Femoral Nerve (L2–4)

The femoral nerve is the nerve of the anterior compartment of the thigh. It arises from the posterior divisions of the second to fourth lumbar ventral rami, descends through psoas major and emerges on its lateral border to pass between psoas and iliacus and enter the thigh behind the inguinal ligament and lateral to the femoral sheath. Its terminal branches form in the femoral triangle approximately 2 cm distal to the inguinal ligament. In the abdomen the nerve supplies small branches to iliacus and a branch to the proximal part of the femoral artery. It subsequently supplies a large cutaneous area on the anterior and medial thigh and medial leg and foot and gives articular branches to the hip and knee. The femoral nerve is described in detail below.

Fig. 20.1 Nerves on the anterior aspect of the lower limb, their cutaneous branches and the muscles they supply.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Fig. 20.2 Nerves on the posterior aspect of the lower limb, their cutaneous branches and the muscles they supply.

(From Aids to the Examination of the Peripheral Nervous System, 4th ed. 2000. Saunders, London. With permission of Guarantors of Brain.)

Obturator Nerve (L2–4)

The obturator nerve is the nerve of the medial compartment of the thigh. It arises from the anterior divisions of the second to fourth lumbar ventral rami, descends through psoas major and emerges from its medial border at the pelvic brim. It crosses the sacroiliac joint behind the common iliac artery and lateral to the internal iliac vessels, runs along the lateral pelvic wall on obturator internus and enters the thigh through the upper part of the obturator foramen. Near the foramen it divides into anterior and posterior branches, separated at first by part of obturator externus and more distally by adductor brevis. It gives articular branches to the hip and knee and may supply skin on the medial thigh and leg. The obturator nerve is described in detail below.

Sciatic Nerve (L4, L5, S1–3)

The sciatic nerve is the nerve of the posterior compartment of the thigh and, via its major branches, of all the compartments of the lower leg and foot. Formed in the pelvis from the ventral rami of the fourth lumbar to third sacral spinal nerves, it is 2 cm wide at its origin and is the thickest nerve in the body. It enters the lower limb via the greater sciatic foramen below piriformis and descends between the greater trochanter and the ischial tuberosity. The nerve passes along the back of the thigh, where it is crossed by the long head of biceps femoris, and divides into the tibial and common peroneal (fibular) nerves proximal to the knee. The actual level of division is very variable because the tibial and common peroneal nerves are structurally separate and only loosely connected throughout their proximal course. The sciatic gives off articular branches that supply the hip joint through its posterior capsule (these are sometimes derived directly from the sacral plexus) and the knee joint. All the hamstring muscles, including the ischial part of adductor magnus but not the short head of biceps femoris, are supplied by the medial (tibial) component of the sciatic nerve. The short head of biceps is supplied by the lateral (common peroneal) component. The sciatic nerve is described in detail below.

Tibial Nerve (L4, L5, S1–3)

The tibial nerve arises from the anterior division of the sacral plexus. It descends along the back of the thigh and popliteal fossa to the distal border of popliteus, then passes anterior to the arch of soleus with the popliteal artery and continues into the leg. In the popliteal fossa it lies lateral to the popliteal vessels, becomes superficial to them at the knee and crosses to the medial side of the artery. In the leg it is the nerve of the posterior compartment and descends with the posterior tibial vessels to lie between the heel and the medial malleolus. It ends beneath the flexor retinaculum by dividing into the medial and lateral plantar nerves. The tibial nerve supplies articular branches to the knee and ankle. Its cutaneous area of supply, including its terminal branches, includes the back of the calf, the whole of the sole, the lateral border of the foot and the medial and lateral sides of the heel. The tibial nerve is described in detail below.

Common Peroneal Nerve (L4, L5, S1, S2)

The common peroneal nerve (common fibular nerve) is derived from the posterior division of the sacral plexus. In the leg it is the nerve of the anterior and lateral compartments. It descends obliquely along the lateral side of the popliteal fossa to the fibular head, lying between the tendon of biceps femoris and the lateral head of gastrocnemius. It curves lateral to the neck of the fibula deep to peroneus longus and divides into superficial and deep peroneal (fibular) nerves; the common peroneal nerve is easily injured at the fibular neck. Before it divides, it gives off articular branches to the knee and the superior tibiofibular joints and cutaneous branches. Its cutaneous area of supply, including its terminal branches, includes the anterolateral and lateral surfaces of the leg and most of the dorsum of the foot. The common peroneal nerve is described in detail below.

Gluteal Nerves

The gluteal nerves arise from the posterior division of the sacral plexus. The superior gluteal nerve (L4, L5, S1) leaves the pelvis through the greater sciatic notch above piriformis and supplies gluteus medius, gluteus minimus, tensor fasciae latae and the hip joint. The inferior gluteal nerve (L5, S1, S2) passes through the greater sciatic notch below piriformis and supplies gluteus maximus. The gluteal nerves are described in detail below.

