Lower Limb Nerves

The Sciatic Nerve

The sciatic nerve is formed in the pelvis by fibres from the lumbosacral trunk (L4,5) and by fibres from S1,2,3. This thick nerve immediately leaves the pelvis through the greater sciatic notch, below the piriformis muscle (P on diagram). The nerve may divide immediately, or may pass either above the piriformis or through the piriformis. In the gluteal region the nerve lies deep to gluteus maximus, between the greater trochanter (GT) and the ischial tuberosity(IT). The nerve then passes down the back of the thigh to the apex of the popliteal fossa. In the thigh the nerve divides into lateral common peroneal and medial tibial divisions. In the upper part of its course the sciatic nerve supplies the semimembranosus, semitendinosus, the ischial head of adductor magnus and long head of biceps femoris from its tibial division. The common peroneal division supplies fibres to the short head of biceps femoris.

The Tibial Nerve

The tibial nerve passes through the popliteal fossa to pass below the arch of soleus. In the popliteal fossa the nerve gives off branches to gastrocnemius, popliteus, soleus and plantaris, and the sural nerve. The sural nerve is joined by fibres from the common peroneal nerve and runs down the calf to supply the lateral side of the foot. Below the soleus muscle the nerve lies close to the tibia and supplies the tibialis posterior, the flexor digitorum longus and the flexor hallucis longus. The nerve passes into the foot below the medial malleolus. Here it is bound down by the flexor retinaculum in company with the posterior tibial artery. The nerve and artery divide into medial and lateral plantar branches. The medial plantar nerve supplies the abductor hallucis, the flexor digitorum brevis, the flexor hallucis brevis and the first lumbrical. Cutaneous distribution of the medial planter nerve is to the medial sole and medial three and one half toes, including the nail beds on the dorsum (like the median nerve in the hand). The lateral plantar nerve supplies quadratus plantae, flexor digiti minimi, adductor hallucis, the interossei , three lumbricals. and abductor digiti minimi. Cutaneous innervation is to the lateral sole and lateral one and one half toes (like the ulnar nerve).

The Common Peroneal Nerve

The common peroneal nerve leaves the popliteal fossa between the tendon of biceps femoris and the lateral head of gastrocnemius. It crosses behind the head of the fibula and passes laterally around the neck of the fibula (where it may be palpated). The nerve gives of the sural communicating branch to the sural nerve, and the lateral cutaneous nerve of the calf. The nerve pierces the peroneus longus muscle to divide into deep and superficial branches.

The deep peroneal nerve supplies the muscles of the anterior compartment - the tibialis anterior, extensor hallucis longus, extensor digitorum longus, peroneus tertius and extensor digitorum brevis. The deep peroneal nerve supplies cutaneous branches to the cleft between the big toe and the second toe.

The superficial peroneal nerve supplies the muscles in the lateral compartment (peroneus longus and brevis) and the skin over the anterior lower leg and dorsum of the foot.

Nerve injury

The sciatic nerve is commonly injured during intramuscular injections into the buttocks. The nerve may also be injured by posterior dislocations or fracture dislocations of the hip joint. Injury to the nerve might result in loss of the hamstrings and calf muscles resulting in loss of knee flexion, and loss of the muscles of the anterior and lateral compartments of the leg resulting in foot drop. Cutaneous sensation would be lost over the calf and dorsum, sole and lateral side of the foot.

The common peroneal nerve may be injured as it winds around the neck of the fibula, resulting in foot drop (anterior compartment muscles), and loss of sensation on the lower anterior leg and dorsum of the foot. The nerve is also at risk in anterior compartment syndrome.

The roots of the nerve may also be compressed by a prolapsed disc, giving pain over part of the distribution of the nerve - sciatica.

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