Chapter 20 Lumbar Plexus and Sacral Plexus

Overview of the Plexuses

The lumbar and sacral plexuses innervate the lower limb. The lumbar plexus lies deep within psoas major, anterior to the transverse processes of the first three lumbar vertebrae. The sacral plexus lies in the pelvis on the anterior surface of piriformis, deep to the pelvic fascia, which separates it from the inferior gluteal and pudendal vessels. The lumbosacral trunk (L4 and L5) emerges medial to psoas major and lies on the ala of the sacrum before crossing the pelvic brim to join the anterior primary ramus of S1.

Overview of the Principal Nerves of the Lower Limb (Figs 20.1, 20.2)

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.

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

L1	Psoas major, psoas minor
L2	Psoas major, iliacus, sartorius, gracilis, pectineus, adductor longus, adductor brevis
L3	Quadriceps, adductors (magnus, longus, brevis)
L4	Quadriceps, tensor fasciae latae, adductor magnus, obturator externus, tibialis anterior, tibialis posterior
L5	Gluteus medius, gluteus minimus, obturator internus, semimembranosus, semitendinosus, extensor hallucis longus, extensor digitorum longus, peroneus tertius, popliteus
S1	Gluteus maximus, obturator internus, piriformis, biceps femoris, semitendinosus, popliteus, gastrocnemius, soleus, peronei (longus and brevis), extensor digitorum brevis
S2	Piriformis, biceps femoris, gastrocnemius, soleus, flexor digitorum longus, flexor hallucis longus, some intrinsic foot muscles
S3	Some intrinsic foot muscles (except abductor hallucis, flexor hallucis brevis, flexor digitorum brevis, extensor digitorum brevis)

Table 20.3 Segmental innervation of joint movements of the lower limb

Hip	Flexors, adductors, medial rotators	L1–3
	Extensors, abductors, lateral rotators	L5, S1
Knee	Extensors	L3, L4
	Flexors	L5, S1
Ankle	Dorsiflexors	L4, L5
	Plantar flexors	S1, S2
Foot	Inverters	L4, L5
	Everters	L5, S1
	Intrinsic muscles	S2, S3

Table 20.4 Movements and muscles tested to determine location of a lesion in the lower limb

With the patient supine and the lower limb externally rotated and partially flexed at the hip and knee, the foot is passively dorsiflexed to stretch the calcaneal tendon, which is then struck with a percussion hammer. Contraction of the calf muscles plantar-flexes the ankle. The reflex can also be examined with the patient kneeling on a chair.

Plantar Reflex

The plantar reflex is a superficial reflex, and its elicitation is an important part of the clinical examination of the central nervous system. With the foot relaxed and warm, the outer edge of the sole is stroked longitudinally with a hard object (traditionally, the examiner’s fingernail or a key). This should elicit flexion of the toes, although the normal adult response varies with the strength of the stimulus. In adults with upper motor neurone lesions, the response includes extension of the great toe (Babinski’s sign).

Lumbar Plexus

The posterior abdominal wall contains the origin of the lumbar plexus (Fig. 20.6) and numerous autonomic plexuses and ganglia that lie close to the abdominal aorta and its branches.

Fig. 20.6 The lumbar plexus, its branches and the muscles they supply. The ventral branches of the ventral rami are coloured yellow, and the dorsal branches are coloured orange.

Lumbar ventral rami increase in size from first to last and are joined, near their origins, by grey rami communicantes from the four lumbar sympathetic ganglia. These rami, long and slender, accompany the lumbar arteries around the sides of the vertebral bodies, behind psoas major. Their arrangement is irregular: one ganglion may give rami to two lumbar nerves, or one lumbar nerve may receive rami from two ganglia. Rami often leave the sympathetic trunk between ganglia. The first, second and sometimes third lumbar ventral rami are each connected with the lumbar sympathetic trunk by a white ramus communicans. The lumbar ventral rami descend laterally into psoas major. The first three and most of the fourth form the lumbar plexus; the smaller moiety of the fourth joins the fifth as a lumbosacral trunk, which joins the sacral plexus. The fourth is often termed the nervus furcalis, being divided between the two plexuses; however, the third is occasionally the nervus furcalis. Alternatively, both the third and fourth may be furcal nerves, in which case the plexus is termed ‘prefixed.’ More frequently, the fifth nerve is furcal, and the plexus is then termed ‘postfixed.’ These variations modify the sacral plexus.

The lumbar plexus lies within the substance of the posterior part of psoas major, anterior to the transverse processes of the lumbar vertebrae. It is formed by the first three and most of the fourth lumbar ventral rami. The first lumbar ramus receives a branch from the last thoracic ventral ramus. The paravertebral part of psoas major consists of posterior and anterior masses, which arise from different attachments. The lumbar plexus lies between these masses and hence is ‘in line’ with the intervertebral foramina. Although there may be minor variations, the most common arrangement of the plexus is described here.

The first lumbar ventral ramus, joined by a branch from the twelfth thoracic ventral ramus, bifurcates, and the upper and larger part divides again into the iliohypogastric and ilioinguinal nerves. The smaller lower part unites with a branch from the second lumbar ventral ramus to form the genitofemoral nerve. The remainder of the second, third and parts of the fourth lumbar ventral rami join the plexus and divide into ventral and dorsal branches. Ventral branches of the second to fourth rami join to form the obturator nerve. The main dorsal branches of the second to fourth rami join to form the femoral nerve. Small branches from the dorsal branches of the second and third rami join to form the lateral femoral cutaneous nerve. The accessory obturator nerve, when it exists, arises from the third and fourth ventral branches. The lumbar plexus is supplied by branches from the lumbar vessels, which supply the psoas major.

The branches of the lumbar plexus are as follows:

Muscular	T12, L1–4
Iliohypogastric	L1
Ilioinguinal	L1
Genitofemoral	L1, L2
Lateral femoral cutaneous	L2, L3
Femoral	L2–4 dorsal divisions
Obturator	L2–4 ventral divisions
Accessory obturator	L2, L3