Autonomic Innervation

The autonomic supply to the limbs is exclusively sympathetic. The preganglionic sympathetic inflow to the lower limb is derived from neurones in the lateral horn of the lower thoracic (T10, T11) and upper lumbar (L1, L2) spinal cord segments. Fibres pass in white rami communicantes to the sympathetic chain and synapse in the lumbar and sacral ganglia. Postganglionic fibres pass in grey rami communicantes to enter the lumbar and sacral plexuses, and many are distributed via the cutaneous branches of the nerves derived from these plexuses. The blood vessels to the lower limb receive their sympathetic supply via adjacent peripheral nerves. Postganglionic fibres accompanying the iliac arteries are destined mainly for the pelvis but may supply vessels in the upper thigh.

Dermatomes

Our knowledge of the extent of individual dermatomes, especially in the limbs, is based largely on clinical evidence. The dermatomes of the lower limb arise from spinal nerves T12 to S3 (Figs 20.3–20.5).

Fig. 20.3 Dermatomes of the lower limb. There is considerable variation in and overlap between dermatomes, but the overlap across axial lines (heavy blue) is minimal.

Fig. 20.4 Dermatomes of the perineum.

Fig. 20.5 Cutaneous nerves of the right lower limb, their areas of distribution and segmental origins. A, Anterior aspect. B, Sole of the foot. C, Posterior aspect. In C, the interrupted line represents the trunk of the posterior cutaneous nerve of the thigh, most of which lies deep to the fascia lata.

The preaxial border starts near the midpoint of the thigh and descends to the knee. It then curves medially, descending to the medial malleolus and the medial side of the foot and hallux. The postaxial border starts in the gluteal region and descends to the centre of the popliteal fossa, then deviates laterally to the lateral malleolus and the lateral side of the foot. The ventral and dorsal axial lines exhibit corresponding obliquity. The ventral axial line starts proximally at the medial end of the inguinal ligament and descends along the posteromedial aspect of the thigh and leg to end proximal to the heel. The dorsal axial line begins in the lateral gluteal region and descends posterolaterally in the thigh to the knee; it inclines medially and ends proximal to the ankle. Considerable overlap exists between adjacent dermatomes innervated by nerves derived from consecutive spinal cord segments.

Myotomes

Tables 20.1 to 20.4 summarize the predominant segmental origin of the nerve supply for each of the lower limb muscles and for movements that take place at the joints of the lower limb. Damage to these segments or to their motor roots results in maximal paralysis.

Table 20.1 Movements, muscles and segmental innervation in the lower limb

Table 20.2 Segmental innervation of muscles of the lower limb

Table 20.3 Segmental innervation of joint movements of the lower limb

Reflexes

Muscular Branches

Small branches are derived from all five lumbar roots.

Femoral Nerve

The femoral nerve (Fig. 20.7), the largest branch of the lumbar plexus, arises from the dorsal branches (posterior divisions) of the second to fourth lumbar ventral rami. It descends through psoas major, emerging low on its lateral border, and then passes between psoas and iliacus, deep to the iliac fascia. Passing behind the inguinal ligament into the thigh, it splits into anterior and posterior divisions. Behind the inguinal ligament it is separated from the femoral artery by part of psoas major. In the abdomen the nerve supplies small branches to iliacus and pectineus and a branch to the proximal part of the femoral artery; the latter branch sometimes arises in the thigh.

Fig. 20.7 Nerves of the left lower limb (anterior aspect).

Nerve to Pectineus

The nerve to pectineus branches from the medial side of the femoral nerve near the inguinal ligament. It passes behind the femoral sheath and enters the anterior aspect of the muscle.

Anterior Division of the Femoral Nerve

The anterior division of the femoral nerve supplies intermediate and medial cutaneous nerves of the thigh and branches to sartorius.

The intermediate cutaneous nerve of the thigh pierces the fascia lata approximately 8 cm below the inguinal ligament, either as two branches or as one trunk that quickly divides into two. These descend on the front of the thigh, supplying the skin as far as the knee and ending in the peripatellar plexus (see later). The lateral branch of the intermediate cutaneous nerve communicates with the femoral branch of the genitofemoral nerve, frequently piercing sartorius and sometimes supplying it.

