The pelvic girdle, formed by the right and left innominate articulating anteriorly at the symphysis pubis and posteriorly by the sacrum via the sacroiliac joint, connects the lower limb to the vertebral column. The sacroiliac joint provides great strength for weight transference from the trunk to the lower limb at the sacrifice of almost all mobility. The human ilium has developed so that it is no longer blade-like but is shortened and tightly curved backwards and outwards (Fig. 3.1), changing the actions of the gluteal muscles. These changes in the pelvis have resulted in a shift from it lying in an essentially horizontal to an essentially vertical position. This has enabled the trunk to be held vertically, but has necessitated a change in the orientation of the sacrum with respect to the ilium: the result is that the axis of the pelvic canal lies almost at right angles to the vertebral column. During evolution there has been a relative approximation of the sacral articular surface to the acetabulum which makes for greater stability in the transmission of the weight of the trunk to the hip joint. This increase in the magnitude has resulted in an increase in the contact area between sacrum and ilium relative to the area of the ilium as a whole. For the same reason the acetabulum and femoral head have also increased in relative size during evolution. The shortening of the ischium that has occurred is an adaptation for speed and rapid movements, which is of great importance in bipeds. Thus power of action has been sacrificed for speed.

Figure 3.1 The evolutionary changes that have occurred in the human pelvis as part of the adaptation to the erect posture and bipedalism.

Changes have also occurred at the knee, with the femoral condyles being more parallel in humans compared with other primates. The major change, however has been in bringing the knees inwards towards the midline, which appears to be part of the overall pattern of centring the body mass, thus reinforcing skeletal rather than muscular equilibrium.

In humans, the tibia and fibula are held tightly together, with the tibia being the weight-bearing component while the fibula is mainly for muscle attachments. There has been a loss of rotation of the fibula with respect to the tibia.

It is the foot, however, which has undergone the greatest change during evolution (Fig. 3.2), reflecting not so much the evolution of a new function as a reduction in the original primate functions, with the foot changing from a grasping, tactile organ to a locomotor prop. Although some non-locomotor function is still possible, the foot has evolved from a generalized to a specialized organ. The joints of the human foot permit much less internal mobility – an adaptation to ground walking. In locomotion, the foot acts as a lever adding propulsive force to that of the leg, with the point of pivot being the subtalar joint. The forefoot has been shortened relative to the hindfoot, where the main thrust in walking is developed – the power capabilities are thus accentuated.

Figure 3.2 The evolutionary changes that have occurred within the foot during its adaptation to bipedalism.

The arches of the foot are formed by the shape of the bony elements, and are supported by ligaments and tendons. They convert the foot into a complex spring under tension and allow it to transmit the stresses involved in walking, both when body momentum is checked at heel-strike and when the foot is used in propelling the body forward. The lateral arch helps to steady the foot on the ground, while the medial arch transmits the main force of thrust in propulsion. The arched foot is important in providing one of the major determinants of gait, i.e. helping to minimize energy expenditure and thus increase the efficiency of walking.

An important consequence of the upright, bipedal posture is that the centre of gravity of the body has been brought towards the vertebral column, so that in humans it lies slightly behind and at about the same level as the hip joint, thus reducing the tendency of gravity to pull the trunk forward. The centre of gravity projection then passes anterior to the knee and ankle joints (see Fig. 3.4); at the knee the line of weight transmission passes towards the outside. Because of the angulation of the femur, during walking the foot, tibia and knee joint of each leg stay close to the line followed by the centre of gravity, and thus energy expenditure is minimal in maintaining the centre of gravity above the supporting limb. Balance is thus improved and there is more time for the free leg to swing forward promoting an increase in stride length. The alteration in the line of weight transmission is carried into the foot, where it passes to the inner side. However, it must be remembered that weight is also transmitted through the outside of the foot, bringing the entire foot into use as a stabilizing element.

Figure 3.4 The regions, bones and joints of the lower limb.

In order to reduce the possibility of collapse or dislocation, due to the forces to which they are subjected, the joints of the lower limb are structurally more stable than those of the upper limb. This increased stability is due to either the shape of the articular surfaces, the number and strength of the ligaments, or the size of the muscles related to the joint. Generally, each of these factors contributes to a varying extent.

