Describe the boundaries and contents of the popliteal fossa. Add a note on the sciatic nerve and the effects of its injury.

Definition

Boundaries

Contents

• From superficial to deep — tibial nerve, popliteal vein, popliteal artery
• Mnemonic — the artery is deepest and most medial; the nerve is most superficial and lateral
• Common peroneal (fibular) nerve — along the medial border of biceps femoris
• Popliteal lymph nodes — drain the lateral side of the foot, the heel and the deep tissues of the leg
• Genicular vessels and nerves; the small saphenous vein pierces the fascia to end in the popliteal vein
• Popliteal fat

The Sciatic Nerve

• Leaves the pelvis through the greater sciatic foramen below piriformis
• Descends midway between the ischial tuberosity and the greater trochanter
• Lies on obturator internus, gemelli, quadratus femoris and adductor magnus
• Divides into the tibial and common peroneal nerves, usually at the upper angle of the popliteal fossa, though the division may be higher
• It is really two nerves in one sheath from the start — the tibial (anterior divisions) and common peroneal (posterior divisions)

Common Peroneal Nerve and Foot Drop

• Winds round the neck OF the fibula, where it is subcutaneous and the most commonly injured nerve in the lower limb
• Divides into superficial peroneal (peroneus longus and brevis; skin of the dorsum) and deep peroneal (anterior compartment; first web space)
• Causes — a tight plaster cast, fracture of the fibular neck, prolonged squatting or crossing the legs, pressure from a lithotomy position, leprosy
• Eversion is lost as well as dorsiflexion, which distinguishes it from an isolated deep peroneal lesion

Sciatic Nerve Injury

• Complete lesion — paralysis of the hamstrings and all muscles below the knee; foot drop; sensory loss below the knee except the medial side (saphenous, from the femoral nerve)
• Knee flexion is weakened but retained, by gracilis and sartorius
• Causes — posterior dislocation of the hip, fracture of the pelvis, badly placed intramuscular injection, penetrating wounds
• Piriformis syndrome — compression by a hypertrophied piriformis, particularly when the nerve pierces the muscle (a variation in about 10%)

The Popliteal Artery and Vein

• Popliteal artery — the continuation of the femoral artery from the adductor hiatus to the lower border of popliteus, where it divides into anterior and posterior tibial arteries
• The deepest structure in the fossa, lying on the popliteal surface of the femur and the capsule
• Branches — five genicular (superior medial and lateral, middle, inferior medial and lateral), muscular and sural
• These form the genicular anastomosis around the knee
• Popliteal vein lies between the artery and the tibial nerve, formed by the union of the venae comitantes of the tibial arteries; receives the small saphenous vein
• The pulse is felt with the knee flexed to relax the popliteal fascia, pressing the artery against the tibia

Applied Aspects

• Intramuscular injections must be given in the upper outer quadrant of the buttock, or better into the ventrogluteal or deltoid site, to avoid the sciatic nerve — injection palsy remains a preventable cause of disability in children
• Sciatica is most often due to a prolapsed intervertebral disc compressing a root (L5 or S1), not to the nerve trunk; the straight leg raising test is positive
• Popliteal aneurysm is the commonest peripheral aneurysm; it may thrombose and threaten the limb, and is often bilateral
• Baker cyst may rupture and mimic a deep vein thrombosis
• Foot drop is treated with an ankle-foot orthosis while the nerve recovers; leprosy remains an important cause in India
• The tibial nerve is rarely injured because it lies deep and protected; when it is, the result is loss of plantar flexion and of sensation over the sole, which is far more disabling for walking than a foot drop
• Deep vein thrombosis of the calf veins is the commonest source of pulmonary embolism, and the popliteal vein is the usual conduit