Popliteal Fossa — MRCS Part B Anatomy

A knee dislocation on the radiograph, which is the injury that turns the popliteal fossa from a diamond of anatomy into an emergency: the popliteal artery is fixed at both ends and tears.

The injury

A dislocated knee is reduced at once, under sedation, and the pulses, capillary refill and ankle-brachial index are documented before and after; any abnormality, or a hard sign of vascular injury, means CT angiography or the vascular surgeons straight away, because the artery is torn or intimally damaged in about a fifth of these injuries and the leg is lost if it is not repaired within about six hours. The common peroneal nerve is injured in a quarter, the tibial nerve less often. After reduction the knee is splinted and the pulses checked repeatedly; the ligaments are reconstructed later.

Boundaries

A diamond behind the knee: above and laterally biceps femoris; above and medially semimembranosus, with semitendinosus on it; below, the two heads of gastrocnemius, with plantaris beside the lateral head. The floor is the popliteal surface of the femur, the back of the joint capsule with the oblique popliteal ligament, and the fascia over popliteus. The roof is the deep fascia, pierced by the small saphenous vein and the posterior cutaneous nerve of the thigh.

Contents, deep to superficial

The popliteal artery, lying on the floor, with its genicular branches and its division into anterior and posterior tibial arteries at the lower border of popliteus; the popliteal vein, formed from the venae comitantes and receiving the small saphenous vein; the tibial nerve, the larger division of the sciatic, running straight down the middle; and the common peroneal nerve, running along the medial edge of biceps femoris to the neck of the fibula. Also the popliteal lymph nodes and fat. The order from deep to superficial and from medial to lateral is artery, vein, nerve.

The tibial nerve

A lesion at the knee paralyses the plantar flexors and the intrinsic muscles of the sole: the patient cannot stand on tiptoe, the toes claw, and sensation is lost on the sole of the foot. It is rarely injured, because it lies deep.

The common peroneal nerve

It winds round the neck of the fibula under the skin, where it is compressed by a plaster, a tight boot, a leg crossed over the other, or the bed rail in a thin patient, and where it is torn by a fracture of the fibular neck, a varus injury or a knee dislocation, or cut in a lateral approach to the knee. The palsy is a foot drop with a high-stepping gait, loss of eversion, and numbness over the dorsum of the foot and the lateral leg. Other causes: a ganglion at the proximal tibiofibular joint, diabetes, and weight loss.

Testing the deep peroneal nerve

Ask the patient to dorsiflex the ankle and extend the great toe against resistance (tibialis anterior and extensor hallucis longus), and test sensation in the first web space between the great and second toes, its only cutaneous territory. The superficial branch is tested by eversion and by sensation on the dorsum of the foot.

Blood supply of the lower limb

The external iliac artery becomes the femoral artery under the inguinal ligament, gives the profunda femoris in the femoral triangle (which supplies the thigh through its perforating and circumflex branches and is the collateral route round a superficial femoral occlusion), runs down the adductor canal, and passes through the adductor hiatus to become the popliteal artery. The popliteal gives the genicular anastomosis around the knee and divides into the anterior tibial artery, which passes forward through the interosseous membrane to the anterior compartment and becomes the dorsalis pedis on the foot, and the posterior tibial artery, which gives the peroneal (fibular) artery and runs behind the medial malleolus to the plantar arteries of the sole. The pulses are femoral, popliteal, posterior tibial and dorsalis pedis.