascend to the anterior region of the thigh, up to the tissue of the femoral triangle.
- Downward (along the popliteal vessels and the tibial nerve) — collections descend into the deep cellular space of the posterior leg region.
Briefly: answers to topical questions
Which specific 5 genicular arteries branch off from the popliteal artery within the popliteal fossa?
Five genicular arteries branch off from the popliteal artery within the popliteal fossa, participating in the formation of the knee articular network. These include:
- Superior lateral genicular artery (a. superior lateralis genus) — curves around the femoral condyle.
- Superior medial genicular artery (a. superior medialis genus) — curves around the femoral condyle.
- Middle genicular artery (a. media genus) — pierces the posterior joint capsule, supplying blood to the cruciate ligaments and the synovial membrane.
- Inferior lateral genicular artery (a. genus inferior lateralis) — curves around the tibial condyle.
- Inferior medial genicular artery (a. genus inferior medialis) — curves around the tibial condyle.
Through which canal do the tibial nerve and popliteal vessels leave the popliteal fossa, passing into the posterior leg region?
The tibial nerve and popliteal vessels pass from the popliteal fossa into the posterior leg region through the cruropopliteal canal (canalis cruropopliteus).
The canal is located between the superficial and deep muscle groups of the leg. Its anterior inlet is bounded by the popliteus muscle (m. popliteus), and its posterior inlet is bounded by the tendinous arch of the soleus muscle (arcus tendineus m. solei). At the inlet, the popliteal artery divides into the anterior and posterior tibial arteries.
What large superficial vein pierces the fascia and empties into the popliteal vein in the area of the popliteal fossa?
The small saphenous vein (v. saphena parva) empties into the popliteal vein in the area of the popliteal fossa.
It forms in the subcutaneous tissue of the posterior leg region and ascends upward. In the middle third of the leg, the vein pierces the fascia, runs between its layers in Pirogov's canal, and then goes beneath the fascia. Upon reaching the popliteal fossa, it passes between the heads of the gastrocnemius muscle and empties directly into the popliteal vein (v. poplitea). Along this path, the vein is accompanied by the medial sural cutaneous nerve (n. cutaneus surae medialis).
To remember the syntopy of the neurovascular bundle (from superficial to deep and from lateral to medial), use the word "NEVA": N — Nerve (n. tibialis), V — Vein (v. poplitea), A — Artery (a. poplitea).
Class and exam questions
Why is it difficult to palpate the pulsation of the popliteal artery when the knee is extended?
The artery lies at the very floor of the fossa (on the joint capsule), and superficially the region is covered by a very dense popliteal fascia, comparable to an aponeurosis.
How do blood vessels enter the popliteal fossa?
The popliteal artery and vein enter the fossa from the anterior surface of the thigh through the exit opening of the adductor canal (Hunter's canal).
How is the sural nerve (n. suralis) formed?
It is formed by the joining of the medial sural cutaneous nerve (a branch of the tibial nerve) and the lateral sural cutaneous nerve (a branch of the common fibular nerve).
What else to study on this topic
- Anatomy and branches of the 5 genicular arteries
- Formation of the deep and superficial arterial networks of the knee joint
- Ligamentous apparatus of the posterior part of the knee joint capsule
- Topography of the sciatic nerve before its entry into the popliteal fossa
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