The medial cutaneous nerve of the thigh is at first lateral to the femoral artery. It crosses anterior to the artery at the apex of the femoral triangle and divides into anterior and posterior branches. Before doing so, it sends a few rami through the fascia lata to supply the skin of the medial side of the thigh, near the long saphenous vein; one ramus emerges via the saphenous opening, and another becomes subcutaneous about mid thigh. The anterior branch descends on sartorius, perforates the fascia lata beyond mid thigh and divides into one branch that supplies the skin as low as the medial side of the knee and another branch that crosses to the lateral side of the patella and connects with the infrapatellar branch of the saphenous nerve. The posterior branch descends along the posterior border of sartorius to the knee, pierces the fascia lata, connects with the saphenous nerve and gives off several cutaneous rami, some as far as the medial side of the leg. The nerve contributes to the subsartorial plexus.

The main nerve to sartorius arises from the femoral nerve in common with the intermediate cutaneous nerve of the thigh.

Posterior Division of the Femoral Nerve

The branches of the posterior division of the femoral nerve are the saphenous nerve and branches to quadriceps femoris and the knee joint.

The saphenous nerve (see Fig. 20.7) is the largest cutaneous branch of the femoral nerve. It descends lateral to the femoral artery into the adductor canal, where it crosses anteriorly to become medial to the artery. At the distal end of the canal it leaves the artery and emerges through the aponeurotic covering with the saphenous branch of the descending genicular artery. As it leaves the adductor canal it gives off an infrapatellar branch that contributes to the peripatellar plexus and then pierces the fascia lata between the tendons of sartorius and gracilis, becoming subcutaneous to supply the prepatellar skin. It descends along the medial tibial border with the long saphenous vein and divides distally into one branch that continues along the tibia to the ankle and another branch that passes anterior to the ankle to supply the skin on the medial side of the foot, often as far as the first metatarsophalangeal joint. The saphenous nerve connects with the medial branch of the superficial peroneal nerve. Near mid thigh, it gives a branch to the subsartorial plexus (see below). The nerve may be subject to an entrapment neuropathy as it leaves the adductor canal.

The muscular branches of the posterior division of the femoral nerve supply quadriceps femoris. A branch to rectus femoris enters its proximal posterior surface and also supplies the hip joint. A larger branch to vastus lateralis forms a neurovascular bundle with the descending branch of the lateral circumflex femoral artery in its distal part and also supplies the knee joint. A branch to vastus medialis descends through the proximal part of the adductor canal, lateral to the saphenous nerve and femoral vessels. It enters the muscle at about its midpoint, sending a long articular filament distally along the muscle to the knee. Two or three branches to vastus intermedius enter its anterior surface about mid thigh; a small branch from one of these descends through the muscle to supply articularis genu and the knee joint.

Vascular branches of the femoral nerve supply the femoral artery and its branches.

CASE 3 Femoral Neuropathy

Five days ago, a 55-year-old woman suddenly developed pain in the right anterior and medial thigh. She has a long-standing history of insulin-dependent diabetes mellitus. She now has difficulty walking due to both pain and perceived weakness of the right leg.

Examination documents right quadriceps muscle weakness with mild wasting, as well as sensory loss involving the anterior and medial thigh, extending distally down the medial aspect of the leg. Her patellar reflex cannot be elicited; reflex activity is otherwise normal.

Discussion: This woman has femoral neuropathy characterized by weakness and wasting of the quadriceps femoris muscle and loss of the patellar reflex. Medial and anterior thigh pain and tingling extending into the medial leg in the saphenous nerve distribution are due to the fact that the saphenous nerve, a sensory nerve, is a branch of the posterior division of the femoral nerve.

Originating in the upper lumbosacral plexus, the femoral nerve travels through psoas major, passing low and deep between psoas and iliacus, then behind the inguinal ligament, separated from the femoral artery by a part of the psoas major. Involvement of the femoral nerve may result from a retroperitoneal haemorrhage or, as in this case, may reflect a diabetic mononeuropathy (compare with Case 4, Proximal Diabetic Amyotrophy). Compression at the inguinal ligament by external or internal pressure can also result in femoral neuropathy.

CASE 4 Proximal Diabetic Neuropathy

A 60-year-old man with type 2 diabetes mellitus develops acute right anterior thigh pain so severe that he requires long-acting narcotics. Several weeks later he notes atrophy of the right thigh with weakness. He has no other sensory complaints. In the past, he has had difficulty controlling his glucose levels, with significant hyperglycemia, but he recently lost 15 pounds with better glucose control. On examination he has mild atrophy of all anterior thigh muscles on the right, with moderate weakness in the same muscles. Hamstring, hip flexors and extensors and all distal muscles exhibit normal strength. On sensory examination he has mild bilateral distal sensory loss, along with decreased Achilles reflexes.

Discussion: Diabetic amyotrophy (proximal diabetic neuropathy) often has an acute or subacute onset, affecting the anterior thigh muscles more than hip, posterior thigh or distal leg musculature. Ischaemic injury to the proximal lumbosacral plexus, probably due to a vasculitic process, is the most likely cause. Recent weight loss is common and suggests an additional metabolic component. Other portions of the lumbosacral plexus or spinal nerve may be involved.

