The sciatic nerve is a large, bilateral fiber formed from the anterior and posterior divisions of nerve roots L4, L5, S1, S2 and S3. It is the largest branch of the lumbosacral plexus. The nerve supplies the posterior compartment of the thigh, the leg and foot. For this reason, it has been considered the nerve of the lower limb. The course of the nerve and its direct and indirect innervations will be evaluated in this article, in addition to limitations arising from damage to the nerve and its branches.
As the nerve is formed by the lumbosacral trunk (L4 and L5) and the sacral outflow from S1 to S3, the nerve travels inferiorly, on the pelvic surface of the ischiococcygeus muscle. Here, the internal pudendal artery lies anterior to the sciatic nerve, the superior gluteal artery lies between the lumbosacral trunk and the S1 outflow, and the inferior gluteal artery lies between the S2 and S3 outflow.
The sciatic nerve leaves the pelvic cavity and enters the gluteal region, deep to gluteus maximus, by way of the greater sciatic foramen. It arises in the region at a slight inferolateral angle, deep to the inferior border of piriformis and the inferior gluteal artery. It travels superior to the gemelli, obturator internus, and quadratus femoris. At the level of the ischial tuberosity, the nerve assumes a more vertical course as it travels caudally, deep to the long head of biceps femoris (the hamstring muscles), semimembranosus and semitendinosus and superficial to the hamstring component of adductor magnus.
At the apex of the popliteal fossa (at the beginning of the adductor hiatus), the sciatic nerve divides into its tibial and common fibular (peroneal) components. These derivative nerves travel lateral to the popliteal vessels as they enter the fossa, with the common peroneal component being the more lateral of the two nerves.
The trunk of the sciatic nerve innervates the proximal parts of the hamstring muscles as well as the ischial component of adductor magnus. Distally, after the nerve bifurcates, it innervates the long head of biceps femoris, semimembranosus, semitendinosus and adductor magnus via the tibial component and the short head of biceps femoris through its common peroneal component. The tibial and common peroneal nerves also provide motor supply to the leg and foot along its caudal route, which will now be discussed.
The tibial branch of the sciatic nerve contains anterior nerve fibers from the lumbosacral trunk and S1 – S3. In the popliteal fossa, it gives off genicular branches to the knee joint (that accompany the genicular arteries) and muscular branches to soleus, plantaris, popliteus and gastrocnemius. Before exiting the fossa, it also gives off a medial sural cutaneous nerve that travels between gastrocnemius, behind the lateral malleolus and terminates at the side of the little toe.
As the tibial nerve exits the popliteal fossa, it crosses medially over the tibial vein (continuation of popliteal vein) to rest between the vein and its artery on the superficial surface of popliteus. The nerve and the accompanying tibial vessels then pierce the tendinous arch of soleus to gain access to the deep compartment of the posterior leg. The nerve then courses caudally between flexor digitorum longus, tibialis posterior and flexor hallucis longus. In the calf, it provides muscular innervation for flexor digitorum and hallucis longus, tibialis posterior and soleus.
It passes deep to the flexor retinaculum with the tendons of the aforementioned muscles (the former two being anterior and the latter being posterior) and the posterior tibial artery. It gives medial calcaneal cutaneous branches while travelling under the flexor retinaculum. As it emerges, it divides into a medial and a lateral plantar nerve. The former branch innervates the medial sole and the medial plantar aspect of 3 1⁄2 toes, along with abductor hallucis, flexor digitorum brevis, the first lumbrical, flexor hallucis brevis. The latter will innervate the rest of the foot muscles along with the remaining part of the sole.
Common Peroneal (Fibular) Nerve
The common peroneal, conversely, has posterior nerve fibers from the lumbosacral trunk and roots S1 and S2. While the tibial nerve was the nerve of the flexor compartment of the leg, the common peroneal nerve is the nerve of the extensor and peroneal compartments of the leg and the dorsum of the foot. The nerve enters the popliteal fossa similar to the tibial nerve. It courses along the medial border of biceps femoris but does not directly supply anything. However, its genicular branches innervate the knee and tibiofibular joints. Before exiting the popliteal fossa, it gives off a lateral sural cutaneous nerve that joins the sural cutaneous nerve of the tibial branch.
The common peroneal nerve gains access to the anterior compartment of the leg by crossing over plantaris and the lateral head of gastrocnemius. Afterwards, it wraps around the neck of the fibula and pierces peroneus longus; where it bifurcates into its superficial and deep branches. The superficial branch continues inferiorly between the fibers of peroneus longus. It emerges anteriorly from the body of the muscle in the lower third of the leg to supply the skin of this region. It also gives motor supply to peroneus longus and peroneus brevis as well as cutaneous supply above these muscles. The nerve then divides at the ankle into its medial and lateral components that innervates the medial aspect of the great toe and second interdigital cleft, as well as the third and fourth clefts, respectively.
The deep peroneal nerve travels inferolaterally towards the interosseous membrane, lateral to the anterior tibial artery and vein, and deep to tibialis anterior. Motor innervation is provided to extensor digitorum longus, extensor hallucis longus, peroneus tertius and tibialis anterior. The nerve then continues caudally until it bifurcates in the dorsum of the foot into a medial and a lateral deep peroneal nerve. The lateral deep peroneal nerve travels laterally with the lateral tarsal artery to supply extensor digitorum brevis. The medial deep peroneal nerve continues alongside dorsalis pedis and ends by giving cutaneous innervation to the first interdigital cleft.
The sciatic nerve is most commonly injured by misplaced injections to the gluteal region. Extreme trauma can also result in injury to the nerve. Damage to the nerve will result in hamstring paralysis as well as loss of innervation to the muscles of the leg and foot. The most obvious clinical signs will be the inability to plantarflex or dorsiflex the foot. As a result, the patient’s foot on the affected side will limp when they are asked to raise their leg such that the sole is no longer in contact with the ground or while walking (foot drop). It should also be noted that the patient may still have sensation to the medial side of the leg and foot (up to the medial big toe) due to supply from the saphenous nerve and posterior cutaneous nerve of thigh.
Fracture of the neck of the fibula is the most common way to injure the common peroneal nerve. The loss of innervation to the extensor muscles of the leg will result in a classic foot drop. The patient makes a conscious effort to raise the foot while walking so that the toes aren’t dragged on the ground. The inability to dorsiflex should also be noted.
It is unlikely to injure the tibial nerve. However, should this occur, paralysis to calf muscles and loss of sensation to the lower calf and soles will result. Because the tibial nerve innervates the posterior leg compartment, a patient with a nerve lesion in the area will not be able to stand on their tiptoe.