Development of the musculoskeletal system

Mesodermal somites

By the end of the third week after fertilization the paraxial mesoderm begins to divide into mesodermal somites which are easily recognizable during the fourth and fifth weeks (Fig. 3.3B). Eventually some 44 pairs of somites develop, although not all are present at the same time; however, the paraxial mesoderm at the cranial end of the embryo remains unsegmented. There are 4 occipital somites, followed by 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8–10 coccygeal somites. The growth and migration of these somitic cells are responsible for the thickening of the body wall, as well as the development of bone and muscle. The deeper layers of the skin are also of somitic origin. Somite-derived tissue spreads medially to form the vertebrae, dorsally to form the musculature of the back, and ventrally into the body wall to form the ribs, and the intercostal and abdominal muscles.

Figure 3.3 (A) Somites in the developing embryo, (B) limb buds and the direction in which they will rotate, (C) rotation of the limbs.

Soon after its formation each somite becomes differentiated into three parts. The ventromedial part forms the sclerotome, which migrates medially towards the notochord and neural tube to take part in the formation of the vertebrae and ribs (Fig. 3.3A). The remainder of the somite is known as the dermomyotome. The cells of the dorsal and ventral edges proliferate and move medially to form the myotome, whose cells migrate widely and become differentiated into myoblasts (primitive muscle cells). The remaining thin layer of cells forms the dermatome, which spreads out to form the dermis of the skin.

The myotome of each somite receives a single spinal nerve which innervates all the muscle derived from that myotome, no matter how far it eventually migrates. The dorsal aortae lie adjacent to the somites and give off a series of intersegmental arteries which lie between them.

Development of the limbs

The limbs initially appear as flipper-like projections (the limb buds), with the forelimbs appearing first, between 24 and 26 days, each bud consisting of a mass of mesenchyme covered by ectoderm with a thickened ectodermal ridge at the tip; the ectodermal ridge controls normal development of the limb. Consequently, damage to it will result in some trauma to the limb. At the beginning of the second month the elbow and knee prominences project laterally and backwards. At about the same time, the hand and foot plates appear as flattened expansions at the end of the limb bud. Between 36 and 38 days, five radiating thickenings forming the fingers and toes can be distinguished; the webs between the thickenings disappear freeing the digits. Appropriate spinal nerves grow into the limbs in association with migration of the myotomes: C5, 6, 7, 8 and T1 for the upper limb, and L4, 5, S1, 2 and 3 for the lower limb. The limb bones differentiate from the mesenchyme of the bud. The limbs grow in such a way that they rotate in opposite directions, the upper limb laterally and the lower limb medially (Fig. 3.3C). Consequently, the thumb becomes the lateral digit of the hand while the great toe is the medial digit of the foot.

During development the lower limb bud appears as a swelling from the body wall (Fig. 3.3C(i)); the lower limb follows the upper limb in appearance and development. At first, each limb bud projects at right angles to the surface of the body, having ventral and dorsal surfaces, and cephalic (preaxial) and caudal (postaxial) borders (Fig. 3.3C(ii)). As the limb increases in length it becomes differentiated, during which time it is folded ventrally so that the ventral surface becomes medial (Fig. 3.3C(iii)), with the convexities of the elbow and knee directed laterally (Fig. 3.3C(iv)). At a later stage, the upper and lower limbs rotate in opposite directions such that the convexity of the knee is directed towards the cranial end of the body (Fig. 3.3C(v)). The ventral rami entering a limb bud pass anterior to the myotomes and eventually divide into dorsal and ventral divisions, which unite with the corresponding branches of the adjacent ventral rami to form the nerves of the dorsal and ventral aspects of the limb bud. These nerves will supply respectively the sheets of muscle and overlying skin of the dorsal (extensor) and ventral (flexor) surfaces of the limbs prior to their rotation into the adult pattern. During the folding and rotation of the limb and the migration of muscle masses, these nerve–muscle connections are maintained.

