Pelvic girdle and lower limb: overview and surface anatomy

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CHAPTER 79 Pelvic girdle and lower limb: overview and surface anatomy

This chapter is made up of two sections. The first is an overview of the general organization of the lower limb, with particular emphasis on the fascial skeleton, distribution of the major blood vessels and lymphatic channels, and the branches of the lumbar and sacral plexuses: it is intended to complement the detailed regional anatomy described in Chapters 80 to 84 Chapter 81 Chapter 82 Chapter 83 Chapter 84. The second section describes the surface anatomy of the lower limb.

The structure of the lower limb is determined by its adaptations for weightbearing, locomotion and the maintenance of equilibrium (stability). Indeed, the adaptations for weightbearing and for stability, and the differing developmental histories of the limbs account for the major structural and functional differences between the upper and lower limbs. The inguinal (pelvicrural) and gluteal (buttock) regions are important anatomical junctional zones between the trunk and the lower limb through which longitudinally running nerves and vessels may pass in either direction. The inguinal region includes the junctional zones between the limb and abdominal cavity via the myopectineal orifice (the gap between the inguinal ligament and hip bone), and between the limb and pelvic cavity via the obturator foramen. The gluteal region includes the junctional zones between the limb and the abdominopelvic cavity via the greater sciatic foramen, and between the limb and the perineum via the lesser sciatic foramen (Fig. 79.1).

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Fig. 79.1 Gateways from the abdomen, pelvis and perineum to the lower limb.

(From Drake, Vogl and Mitchell 2005.)

SKIN, FASCIA AND SOFT TISSUES

In the young adult the skin of the lower limb is generally stronger and thicker than that of the upper limb: weightbearing skin, e.g. of the sole of the foot, is particularly thickened. The skin of the buttocks and posterior thigh bears weight in the sitting position, and consequently is relatively thick. The skin over the anteromedial aspect of the lower leg is particularly fragile and vulnerable in the elderly. Body hair is usually well developed in all areas except the sole and posterior ankle.

Fascial skeleton

The ‘fascial skeleton’ (deep fascia) of the lower limb is well-defined and forms a tough circumferential ‘stocking-like’ structure that constrains the musculature (Fig. 79.2). Septa pass from the deep surface of this fascial sheath to the bones within, confining the functional muscle groups within osteofascial compartments. The tough fascia gives additional areas of attachment to the muscles and ensures that they work to maximal effect. Thickenings in the ensheathing layer may act as additional tendons (e.g. iliotibial tract). Elsewhere, thickenings in the fascial skeleton form fibrous retinacula where tendons cross joints. Although they are particularly prominent in the embalmed cadaver, these fascial layers are also readily demonstrable in the living, and are of considerable functional significance. The pattern of soft-tissue organization has a bearing on the physiological effects of the muscles and is crucial for efficient venous return from the limb. The fascial planes also control and direct the spread of pathological fluids (blood, pus) within the limb and play an important part in determining the degree and direction of displacement seen in long bone fractures.

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Fig. 79.2 Deep fascia of the lower limb. A, Anterior view. B, Posterior view.

(From Sobotta 2006.)

Fasciocutaneous system

The fascial septa dictate the pathways of cutaneous arteries, which subsequently perforate and ramify on the fascial ‘stocking’ before supplying the skin.

Osteofascial compartments in the lower limb

The muscles of the thigh may be grouped into three compartments according to their function, namely anterior (extensor), posterior (flexor) and medial (adductor): only the anterior and posterior compartments possess distinct fascial boundaries. A very definite fascial separation into anterior (extensor), posterior (flexor) and lateral (evertor) compartments exists in the leg, and compartment syndrome is most common in this region (see below). Osteofascial compartments in the foot are described in Chapter 84.

Vessels and nerves run through all the osteofascial compartments and supply the muscles contained within them. The muscles acting within these closed compartments assist in maintaining the anti-gravity flow of venous blood.

Compartment syndrome

The fascial boundaries that limit the osteofascial compartments are largely inelastic, which means that any condition that leads to an increase in the volume of the compartmental contents, e.g. muscle swelling caused by trauma or unaccustomed overuse, haemorrhage and local infection, is likely to cause an increase in intracompartmental pressure. If unrelieved, this increased pressure will lead to compressive occlusion of the vessels in the compartment and consequent ischaemic damage to the nerves and muscles of the compartment, a phenomenon known as compartment syndrome.

