Part twelve. Summary of lower limb innervation

Cutaneous innervation

The segmental supply (dermatomes) of the lower limb has been considered on page 14. The cutaneous nerves have been described in the preceding pages but for convenience are summarized here (Figs 3.46 and 3.47).

The skin of the buttock receives fibres that run down from the subcostal and iliohypogastric nerves, the posterior rami of the first three lumbar and first three sacral nerves, and the perforating cutaneous nerve, with an upward contribution from the posterior femoral cutaneous nerve. The latter supplies a long strip down the back of the limb to the midcalf, with lateral and medial femoral cutaneous nerves on either side and a small contribution from the obturator nerve on the medial side of the upper thigh. On the front of the thigh, the subcostal, femoral branch of genitofemoral and ilioinguinal nerves supply skin below the inguinal ligament, while the lateral, intermediate and medial femoral cutaneous nerves supply the skin of the rest of the thigh, with the obturator nerve contributing to the supply of the medial side.

The skin of the front of the knee receives branches from the medial, intermediate and lateral femoral cutaneous nerves, the lateral cutaneous nerve of the calf and the saphenous nerve. The last two supply the leg, with the saphenous nerve reaching as far as the level of the metatarsophalangeal joint of the great toe. The superficial peroneal extends over the front of the lower leg and dorsum of the foot, with the deep peroneal supplying the first toe cleft. The sural nerve takes over from the lateral cutaneous nerve of the calf on the lateral side of the back of the leg and extends along the lateral side of the foot to the little toe. Medial calcanean branches of the tibial nerve supply the heel, and medial and lateral plantar nerves the sole. The medial plantar, like the median nerve in the hand, usually supplies three and a half and the lateral plantar the rest of the digits.

Muscular innervation

The segmental innervation of lower limb muscles has been considered on page 15. In the thigh the anterior compartment is supplied by the femoral nerve and the adductor group by the obturator nerve. The tibial part of the sciatic nerve is the nerve of the posterior compartment, with only the short head of biceps supplied by the common peroneal part. In the gluteal region, the inferior gluteal nerve innervates gluteus maximus with the other two glutei receiving their supply from the superior gluteal which also supplies tensor fasciae latae. The short lateral rotator muscles behind the hip have their own nerves, with the obturator externus supplied by the obturator nerve. The tibial nerve is the nerve of the flexor compartment of the leg and its plantar branches supply the muscles of the sole. The common peroneal nerve divides into the superficial peroneal for the peroneal compartment and the deep peroneal for the anterior or extensor compartment.

Sympathetic innervation

As with the brachial plexus, a grey ramus communicans connects each nerve root of the lumbar and sacral plexuses with the appropriate ganglion of the sympathetic trunk so that postganglionic fibres can be distributed to each nerve. The preganglionic fibres for the lower limb have come from cell bodies in the lateral horn of spinal cord segments T11–L2, for the supply of blood vessels, sweat glands and arrectores pilorum muscles.

Lumbar and sacral plexuses

Summaries of these plexuses, including all the lower limb branches, begin on page 325, following descriptions of the abdomen and pelvis.

Part thirteen. Summary of lower limb nerve injuries

Peripheral nerve injuries are much less common in the lower limb than in the upper and damage to the lumbar and sacral plexuses is most unusual. The most common injury is to the common peroneal nerve, and the main features of this and other nerve lesions are summarized below, together with notes on the more common exposures if exploration and repair are required.

Femoral nerve

Since the nerve breaks up into a sheaf of branches as soon as it enters the thigh, it is more subject to damage by penetrating injuries of the lower abdomen than of the limb. Pelvic masses such as a haematoma or neoplasm may affect it, and it has been known to be damaged by catheterization of the femoral artery and during laparoscopic repair of inguinal hernia. In a complete lesion extension of the knee by the quadriceps will be lost, with some weakness of hip flexion. There is sensory loss over the front of the thigh; with lesions that cause pain in the nerve the pain may extend as far as the medial side of the foot (saphenous branch). Test for the action of rectus femoris (see p. 120).

Lateral femoral cutaneous nerve

The nerve may be compressed in the iliac fossa or as it passes from abdomen to thigh deep to or through the inguinal ligament and medial to the anterior superior iliac spine (see p. 112), producing meralgia paraesthetica which is recognized by paraesthesia in the lateral part of the thigh. If necessary the nerve may have to be freed from the iliac fascia and inguinal ligament.

Obturator nerve

On account of its deep position, trauma to this nerve is extremely rare, but it may be damaged by obstetric procedures or involved in pelvic disease; e.g. an ovarian tumour may cause pain in the skin on the medial side of the thigh. In obturator paralysis the loss of adduction at the hip is not noticed during walking but when sitting the affected limb cannot be crossed over the other.

Sciatic nerve

The most common cause of damage is (regrettably) by misplaced gluteal injections. Other causes include pelvic disease and severe trauma to the hip (in 7% of dislocations and 16% of fracture dislocations). There is paralysis of the hamstrings and all the muscles of the leg and foot (supplied by the tibial and common peroneal nerves). Even when paralysis of the hamstrings may be difficult to test for clinical reasons foot drop will be obvious, and there will be sensory loss below the knee but not on the medial side of the leg or on the upper part of the calf due to the supply from the saphenous branch of the femoral nerve and the posterior femoral cutaneous nerve respectively. Test for plantarflexion and dorsiflexion of the foot.

Surgical approach. The sciatic nerve is explored by exposure at the lower border of gluteus maximus, retracting semitendinosus and the long head of biceps medially; from there it can be followed upwards or downwards (retracting biceps laterally).

Common peroneal (fibular) nerve

Direct trauma or pressure by plaster casts at the neck of the fibula make this the most commonly

damaged nerve in the lower limb. Foot drop is the most obvious sign, due to paralysis of the extensor muscles supplied by the deep peroneal branch. This results in a high-stepping gait to ensure that the toes do not scrape along the ground. Peroneus longus and brevis in the lateral compartment will also be affected, being supplied by the superficial peroneal branch. Sensory loss in a common peroneal nerve lesion extends over the lower lateral part of the leg and the dorsum of the foot. Test for dorsiflexion.

Surgical approach. The nerve can be exposed by following it down from the lateral side of the popliteal fossa, where it lies medial to the biceps tendon.

Tibial nerve

Damage to this nerve is uncommon. The main effect is paralysis of the calf muscles; sensory loss is on the lower part of the calf and on the sole. Test for standing on tiptoe.

Surgical approach. The nerve is exposed in the middle of the popliteal fossa; it can be followed lower down by splitting gastrocnemius and soleus vertically in the midline.

Saphenous nerve

The lower part of this nerve, in front of the medial malleolus, is at risk of damage during varicose vein surgery and when the great saphenous vein is harvested for arterial bypass procedures.