The rotation of the lower limb in the course of development results in its extensor and flexor surfaces coming to lie anteriorly and posteriorly respectively. This rotation is reflected in the arrangement of the innervation. The muscles on the front of the thigh and leg are supplied by nerves coming from the posterior part of the lumbar and lumbosacral plexuses, the femoral and common fibular (peroneal) nerves, while those at the back of the thigh and leg and in the sole of the foot are from the anterior aspect of the lumbosacral plexus, the tibial nerve.

As in the upper limb, many muscles cross several joints and exert their actions on each. It is unusual for one joint of the lower limb to be moved in isolation. In standing and walking the joints and muscles form a coordinated mechanism.

The uppermost limit of the lower limb is a line joining the iliac crest, inguinal ligament, symphysis pubis, ischiopubic ramus, ischial tuberosity, sacrotuberous ligament, and the dorsum of the sacrum and coccyx. The bulge of tissue running between the innominate and the upper part of the femur forms the buttock, but the region is usually named gluteal after the underlying muscles. The lower limb is divided into the thigh between the hip and knee, the leg between the knee and ankle, and the foot distal to the ankle joint. The foot is divided into the foot proper and the toes, and has a superior surface (dorsum) and an inferior (plantar) surface, which is the sole of the foot (Fig. 3.4).

The bones of the lower limb are those of the pelvic girdle (innominate and sacrum), the femur in the thigh, the medial tibia and lateral fibula in the leg, the seven tarsal bones and five metatarsals in the foot, and the phalanges of the toes – two in the big toe and three in each remaining toe (Fig. 3.4).

Bones

Pelvic girdle

Introduction

The pelvic girdle (Fig. 3.5) comprises the two innominates and sacrum; the ring of bone formed uniting the trunk and the lower limbs. A large, irregular bone, the innominate consists of two expanded triangular blades twisted 90° to each other in the region of the acetabulum; each blade is also twisted within itself. The innominate is formed from three separate bones, the ilium, ischium and pubis, which come together and fuse in the region of the acetabulum such that in the adult it appears as a single bone. The sacrum consists of five fused vertebrae and is roughly triangular. The coccyx, which is the remnant of the tail, consists of four fused coccygeal vertebrae. Each innominate articulates with the sacrum posteriorly by joints which are synovial anteriorly and fibrous posteriorly. Each innominate also articulates with its counterpart anteriorly at the pubic symphysis, by a secondary cartilaginous joint.

Figure 3.5 Transfer of weight from the vertebral column, through the pelvis to the femur.

The pelvic girdle has several functions:

1. It supports and protects the pelvic viscera.

2. It supports body weight transmitted through the vertebrae, thence through the sacrum, across the sacroiliac joints to the innominate and then to the femora in the standing position, or to the ischial tuberosities when sitting.

3. During walking, the pelvis swings from side to side by a rotatory movement at the hip joint which occurs together with small movements at the lumbar intervertebral joints; when the hip joints are fused pelvic rotation is taken up by the thoracic spine enabling a patient to walk reasonably well.

4. As with all but a few small bones in the hand and foot, the pelvis provides attachments for muscle.

5. In females it provides bony support for the birth canal.

The pelvic girdle transmits the weight from the vertebral column to the lower limbs (Fig. 3.5), its bony and ligamentous components reflecting this function. There are some differences in the pelvis between females and males. In the female, these are due to adaptation for childbearing and the transmission of the relatively large fetal head during childbirth. The pelvis is essentially a basin, the upper part being known as the greater or false pelvis containing abdominal viscera. That part below the pelvic brim or pelvic inlet is the lesser or true pelvis. In the anatomical position, the pelvic inlet forms an angle of about 60° with the horizontal (Fig. 3.6). The acetabulum is directed laterally and inferiorly with the acetabular notch directed inferiorly. The anterior superior iliac spines and the pubic tubercles lie in the same vertical frontal plane. The lowest part of the sacrum lies above the level of the symphysis pubis.

Figure 3.6 Relative positions of the sacrum and pubis in the anatomical position, lateral view.

The anterior superior iliac spines can be easily palpated in the living subject, particularly in females, where they tend to be further apart than in males. The iliac crest can also be palpated about 10 cm above the greater trochanter of the femur. In the sitting position, the ischial tuberosities can be felt, with the weight of the body resting on them. The body of the right and left pubic bones can also be palpated; they separate the anterior abdominal wall from the genitalia.