BONES AND JOINTS

The bones of the lower limb are the three fused components of the pelvic girdle; the femur and patella (thigh); the tibia and fibula (leg); the tarsus, metatarsus and phalanges (foot) (Fig. 79.3). The hip bones (especially the ilium and ischium), femur, tibia and bones of the hindfoot are strong and their external (cortical) and internal (trabecular) structure is adapted for weightbearing.

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Fig. 79.3 Overview of bones of the lower limb. A, posterior aspect. B, anterior aspect.

The pelvic girdle connects the lower limb to the axial skeleton via the sacroiliac joint, a synovial joint in which mobility has been sacrificed for stability and strength, to allow for effective weight transmission from the trunk to the lower limb. Anteriorly, the pelvic girdle articulates with the contralateral girdle at the pubic symphysis, a secondary cartilaginous joint that may display a slight degree of mobility during hip and sacroiliac movement and during childbirth. The hip joint, a synovial ball-and-socket joint, exhibits a very effective compromise between mobility and stability that allows movement in all three orthogonal planes. The more distal joints have gained mobility at the expense of stability. The knee joint includes the patellofemoral articulation, which allows the patella to glide over the distal femur. However, the main component of the knee joint is a bicompartmental synovial articulation between the femur and the tibia that allows flexion, extension and some medial and lateral rotation of the leg. It is not a true hinge joint because its axes of flexion and extension are variable and there is coupled rotation. The tibia and fibula articulate with each other at the superior and inferior tibiofibular joints. The superior joint, a plane synovial joint, allows slight gliding movement only. The inferior joint, a fibrous joint, lies just above the ankle and allows a degree of fibular rotation linked to ankle motion. The ankle (talocrural) joint is formed by the distal ends of the tibia and fibula ‘gripping’ the talus, and allows dorsiflexion and plantarflexion. There are multiple joints in the foot that may be classified topographically on the basis of whether they are in the hindfoot, midfoot or forefoot. Collectively, these joints allow the complex movements required as the foot fulfils its functional roles as a platform for standing and for shock absorption and propulsion in gait.

Both knee and ankle are commonly subject to closed injuries, and the relatively superficial location of the knee renders it susceptible to open injury and infection. Although the ankle is frequently injured and is a major loadbearing joint, the incidence of clinically significant degenerative arthritis is surprisingly low when compared with that found in the hip and knee.

MUSCLES

The effects of extension and medial rotation of the limb that occur during fetal development are manifest in the relative positions of the muscle groups in the thigh and the leg, and in the adult pattern of segmental innervation (dermatomes). In broad outline, the anterior aspect of the adult limb is the extensor aspect, while the flexors lie posteriorly; the reverse is true at the hip.

The role of the muscles of the lower limb in the maintenance of equilibrium during locomotion and in stance is rarely emphasized sufficiently. Many of the muscles act frequently or predominantly from their distal attachments. During both stance and locomotion, the distal attachment is often fixed and the proximal attachment is mobile, e.g. the predominant action of gluteus medius is as a pelvic stabilizer rather than as a hip abductor. (In contrast, in the upper limb the proximal muscle attachments are usually fixed and the distal attachments are mobile, an arrangement that is consistent with the prehensile function of the hand.) The lower limb contains many muscles that act upon more than one joint, and it is unusual for any joint of the lower limb to move in isolation.

Muscles of the lower limb may be subdivided into muscles of the iliac and gluteal regions, and of the thigh, leg and foot. (Note that in anatomical nomenclature, ‘leg’ refers to that part of the lower limb between the knee and ankle.) The main muscles of the iliac region are psoas major and iliacus, the major flexors of the hip that run from the lumbar spine and inner surface of the ilium respectively to the lesser trochanter of the femur. The much less important psoas minor (when present) runs from the lumbar spine to the pubis. The muscles of the gluteal region include the three named gluteal muscles and the deeper short lateral rotators of the hip joint. Gluteus maximus lies most superficially, running from the posterior pelvis to the proximal femur and fascia lata. It is a powerful extensor of the hip joint, acting more often to extend the trunk on the femur than to extend the limb on the trunk. Gluteus medius and minimus, attaching proximally to the outer iliac surface and distally to the greater trochanter of the femur, are abductors of the hip; their most important action is to stabilize the pelvis on the femur during locomotion, and they are helped in this function by tensor fasciae latae, a more anteriorly placed muscle that arises from the anterolateral ilium and inserts into the fascia lata. Two of the short lateral rotators of the hip, piriformis and obturator internus, arise from within the pelvic walls, while the others, obturator externus, the gemelli and quadratus femoris, are attached externally: all of these muscles are attached distally to the proximal femur.