Sciatic Nerve

The sciatic nerve is 2 cm wide at its origin and is the thickest nerve in the body (Fig. 20.9). It leaves the pelvis via the greater sciatic foramen below piriformis and descends between the greater trochanter and the ischial tuberosity and along the back of the thigh, dividing into the tibial and common peroneal (fibular) nerves at a varying level proximal to the knee. Superiorly, it lies deep to gluteus maximus, resting first on the posterior ischial surface with the nerve to quadratus femoris between them. It then crosses posterior to obturator internus, the gemelli and quadratus femoris, separated by the last from obturator externus and the hip joint. It is accompanied medially by the posterior femoral cutaneous nerve and the inferior gluteal artery. More distally, it lies behind adductor magnus and is crossed posteriorly by the long head of biceps femoris. It corresponds to a line drawn from just medial to the midpoint between the ischial tuberosity and the greater trochanter to the apex of the popliteal fossa.

Fig. 20.9 Nerves of the left lower limb (posterior aspect).

Articular branches arise proximally to supply the hip joint through its posterior capsule; these are sometimes derived directly from the sacral plexus. Muscular branches are distributed to biceps femoris, semitendinosus, semimembranosus and the ischial part of adductor magnus.

The point of division of the sciatic nerve into its major components (tibial and common peroneal) is very variable. A common site is at the junction of the middle and lower thirds of the thigh, near the apex of the popliteal fossa. The division may occur at any level above this, but rarely below it. It is not uncommon for the major components to leave the sacral plexus separately, in which case the common peroneal component usually passes through piriformis at the greater sciatic notch, while the tibial component passes below the muscle.

Lesions of the Sciatic Nerve

The sciatic nerve supplies the knee flexors and all the muscles below the knee, so that a complete palsy of the sciatic nerve results in a flail foot and severe difficulty walking. This is rare and usually related to trauma. The nerve is vulnerable in posterior dislocation of the hip. As it leaves the pelvis it passes either behind piriformis or sometimes through the muscle, and at that point it may become entrapped (piriformis syndrome); this is a common anatomical variant, but an extremely rare entrapment neuropathy. External compression over the buttock (e.g. in patients who lie immobile on a hard surface for a considerable length of time) can injure the nerve. It may be damaged by misplaced therapeutic injections into gluteus maximus. The safe zone for deep intramuscular injections is the upper outer quadrant of the buttock. Perhaps safer still is to inject into quadriceps, although this can produce other problems such as haemorrhage, leading to contracture of the muscle, which limits knee motion. Sciatic nerve palsy occurs after total hip replacement or similar surgery in approximately 1% of cases. Haematoma is characterized by the development of severe pain in the immediate postoperative period. Early surgical exploration and evacuation of haematoma can reverse the nerve lesion. Unfortunately, the other causes may not be treatable; however, the majority are temporary. Complete sciatic nerve palsy is very rare. For some reason, possibly anatomical, the common peroneal part is usually affected alone. The patient so afflicted has a footdrop and a high-stepping gait.

CASE 5 Sciatica

A 35-year-old man presents with the acute onset of low back pain radiating into his left hip and down the left leg. The pain began suddenly 2 days ago when he bent over to pick something up off the floor. He cannot stand erect due to increasing pain and is most comfortable leaning to the left. He also observes tingling discomfort in the left leg involving the sole, lateral foot and fifth toe. Pain and paraesthesia are worse with Valsalva’s manoeuvres such as coughing, sneezing or straining.

Examination is hindered by the patient’s pain, but weakness of plantar and toe flexion, of extension (dorsiflexion) of the great toe, and of hip extension and abduction is observed on the left. The left Achilles reflex is absent. There is decreased pinprick sensation over the left lateral foot and fifth toe. Straight leg testing on the left increases symptoms in the leg. He is very tender on percussion over the lumbar spine and exhibits moderate paravertebral muscle spasm.

Discussion: Sciatica is a term used to describe pain radiating down the leg, with or without back pain. It is most commonly associated with nerve root compression at L5 or S1. The most frequent cause of acute sciatica is disc herniation. L5 and S1 roots are most commonly compressed; both roots can be involved in dorsolateral or central disc herniation, with the disc compressing the roots in the lateral recess. A large and more lateral herniation may result in only a single root being compressed, usually within the intervertebral foramen. Owing to mixed root innervation to lower extremity muscles, motor and sensory symptoms and signs do not always precisely match the root injured. Sensory loss in the foot (lateral foot and fifth toe involvement) indicates S1 localization (Fig. 20.10), but in some instances, sensation is normal. Straight leg raising increases symptoms by stretching the roots already compromised. Loss of the Achilles reflex with a normal patellar reflex is typical of an S1 root lesion.