The innominate (hip) bone

An irregularly shaped bone (Figs. 3.7 and 3.8) consisting of three bones fused together, the ilium, the pubis and the ischium.

Figure 3.7 Left innominate, lateral view.

Figure 3.8 Left innominate, medial view.

The sacrum

A triangular bone with the apex inferior, it consists of five fused vertebrae broadened by the incorporation of large costal elements and transverse processes into heavy lateral masses, which lie lateral to the transverse tubercles on the back of the sacrum extending between the anterior sacral foramina onto the front of the bone; the auricular surface lies entirely on the lateral mass. It is wedged between the posterior parts of the two innominates with which it articulates at the sacroiliac joints. The pelvic (anterior) surface (Fig. 3.9A) is concave and relatively smooth, being marked by four transverse ridges separating the original bodies of the five sacral vertebrae. Lateral to each ridge is the anterior sacral foramen, representing the anterior part of the intervertebral foramen; the foramina are directed laterally and anteriorly.

Figure 3.9 Sacrum: (A) anterior view, (B) posterior view.

The dorsal surface (Fig. 3.9B) is convex and highly irregular with posterior sacral foramina, medial to which the vertebral canal is closed over by the fused laminae. However, the spinous processes and laminae of the fourth and fifth sacral vertebrae are usually absent, leaving the vertebral canal open. This is the sacral hiatus, an inferior entrance to the vertebral canal, which may be used, for example during labour, to introduce an anaesthetic agent to block the sacral nerves. Posteriorly, in the midline, the reduced spinous processes form the median sacral crest. Lateral to the posterior sacral foramina are the prominent lateral sacral crests, representing the transverse processes, which provide attachment for the dorsal sacroiliac ligaments, and inferiorly for the sacrotuberous and sacrospinous ligaments. Medial to the posterior sacral foramina are the indistinct intermediate sacral crests, representing the fused articular processes. The superior articular processes of the first sacral vertebra are large and oval, being supported by short heavy pedicles. Their facets, for articulation with the inferior articular surfaces of the fifth lumbar vertebra, are concave from side to side and face posteromedially. The tubercles of the inferior articular processes of the fifth sacral vertebra form the sacral cornua and are connected to the cornua of the coccyx.

The lateral surface (Fig. 3.10A) is triangular, being narrower below. The upper part is divided into an anterior smoother pitted auricular surface, covered in cartilage, for articulation with a similar area on the ilium. The rougher posterior area has three deep impressions for attachment of the powerful posterior sacroiliac ligaments. The superior surface (Fig. 3.10B) faces anterosuperiorly and has a central oval area which is the upper surface of the first sacral vertebra; it is separated from the fifth lumbar vertebra by a thick intervertebral disc. Its anterior projecting border is the sacral promontory. On each side of the body of the sacrum is the ala, formed by the fusion of the costal and transverse processes of the first sacral vertebra. When articulated with the innominate, the ala of the sacrum is continuous with the ala of the ilium.

Figure 3.10 Sacrum: (A) lateral view, (B) superior view, (C) coccyx, anterior view.

Ossification

Primary centres appear in the sacrum between the third and eighth month in utero; one for each vertebral body, one for each half of each vertebral arch, and one for each costal element in the upper four vertebrae. The costal elements fuse with the arches by the age of 5 years, the arches with the body slightly later, with the two parts of each arch uniting between the ages of 7 and 10. The segments of the lateral masses fuse together during puberty, with secondary centres appearing for the vertebral bodies at about the same time. Bodies and epiphyses fuse between 18 and 25 years. Several secondary centres appear at the ends of the costal and transverse processes from which two epiphyses are formed, one of which covers the auricular surface while the other completes the lower margin of the sacrum.

The coccyx

It usually consists of four fused vertebrae forming a single piece or two pieces of bone (Fig. 3.10C), which mainly represent the bodies. The pelvic surface is concave and relatively smooth, while on the posterior surface the rudimentary articular processes are present as a row of tubercles. Superiorly the larger pair form the coccygeal cornua which articulate with the sacral cornua, and enclose the fifth sacral intervertebral foramen. The posterior wall of the vertebral canal of the coccyx is absent so that the sacral hiatus continues downwards over the back of the coccyx.