The muscles of the thigh lie in three functional compartments. The anterior or extensor compartment includes sartorius and the quadriceps group. Sartorius and rectus femoris are attached proximally to the pelvis and can thus act on the hip joint as well as on the knee, whereas the vasti are attached proximally to the femoral shaft, and, acting as a unit, are powerful knee extensors. The medial or adductor compartment contains the named adductor muscles and gracilis; pectineus may also be included. These muscles are attached proximally to the anterior aspect of the pelvis, and distally to the femur; gracilis has no femoral attachment, being attached distally to the proximal tibia, while a part of adductor magnus has a proximal attachment to the ischial tuberosity. The posterior (‘hamstring’) compartment includes semitendinosus, semimembranosus and biceps femoris. These muscles are attached proximally to the ischial tuberosity and act both to extend the trunk on the femur and to flex and rotate the knee. Adductor magnus, as may be inferred from the extent of its proximal attachment and its dual innervation, shares the first of these functions with the hamstrings. Biceps femoris is the only muscle of the thigh that is attached distally to the fibula, and has no tibial attachment.

In the leg the anterior or extensor compartment includes the extensors (dorsiflexors) of the ankle and the long extensors of the toes. Tibialis anterior, the main ankle dorsiflexor, also inverts the foot at the subtalar joint, while the smallest muscle of the compartment, fibularis (peroneus) tertius, is a dorsiflexor that everts the foot. The posterior or flexor (plantarflexor) compartment has superficial and deep components. The superficial component contains gastrocnemius and soleus, powerful plantar flexors of the ankle, and the slender plantaris. Gastrocnemius and soleus are attached distally via the calcaneal (Achilles) tendon. The deep component of the flexor compartment contains popliteus, a rotator of the knee, the long flexors of the toes and tibialis posterior, the main invertor of the foot. The lateral compartment contains the main evertors of the foot, fibularis (peroneus) longus and brevis; both muscles are also plantar flexors at the ankle.

Gastrocnemius, plantaris and popliteus are the only muscles of the leg that are attached proximally to the femur and they can therefore act on the knee as well as at the ankle. The remaining leg muscles are attached proximally to the tibia, fibula or both, and to the interosseous membrane. The intrinsic muscles of the sole of the foot are arranged in layers. They facilitate the actions of the long flexors of the toes, and by effecting subtle changes in the shape of the foot they help control foot posture in stance and locomotion.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

ARTERIAL SUPPLY

The femoral artery (the continuation of the external iliac artery) provides the principal arterial supply to the lower limb distal to the inguinal ligament and the gluteal fold (Figs 79.4, 79.5). The femoral artery courses within the anteromedial aspect of the thigh, becoming the popliteal artery on entering the posterior compartment of the thigh and dividing into its terminal branches in the posterior compartment of the leg. The obturator and inferior gluteal vessels also contribute to the supply of the proximal part of the limb. In the embryo the inferior gluteal artery supplied the main axial artery of the limb, which is represented in the adult by the arteria comitans nervi ischiadici (artery to the sciatic nerve).

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Fig. 79.4 Overview of arteries of the lower limb. A, Anterior aspect. B, Posterior aspect.

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Fig. 79.5 The anatomical territories served by the cutaneous blood supply to the lower limb.

The bones of the lower limb receive their arterial supply from nutrient vessels, metaphysial arterial branches of the peri-articular anastomoses, and the arteries supplying the muscles that attach to their periosteum. The pattern of arterial supply is particularly relevant to fracture healing, the spread of infection and malignancy, and to the planning of reconstructive surgical procedures. For further details consult Cormack & Lamberty (1994), Taylor & Razaboni (1994) and Crock (1996).