Fig. 20.10 Footdrop (right) in a case of sciatica. The foot cannot be dorsiflexed on command.

Tibial Nerve

The tibial nerve, the larger sciatic component, is derived from the ventral branches (anterior division) of the fourth and fifth lumbar and first to third sacral ventral rami. It descends along the back of the thigh and popliteal fossa to the distal border of popliteus. It then passes anterior to the arch of soleus with the popliteal artery and continues into the leg. In the thigh it is overlapped proximally by the hamstring muscles, but it becomes more superficial in the popliteal fossa, where it is lateral to the popliteal vessels. At the level of the knee the tibial nerve becomes superficial to the popliteal vessels and crosses to the medial side of the artery. In the distal popliteal fossa it is overlapped by the junction of the two heads of gastrocnemius.

In the leg the tibial nerve descends with the posterior tibial vessels to lie between the heel and the medial malleolus. Proximally, it is deep to soleus and gastrocnemius, but in its distal third it is covered only by skin and fasciae, overlapped sometimes by flexor hallucis longus. At first medial to the posterior tibial vessels, it crosses behind them and descends lateral to them until it bifurcates. It lies on tibialis posterior for most of its course except distally, where it adjoins the posterior surface of the tibia. The tibial nerve ends under the flexor retinaculum by dividing into the medial and lateral plantar nerves.

Common Peroneal Nerve

The common peroneal nerve (common fibular nerve) is approximately half the size of the tibial nerve and is derived from the dorsal branches of the fourth and fifth lumbar and first and second sacral ventral rami. It descends obliquely along the lateral side of the popliteal fossa to the fibular head, medial to biceps femoris. It lies between the bicipital tendon, to which it is bound by fascia, and the lateral head of gastrocnemius. The nerve then passes into the anterolateral muscle compartment through a tight opening in the thick fascia overlying tibialis anterior. It curves lateral to the fibular neck, deep to peroneus longus, and divides into superficial and deep peroneal nerves.

The course of the common peroneal nerve can be indicated by a line from the apex of the popliteal fossa, passing distally, medial to the biceps tendon, to the back of the head of the fibula, where the nerve can be rolled against the bone.

Superficial Peroneal Nerve

The superficial peroneal nerve (superficial fibular nerve) begins at the common peroneal bifurcation. It is at first deep to peroneus longus and passes anteroinferiorly between the peronei and extensor digitorum longus to pierce the deep fascia in the distal third of the leg, where it divides into medial and lateral branches. Between the muscles it supplies peroneus longus, peroneus brevis and the skin of the lower leg.

Deep Peroneal Nerve

The deep peroneal nerve (deep fibular nerve) begins at the common peroneal bifurcation, between the fibula and the proximal part of peroneus longus. It passes obliquely forward, deep to extensor digitorum longus, to the front of the interosseous membrane and reaches the anterior tibial artery in the proximal third of the leg. It descends with the artery to the ankle, dividing there into lateral and medial terminal branches. It is first lateral to the artery, then anterior and again lateral at the ankle.

Sural Nerve

The sural nerve arises from the tibial nerve and descends between the heads of gastrocnemius, pierces the deep fascia proximally in the leg and is joined at a variable level by the sural communicating branch of the common peroneal nerve (Fig. 20.2). Some authors term this branch the lateral sural cutaneous nerve, and they call the main trunk (from the tibial nerve) the medial sural cutaneous nerve. The sural nerve descends lateral to the calcaneal tendon, near the short saphenous vein, to the region between the lateral malleolus and the calcaneus and supplies the posterior and lateral skin of the distal third of the leg. It then passes distal to the lateral malleolus along the lateral side of the foot and little toe, supplying the overlying skin. It connects with the posterior femoral cutaneous nerve in the leg and with the superficial peroneal nerve on the dorsum of the foot. The surface marking at the ankle is a line parallel to the calcaneal tendon halfway between the tendon and the lateral malleolus. However, its position is variable, and it is at risk from any surgery in this region. Rather like the radial nerve at the wrist, the sural nerve has a tendency to form painful neuromas. The nerve is harvested for grafting on occasion because it is sensory only, superficial, and easily identified.

Inferior Gluteal Nerve

The inferior gluteal nerve arises from the dorsal branches of the fifth lumbar and first and second sacral ventral rami. It leaves the pelvis via the greater sciatic foramen below piriformis and divides into branches that enter the deep surface of gluteus maximus.

Superior Gluteal Nerve

The superior gluteal nerve arises from the dorsal branches of the fourth and fifth lumbar and first sacral ventral rami. It leaves the pelvis via the greater sciatic foramen above piriformis, with the superior gluteal vessels, and divides into superior and inferior branches. The superior branch accompanies the upper branch of the deep division of the superior gluteal artery to supply gluteus medius and occasionally gluteus minimus. The inferior branch runs with the lower ramus of the deep division of the superior gluteal artery across gluteus minimus, supplying glutei medius and minimus and ending in tensor fasciae latae.

