Popliteal Fossa Anatomy
The popliteal fossa is a diamondshaped space behind the knee joint. It contains the principal blood vessels and nerves passing between the thigh and the leg. It has a roof, four walls and a floor.
The roof is formed by the investing layer of deep fascia. In the subcutaneous tissue overlying the roof are the posterior cutaneous nerve of the thigh, which continues into the proximal part of the leg, and the small saphenous vein (Fig. 6.37). The vein usually penetrates the roof to drain into the popliteal vein but may drain more proximally into the great saphenous vein (Fig. 6.34).
The walls overhang the fossa. Superiorly, they are formed by the diverging tendons of the hamstrings, namely semitendinosus and semimembranosus lying medially and biceps laterally. Inferiorly are the medial and lateral heads of gastrocnemius, which converge at the inferior angle (Fig. 6.38). Adjacent to the lateral head of gastrocnemius is the small plantaris muscle. On each side of the popliteal fossa the hamstring tendons overlap the heads of gastrocnemius, and between the medial head of gastrocnemius and semimembranosus there is frequently a bursa (Fig. 6.39).
The principal contents of the fossa are embedded in fat and comprise the popliteal artery and vein together with the two terminal branches of the sciatic nerve, the common fibular and tibial nerves (Fig. 6.40). These vessels and nerves are responsible for the blood and nerve supply of most of the leg and foot.
The popliteal artery lies deepest and is the continuation of the femoral artery from the thigh. It enters through the opening in adductor magnus and descends vertically on the floor of the fossa to the inferior angle, where it leaves beneath the fused heads of gastrocnemius. The artery is so deep that it is difficult to palpate unless the knee is flexed to relax the boundaries and roof of the fossa. The artery supplies the surrounding muscles and also forms a substantial plexus of articular branches anastomosing symmetrically around the knee joint (Fig. 1.27).
The popliteal vein lies superficial to the artery and is formed at the inferior angle of the fossa by the union of the venae comitantes that accompany the tibial arteries in the leg. It continues proximally with the artery through the opening in adductor magnus to enter the adductor canal and become the femoral vein.
The tibial and common fibular nerves (Fig. 6.40), entering the fossa from the posterior compartment of the thigh, lie just beneath the roof, superficial to the popliteal vessels. The tibial nerve enters from beneath the hamstrings and descends vertically, bisecting the fossa, and leaves beneath the gastrocnemius at the inferior angle of the fossa, where it enters the posterior compartment of the leg. The tibial nerve is mainly motor in its distribution, supplying gastrocnemius, plantaris, popliteus and soleus. All these branches arise within the fossa. The nerve also gives sensory branches to the knee joint and a large cutaneous branch, which passes into the calf to form the sural nerve. The common fibular nerve descends under cover of the tendon of biceps to reach the lateral angle of the fossa, where it enters the lateral (fibular or peroneal) compartment of the leg. Here it winds around the neck of the fibula where it is vulnerable to damage and compression, resulting in footdrop. It supplies sensory branches to the knee joint and two cutaneous nerves, one to the lateral side of the calf and the other, the lateral sural cutaneous nerve, joining the sural nerve in the calf.
The remaining contents of the popliteal fossa are the deeply placed popliteal lymph nodes, which lie close to the popliteal artery. They drain the deep structures of the leg and foot and the knee joint and receive superficial lymphatics, which accompany the short saphenous vein from the lateral side of the foot and leg.
The floor of the fossa is formed, from above downwards, by the popliteal surface of the femur, the capsule of the knee joint reinforced by the oblique popliteal ligament and popliteus (Fig. 6.39).
This muscle is attached to a triangular area on the posterior surface of the proximal end of the tibia above the soleal line. The tendon passes upwards and laterally, penetrating the capsule of the knee joint (Fig. 6.77) to become attached to a pit below the lateral epicondyle of the femur. Its action is to ‘unlock’ the knee joint by producing lateral rotation of the femur on the tibia during flexion of the joint from the fully extended position. Popliteus is supplied by the tibial nerve.