Arterial perforators of the lower limb

Achieving adequate and aesthetically satisfactory skin and soft tissue cover for large, superficial tissue defects is a perennial challenge in the field of plastic and reconstructive surgery, and accounts for a substantial part of the plastic surgeon’s workload. Generally, split-thickness and full-thickness skin grafts are suitable only for very superficial defects. To achieve tissue cover for deeper and larger tissue defects, the plastic surgeon employs one of a variety of autologous tissue flaps. The viability of a flap transplanted from one part of the body to another is crucially dependent on the blood supply of the flap. An appreciation of the angiosome concept (see Ch. 6), coupled with technological advances in reconstructive microsurgery, has stimulated the development and use of perforator (or perforator-based) flaps. These are flaps of skin or subcutaneous tissue supplied by one or more fascial ‘perforators’, i.e. arteries which reach the suprafascial plexus either directly from a source vessel, or indirectly from some other neighbouring tissue (Fig. 79.6). Perforator-based flaps are typically harvested with sparing of underlying muscle tissue and minimal trauma: their use is said to reduce postoperative pain, donor site morbidity and functional loss.

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Fig. 79.6 Types of arterial perforators.

The lower limb is the largest donor site in the body for perforator-based flaps. Commonly used flaps are antero-lateral thigh flaps which provide a large amount of skin; superior and inferior gluteal artery perforator flaps used in breast reconstruction; vascularized fibula flaps for reconstruction of deficient bone; vascularized tensor fasciae latae flaps for tendon reconstruction; vascularized sural nerve flaps for nerve reconstruction; and the gracilis muscle flap, used in reanimation of paralysed muscle.

In the context of perforator flap surgery, the lower limb may be considered in terms of four anatomic regions: gluteal; anterior hip and thigh; knee and leg; ankle and foot. Each lower limb accounts for approximately 23% of the total body surface area (thigh 10.5%, leg 6.5%, buttock 2.5% and foot 3.5%) and contains an average of 90 arterial perforators (Fig. 79.7).

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Fig. 79.7 Source and location of arterial perforators of the lower limb. A, Anterior aspect. B, Posterior aspect.

VENOUS DRAINAGE

The veins of the lower limb can be subdivided, like those of the upper limb, into superficial and deep groups (Figs 79.8, 79.9). The superficial veins are subcutaneous and lie in the superficial fascia; the deep veins (beneath the deep fascia) accompany the major arteries. Valves are present in both groups, but are more numerous in the deep veins. (Valves are more numerous in the veins of the lower limb than in the veins of the upper limb.) Venous plexuses occur within and between some of the lower limb muscles. The principal named superficial veins are the long and short saphenous veins; their numerous tributaries are mostly unnamed. For details and variations consult Kosinski (1926).

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Fig. 79.8 Overview of veins of the lower limb.

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Fig. 79.9 A, The long saphenous vein and its tributaries. B, The short saphenous vein and its tributaries.

Deep veins of the lower limbs accompany the arteries and their branches. Plantar digital veins arise from plexuses in the plantar regions of the toes, connect with dorsal digital veins and unite to form four plantar metatarsal veins. These run in the intermetatarsal spaces and connect with dorsal veins by means of perforating veins. They then connect with each other to constitute a deep plantar venous arch adjacent to the plantar arterial arch. From this arch, medial and lateral plantar veins run near the corresponding arteries: they communicate with the long and short saphenous veins before forming the posterior tibial veins behind the medial malleolus. The posterior tibial veins accompany the posterior tibial artery. They receive veins from the calf muscles, especially the venous plexus in soleus, and connect with superficial veins and with the fibular veins. The latter, running with their artery, receive tributaries from soleus and from superficial veins.

The anterior tibial veins are continuations of the venae comitantes of the dorsalis pedis artery. They leave the extensor region between the tibia and fibula, pass through the proximal end of the interosseous membrane, and unite with the posterior tibial veins, at the distal border of popliteus, to form the popliteal vein.

Venous (muscle) pumps

In a standing position, venous return from the lower limb depends largely on muscular activity, especially contraction of the calf and foot muscles, known as the ‘muscle pump’, whose efficiency is aided by the tight sleeve of deep fascia. ‘Perforating’ veins connect the long saphenous vein with the deep veins, particularly near the ankle, distal calf and knee. Their valves are arranged so as to prevent flow of blood from the deep to the superficial veins. At rest, pressure in a superficial vein is equal to the height of the column of blood extending from that vein to the heart. When calf muscles contract, blood is pumped proximally in the deep veins and is normally prevented from flowing into the superficial veins by the valves in the perforating veins. During muscular relaxation, blood is drawn into the deep veins from the superficial veins. If the valves in the perforating veins become incompetent, these veins become sites of ‘high pressure leaks’ during muscular contraction, and the superficial veins become dilated and varicose. Similar perforating connections occur in the anterolateral region, where varicosities may also occur. Valvar incompetence in the connecting veins between the long saphenous vein and femoral vein in the adductor canal may predispose to superficial varicosities in the medial aspect of the thigh (Dodd & Cockett 1976).

