The sciatic nerve (Figure C3-1) leaves the pelvis via the sciatic notch and then passes, usually, under the piriformis muscle, which is covered by the gluteus maximus. In healthy individuals, the sciatic nerve passes underneath the piriformis muscle in 85 to 90+ of cases, while in the rest, the peroneal division only passes above or through the muscle. Rarely (1 to 2+ of persons), the entire sciatic nerve pierces the piriformis muscle.

Figure C3-1 The sciatic nerve and its main branches.

(Reprinted with permission from Haymaker W, Woodhall B. Peripheral nerve injuries: principles of diagnosis. Philadelphia, PA: WB Saunders, 1953.)

The superior gluteal nerve, which innervates the gluteus medius and minimus and tensor fascia lata, branches off the sciatic trunk before the piriformis. However, the inferior gluteal nerve, which innervates the gluteus maximus, passes under the muscle (Figure C3-2). In the thigh, the tibial nerve innervates most hamstring muscles (semitendinosus, semimembranosus, and the long head of the biceps femoris), except the short head of the biceps femoris; the latter is the only hamstring muscle innervated by the common peroneal nerve. Also, the tibial nerve contributes, with the obturator nerve, to innervation of the adductor magnus muscle.

Figure C3-2 The nerve of the sciatic notch and the piriformis muscle.

(Reprinted from Stewart JD. Focal peripheral neuropathies, 2nd ed. New York: Raven Press, 1993, with permission.)

Lesions of the proximal sciatic nerve at the hip or in the upper thigh affect usually the lateral division (common peroneal nerve) more severely than the medial division (tibial nerve). The greater vulnerability of the peroneal division is caused by the following:

• The difference in both the fascicular pattern and the cushioning effect of the epineurium between the two divisions. The tibial nerve has many fascicles that are distributed throughout elastic epineural tissue while the peroneal nerve is composed of fewer fascicles and has limited supportive tissue.

• The difference in anatomic course between the two nerves. The tibial nerve is loosely fixed posteriorly while the peroneal nerve is taut and is secured at the sciatic notch proximally and the fibular neck distally. Consequently, traction of the sciatic nerve results in more damage to the peroneal than the tibial nerve in the thigh.

The sciatic nerve divides into its two terminal branches near the midthigh, although this is extremely variable and the separation may be as low as the popliteal fossa. In the popliteal fossa and before it winds around the fibular neck, the common peroneal nerve gives off first the lateral cutaneous nerve of the calf, which innervates the skin over the upper third of the lateral aspect of the leg. At the fibular neck, the common peroneal nerve lies in close contact with the bone, and passes through a tendinous tunnel, sometimes referred to as the fibular tunnel, which is formed between the edge of the peroneus longus muscle and the fibula. Near that point, the common peroneal nerve divides into superficial and deep branches (see Figure C8-3, Case 8). The superficial peroneal nerve innervates the peroneus longus and brevis, as well as the skin of the lower two-thirds of the lateral aspect of the leg and the dorsum of the foot. The deep peroneal nerve is primarily a motor nerve; it innervates the ankle and toe extensors (the tibialis anterior, extensor hallucis, extensor digitorum longus, and brevis) and the peroneus tertius, in addition to a small area of skin in the web space between the first and second toes.

In the popliteal fossa, the tibial nerve gives off the sural nerve, a purely sensory nerve that innervates the lateral aspect of the lower leg and foot, including the little toe. In 40 to 80+ of individuals, there is a communication between the common peroneal nerve and the sural nerve in the popliteal fossa. This nerve is referred to as the sural communicating nerve but also is called the lateral sural nerve (with the main sural trunk being the medial sural nerve). This nerve may play role in preserving sensation in the lateral foot and little toe in proximal tibial nerve lesions, and causing sensory loss in this distribution following a common peroneal or proximal deep peroneal lesion.

While in the calf, the tibial nerve innervates the gastrocnemius, soleus, tibialis posterior, flexor digitorum profundus, and flexor hallucis longus. At the medial aspect of the ankle, the tibial nerve passes through the tarsal tunnel and divides at, or slightly distal to, the tunnel into its three terminal branches (see inset of Figure C3-1): (1) the calcaneal branch, a purely sensory nerve that innervates the skin of the sole of the heel; (2) the medial plantar nerve, which innervates the abductor hallucis, the flexor digitorum brevis, and the flexor hallucis brevis, in addition to the skin of the medial sole and, at least, the medial three toes; and (3) the lateral plantar nerve, which innervates the abductor digiti quinti pedis, the flexor digiti quinti pedis, the adductor hallucis, and the interossei, in addition to the skin of the lateral sole and two lateral toes.