Perforating Cutaneous Nerve

The perforating cutaneous nerve usually arises from the posterior aspects of the second and third sacral ventral spinal rami. It pierces the sacrotuberous ligament, curves around the inferior border of gluteus maximus and supplies the skin over the inferomedial aspect of this muscle. The nerve may arise from the pudendal nerve or, if absent, may be replaced by a branch from either the posterior femoral cutaneous nerve or the third and fourth, or fourth and fifth, sacral ventral rami.

Nerve to Quadratus Femoris and Gemellus Inferior

The nerve to quadratus femoris and gemellus inferior arises from the ventral branches of the fourth lumbar to first sacral ventral rami. It leaves the pelvis via the greater sciatic foramen below piriformis; descends on the ischium deep to the sciatic nerve, gemelli and the tendon of obturator internus; and supplies gemellus inferior, quadratus femoris and the hip joint.

Nerve to Obturator Internus and Gemellus Superior

The nerve to obturator internus and gemellus superior arises from the ventral branches of the fifth lumbar and first and second sacral ventral rami. It leaves the pelvis like the nerve to quadratus femoris and gemellus inferior, supplies a branch to the upper posterior surface of gemellus superior, crosses the ischial spine lateral to the internal pudendal vessels, reenters the pelvis via the lesser sciatic foramen and enters the pelvic surface of obturator internus.

Posterior Cutaneous Nerve of the Thigh

The posterior cutaneous nerve of the thigh arises from the dorsal branches of the first and second sacral rami and the ventral branches of the second and third sacral rami. It leaves the pelvis via the greater sciatic foramen below piriformis and descends under gluteus maximus with the inferior gluteal vessels, lying posterior or medial to the sciatic nerve. It descends in the back of the thigh superficial to the long head of biceps femoris, deep to the fascia lata. Behind the knee it pierces the deep fascia and accompanies the short saphenous vein to midcalf, its terminal twigs connecting with the sural nerve. Its branches are all cutaneous and are distributed to the gluteal region, perineum and flexor aspect of the thigh and leg. Three or four gluteal branches curl around the lower border of gluteus maximus to supply the skin over its inferolateral area. The perineal branch supplies the superomedial skin in the thigh, curves forward across the hamstrings below the ischial tuberosity, pierces the fascia lata and runs in the superficial perineal fascia to the scrotal or labial skin, communicating with the inferior rectal and posterior scrotal branches of the perineal nerve. It gives numerous branches to the skin of the back and medial side of the thigh, the popliteal fossa and the proximal part of the back of the leg.

Nerve to Piriformis

The nerve to piriformis usually arises from the dorsal branches of the first and second sacral ventral rami (sometimes only the second) and enters the anterior surface of piriformis.

Superficial Peroneal Nerve

The superficial peroneal nerve was described earlier.

Deep Peroneal Nerve

The deep peroneal nerve was described earlier.

Tibial Nerve

The branches of the tibial nerve are the articular, muscular, sural, medial calcaneal and medial and lateral plantar nerves. The articular, muscular and sural nerves were described earlier.

Medial Plantar Nerve

The medial plantar nerve (Fig 20.11) is the larger terminal division of the tibial nerve, and it lies lateral to the medial plantar artery. From its origin under the flexor retinaculum, it passes deep to abductor hallucis, then appears between it and flexor digitorum brevis, gives off a medial proper digital nerve to the hallux and divides near the metatarsal bases into three common plantar digital nerves.

Fig. 20.11 Plantar nerves of the left foot.

Cutaneous branches pierce the plantar aponeurosis between abductor hallucis and flexor digitorum brevis to supply the skin of the sole of the foot. Muscular branches supply abductor hallucis, flexor digitorum brevis, flexor hallucis brevis and first lumbrical. The first two arise near the origin of the nerve and enter the deep surfaces of the muscles. The branch to flexor hallucis brevis is from the hallucal medial digital nerve, and that to the first lumbrical is from the first common plantar digital nerve. Articular branches supply the joints of the tarsus and metatarsus.

Three common plantar digital nerves pass between the slips of the plantar aponeurosis, each dividing into two proper digital branches. The first supplies adjacent sides of the hallux and second toe, and the second supplies adjacent sides of the second and third toes; the third supplies adjacent sides of the third and fourth toes and also connects with the lateral plantar nerve. The first gives a branch to the first lumbrical. Each proper digital nerve has cutaneous and articular branches; near the distal phalanges, a dorsal branch supplies structures around the nail, and the termination of each nerve supplies the ball of the toe. The common digital branches of the medial plantar nerve are distributed in a manner similar to those of the median nerve, as are the motor branches of the two nerves. In the hand, the median nerve supplies abductor and flexor pollicis brevis, opponens pollicis and first and second lumbricals. An opponens is absent in the foot, but abductor hallucis, flexor hallucis brevis and first lumbrical are all supplied by the medial plantar nerve. Because flexor digitorum brevis and flexor digitorum superficialis (median nerve) correspond, only the innervation of the second lumbrical differs.

