Background
The popliteal fossa has the semitendinosus and semimembranosus medially and the biceps femoris laterally. The sciatic nerve divides into the tibial and common peroneal nerve about 5–12 cm proximal to the popliteal crease. The sciatic nerve has a common epineural sheath that envelops the nerve trunks of the tibial and common peroneal nerve from their origin in the pelvis.
The sciatic nerve is formed by the union of the first 3 sacral spinal nerves and the fourth and fifth lumbar nerves (see the image below). It is the largest nerve supplying the leg. It leaves the pelvis through the greater sciatic foramen and runs toward the posterior aspect of the thigh between the greater trochanter and the ischial tuberosity. It separates into its terminal branches about 6 cm proximal to the popliteal crease into the tibial nerve and the common peroneal nerve.
Sciatic nerve anatomy.
The tibial nerve supplies the heel and the sole of the foot. The common peroneal, also known as the common fibular nerve, innervates the lateral aspect of the leg and dorsum of the foot. The medial aspect of the leg below the knee is the only area of the lower leg not innervated by one of the sciatic nerve branches; it is innervated by the saphenous nerve. The saphenous nerve is a cutaneous extension of the femoral nerve.
At the popliteal crease, the nerves are midway between skin and bone. They are lateral and superficial to the popliteal artery and vein in a separate sheath.
The tibial nerve is the larger of the 2 divisions and runs in the middle of popliteal fossa passing inferiorly through the 2 heads of the gastrocnemius. The common peroneal nerve follows the tendon of the bicep femoris along the lateral margin of the popliteal fossa. It is more lateral and superficial than the tibial nerve.
Patients do experience moderate discomfort during a popliteal block because the needle traverses through the biceps femoris.