Venous plexuses

Venous plexuses may be intramuscular (soleus) or intermuscular (in the foot and gluteal region). The plexuses communicate with the axially running deep veins and are components of the ‘muscle pump’ mechanism.

LYMPHATIC DRAINAGE

Most lymph from the lower limb traverses a large intermediary inguinal group of nodes (Fig. 79.10). Peripheral nodes are few and all are deeply sited. Except for an inconsistent node lying proximally on the interosseous membrane near the anterior tibial vessels, they occur only in the popliteal fossa. Enlarged popliteal nodes may be palpated along the line of the popliteal vessels while the passively supported knee is gradually moved from extension to semi-flexion. Inguinal nodes are found superficial and deep to the deep fascia. The deep nodes are few and lie alongside the medial aspect of the femoral vein. The superficial nodes may be divided into a lower vertical group that clothe the proximal part of the long saphenous vein, and an upper group that lie parallel to, but below, the inguinal ligament and which are related to the superficial circumflex iliac and superficial external pudendal vessels. Lymph from the lower limb passes from the inguinal nodes to the external and common iliac nodes, and ultimately drains to the lateral aortic group. Deep gluteal lymph reaches the same group through the internal and common iliac chains.

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Fig. 79.10 Overview of lymphatics of the lower limb.

Superficial tissues

The superficial lymph vessels begin in subcutaneous plexuses. Collecting vessels leave the foot medially, along the long saphenous vein, or laterally with the short saphenous vein. Medial vessels are larger and more numerous; they start on the tibial side of the dorsum of the foot, ascend anterior or posterior to the medial malleolus and accompany the long saphenous vein. They drain to the distal superficial inguinal nodes. Lateral vessels begin on the fibular side of the dorsum of the foot; some cross anteriomedially in the leg to join the medial vessels and so pass to the distal superficial inguinal lymph nodes, others accompany the short saphenous vein and drain to the popliteal nodes. Superficial lymph vessels from the gluteal region run anteriorly to the proximal superficial inguinal nodes.

Deeper tissues

The deep lymph vessels accompany the anterior and posterior tibial, fibular, popliteal and femoral vessels. The deep vessels from the foot and leg are interrupted by popliteal nodes; those from the thigh pass to the deep inguinal nodes. The deep vessels of the gluteal and ischial regions follow their corresponding blood vessels. Those accompanying the superior gluteal vessels end in a node near the intrapelvic part of the superior gluteal artery, adjacent to the superior border of the greater sciatic foramen, while those which follow the inferior gluteal vessels traverse one or two of the small nodes below piriformis and pass to the internal iliac nodes.

INNERVATION

OVERVIEW OF THE LUMBAR AND SACRAL PLEXUSES

Nerves derived from the lumbar and sacral plexuses innervate the lower limb (Figs 79.11). 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, external 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 on the posterior abdominal wall and lies on the ala of the sacrum before crossing the pelvic brim to join the ventral ramus of S1.

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Fig. 79.11 Nerves of the lower limb, their cutaneous branches and muscular branches. A, anterior aspect. B, Posterior aspect. Adductor magnus has dual innervation.

Lesions of the lumbar and sacral plexuses

The deep and protected situation of the plexuses means that lesions are not common. The lumbar plexus may be involved in retroperitoneal pathology, and the sacral plexus may be invaded by spreading pelvic malignancy. Both may be involved in the reticuloses, affected by plexiform neuromas, or damaged in fractures of the lumbar spine and pelvis or in other conditions that cause severe retroperitoneal and pelvic haemorrhage. Temporary lesions may occur after pregnancy and childbirth, e.g. after difficult forceps delivery of a large baby. Pain, which may be diffuse, is the most common feature, and there is often clinical involvement of several roots.

OVERVIEW OF THE PRINCIPAL NERVES OF THE LOWER LIMB

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. It enters the thigh behind the inguinal ligament and lateral to the femoral sheath. Its terminal branches form in the femoral triangle about 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, medial leg and foot, and gives articular branches to the hip and knee. The femoral nerve is described in detail on page 1383.

Obturator nerve (L2–4)

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