Lateral Plantar Nerve

The lateral plantar nerve (see Fig. 20.11) supplies the skin of the fifth toe, the lateral half of the fourth toe and most of the deep muscles of the foot. Its distribution therefore closely resembles that of the ulnar nerve in the hand. It passes laterally forward, medial to the lateral plantar artery, toward the tubercle of the fifth metatarsal. It next passes between flexor digitorum brevis and accessorius and ends between brevis and abductor digiti minimi by dividing into superficial and deep branches. Before division, it supplies flexor digitorum accessorius and abductor digiti minimi and gives rise to small branches that pierce the plantar fascia to supply the skin of the lateral part of the sole (Fig. 20.12). The superficial branch splits into two common plantar digital nerves: the lateral supplies the lateral side of the fifth toe, flexor digiti minimi brevis and the two interossei in the fourth intermetatarsal space; the medial connects with the third common plantar digital branch of the medial plantar nerve and divides into two to supply the adjoining sides of the fourth and fifth toes. The deep branch accompanies the lateral plantar artery deep to the flexor tendons and adductor hallucis and supplies the second to fourth lumbricals, adductor hallucis and all interossei (except those of the fourth intermetatarsal space). Branches to the second and third lumbricals pass distally, deep to the transverse head of adductor hallucis, and curl around its distal border to reach them (Fig. 20.13).

Fig. 20.12 Cutaneous innervation of the foot: dorsum and plantar aspect.

Fig. 20.13 Dissection of the lateral and medial plantar nerves of the left foot. Most of flexor digitorum brevis has been removed. Flexor digitorum longus has been partially divided, and its distal end has been displaced, together with the second, third, and fourth lumbricals.

Nerve Entrapment Syndromes in the Foot

Any nerve of the foot can be affected by entrapment, classically leading to a burning sensation in the distribution of that nerve. Tarsal tunnel syndrome (see Case 6) is much less common than carpal tunnel syndrome. The flexor retinaculum can compress the tibial nerve or either of its branches (medial and lateral plantar nerves), but it is most commonly compressed by a space-occupying lesion (e.g. ganglion), a leash of vessels or the deep fascia associated with abductor hallucis. Compression of the first branch of the lateral plantar nerve by the deep fascia of abductor hallucis can lead to heel pain. The medial plantar nerve can be irritated at the master knot of Henry; this is usually related to jogging. The superficial peroneal nerve can be damaged in severe inversion injuries of the ankle, and the deep peroneal nerve is sometimes compressed by osteophytes in the region of the second tarsometatarsal joint. Sural nerve entrapment does not occur from compression by fascial elements; rather, it follows trauma and subsequent scar formation around the nerve. Entrapment of the common digital nerve as it passes under the intermetatarsal ligament of the third (or, less commonly, second) web space can result in Morton’s neuroma, which is probably the most common form of nerve entrapment in the foot.

Pudendal Nerve

The pudendal nerve arises from the ventral divisions of the second, third and fourth sacral ventral rami and is formed just above the superior border of the sacrotuberous ligament and the upper fibres of ischiococcygeus. It leaves the pelvis via the greater sciatic foramen between piriformis and ischiococcygeus, enters the gluteal region and crosses the sacrospinous ligament close to its attachment to the ischial spine. The nerve lies medial to the internal pudendal vessels on the spine. It accompanies the internal pudendal artery through the lesser sciatic foramen into the pudendal (Alcock’s) canal on the lateral wall of the ischioanal fossa. In the posterior part of the canal it gives rise to the inferior rectal nerve, the perineal nerve and the dorsal nerve of the penis or clitoris.

Sacral Visceral Branches

These arise from the second to fourth sacral ventral rami to innervate the pelvic viscera; they are termed pelvic splanchnic nerves.

Sacral Muscular Branches

Several muscular branches arise from the fourth sacral ventral ramus to supply the superior surface of levator ani and the upper part of the external anal sphincter. The branches to levator ani enter the superior (pelvic) surface of the muscle, while the branch to the external anal sphincter (also referred to as the perineal branch of the fourth sacral nerve) reaches the ischioanal fossa by running through ischiococcygeus or between ischiococcygeus and iliococcygeus. It supplies the skin between the anus and coccyx via its cutaneous branches.

Coccygeal Plexus

The coccygeal plexus is formed by a small descending branch from the fourth sacral ramus and by the fifth sacral and coccygeal ventral rami. The fifth sacral ventral ramus emerges from the sacral hiatus, curves around the lateral margin of the sacrum below its cornu and pierces ischiococcygeus from below to reach its upper, pelvic surface. There it is joined by a descending branch of the fourth sacral ventral ramus, and the small trunk so formed descends on the pelvic surface of ischiococcygeus. They join the minute coccygeal ventral ramus, which emerges from the sacral hiatus and curves around the lateral coccygeal margin to pierce coccygeus to reach the pelvis. This small trunk is the coccygeal plexus. Anococcygeal nerves arise from it and form a few fine filaments that pierce the sacrotuberous ligament to supply the adjacent skin.

Pelvic Part of the Sympathetic System

The pelvic sympathetic trunk lies in the extraperitoneal tissue anterior to the sacrum beneath the presacral fascia. It lies medial or anterior to the anterior sacral foramina and has four or five interconnected ganglia. Above, it is continuous with the lumbar sympathetic trunk. Below the lowest ganglia, the two trunks converge to unite in the small ganglion impar anterior to the coccyx. Grey rami communicantes pass from the ganglia to sacral and coccygeal spinal nerves, but there are no white rami communicantes. Medial branches connect across the midline, and twigs from the first two ganglia join the inferior hypogastric plexus or the hypogastric ‘nerve.’ Other branches form a plexus on the median sacral artery.

Vascular Branches

Postganglionic fibres pass through the grey rami communicantes to the roots of the sacral plexus. Those forming the tibial nerve are conveyed to the popliteal artery and its branches in the leg and foot, while those in the pudendal and superior and inferior gluteal nerves accompany the same named arteries to the gluteal and perineal tissues. Branches may also supply the pelvic lymph nodes.

Preganglionic fibres for the rest of the lower limb are derived from the lower three thoracic and upper two or three lumbar spinal segments. They reach the lower thoracic and upper lumbar ganglia through white rami communicantes and descend through the sympathetic trunk to synapse in the lumbar ganglia. Postganglionic fibres pass from these ganglia via grey rami communicantes to the femoral nerve, which carries them to the distribution of the femoral artery and its branches. Some fibres descend through the lumbar ganglia to synapse in the upper two or three sacral ganglia, from which postganglionic axons join the tibial nerve to supply the popliteal artery and its branches in the leg and foot.

Sympathetic denervation of vessels in the lower limb can be effected by removing or ablating the upper three lumbar ganglia and the intervening parts of the sympathetic trunk, which is rarely useful in treating vascular insufficiency of the lower limb.

Inferior Rectal Nerve

The inferior rectal nerve runs through the medial wall of the pudendal canal with the inferior rectal vessels. It crosses the ischioanal fossa to supply the external anal sphincter, the lining of the lower part of the anal canal and the circumanal skin. It frequently breaks into terminal branches just before reaching the lateral border of the sphincter. Its cutaneous branches distributed around the anus overlap the perineal branch of the posterior femoral cutaneous nerve and the scrotal or labial nerves. The inferior rectal nerve occasionally arises directly from the sacral plexus and crosses the sacrospinous ligament or reconnects with the pudendal nerve. In females the inferior rectal nerve may supply sensory branches to the lower part of the vagina.

Perineal Nerve

The perineal nerve is the inferior and larger terminal branch of the pudendal nerve in the pudendal canal. It runs forward, below the internal pudendal artery, and accompanies the perineal artery, dividing into posterior scrotal or labial and muscular branches. The posterior scrotal or labial nerves are usually double and have medial and lateral branches that run over the perineal membrane and pass forward in the lateral part of the urogenital triangle with the scrotal (or labial) branches of the perineal artery. They supply the skin of the scrotum or labia majora, overlapping the distribution of the perineal branch of the posterior femoral cutaneous and inferior rectal nerves. In females, the posterior labial branches also supply sensory fibres to the skin of the lower vagina.

Muscular branches arise directly from the pudendal nerve to supply the superficial transverse perinei, bulbospongiosus, ischiocavernosus, deep transverse perinei, sphincter urethrae and anterior parts of the external anal sphincter and levator ani. In males, a nerve to the bulb of the urethra leaves the nerve to the bulbospongiosus, piercing it to supply the corpus spongiosum penis, and ends in the urethral mucosa.

Dorsal Nerve of the Penis or Clitoris

The dorsal nerve of the penis or clitoris runs anteriorly above the internal pudendal artery along the ischiopubic ramus deep to the inferior fascia of the urogenital diaphragm. It supplies the corpus cavernosum and accompanies the dorsal artery of the penis or clitoris between the layers of the suspensory ligament. In males, it runs on the dorsum of the penis to end in the glans. In females, the dorsal nerve of the clitoris is very small.