The femur

The longest and strongest bone in the body (Fig. 3.11), it transmits body weight from the ilium to the upper end of the tibia. It has a shaft and two extremities.

Figure 3.11 (A) Left femur, anterior view, (B) right femur, posterior view.

The upper end of the femur consists of a head, neck, and greater and lesser trochanters. The head is slightly more than half a sphere, is entirely smooth and covered with articular cartilage except for a small hollow just below its centre, the fovea capitis, which provides attachment for the ligament of the head of the femur. Connecting the head to the shaft is the neck, which is approximately 5 cm long and forms an angle of 125° with the shaft. The angle varies a little with age and sex. The neck is flattened anteroposteriorly, giving upper and lower rounded borders; the upper border is concave along its long axis and the lower is straight. The anterior surface of the neck joins the shaft at the intertrochanteric line and the posterior at the intertrochanteric crest, marked at its centre by the large quadrate tubercle.

The large quadrilateral greater trochanter is situated on the lateral aspect of the upper part of the shaft lateral to the neck. It has an upper border marked by a tubercle, an anterior border marked by a depression and posterior and inferior borders both roughened for muscular attachment. Its lateral surface is crossed by a diagonal roughened line running downwards and forwards having above it a smooth area covered by a bursa. The medial surface, above the neck, is small and has a deep trochanteric fossa at its centre.

The conical lesser trochanter is situated medially, behind and below the neck, and is smaller than the greater trochanter. Its tip is drawn forwards and presents a roughened ridge running downwards and forwards.

The shaft is strong, and except for a prominent posterior border, almost cylindrical in cross-section. It is gently convex anteriorly, being narrowest at its centre becoming stouter as it approaches the upper and lower extremities. Its posterior border, the linea aspera, is rough for muscle attachments, has medial and lateral lips with a central flattened area between. In the upper and lower quarters of the shaft the two lips diverge, producing a posterior surface. The upper surface is marked medially by the narrow vertical pectineal line, whereas the lateral truncated border is continuous upwards with the posterior border of the greater trochanter to form the gluteal tuberosity. The lower surface, between the supracondylar lines above and the condyles below, forms the popliteal surface of the femur. The rest of the shaft is slightly flattened on its anterior, posteromedial and posterolateral aspects.

The lower end of the femur consists of two large condyles, each projecting backwards beyond the posterior surface of the shaft, with the lateral being stouter than the medial. The inferior, posterior and posterosuperior surfaces of the condyles are smooth and continuous anteriorly with the triangular shaped patellar surface, which is grooved vertically, giving larger lateral and smaller medial regions. The two condyles are separated posteriorly and inferiorly by the intercondylar notch, marked on its lateral wall posteriorly by the attachment of the anterior cruciate ligament (ACL) and on its medial wall anteriorly by the posterior cruciate ligament (PCL). The separating lips of the linea aspera continue downwards onto the upper aspect of the medial and lateral condyles as the supracondylar lines, the medial presenting at its lower end as the adductor tubercle.

The lateral surface of the lateral condyle is roughened, marked just below its centre by the lateral epicondyle below which is a smooth groove for the popliteus tendon. The medial surface of the medial condyle is also roughened and again marked just below its centre by the medial epicondyle.

Ossification

The primary ossification centre for the shaft appears at 7 weeks in utero. At birth, growth plates separate the bony shaft from the upper and lower cartilaginous epiphyses (Fig. 3.12). A secondary ossification centre appears in the lower epiphysis shortly before birth. Secondary ossification centres appear in the upper epiphysis for the head at 1 year and in the greater trochanter at 4 years (Fig. 3.12). The last secondary ossification centre appears in the cartilaginous lesser trochanter at 12 years. The upper epiphysis fuses with the shaft at about the 18th year, the last to do so being the head. The lower epiphysis fuses with the shaft at about 20 years. The neck of the femur is ossified as part of the body (shaft) and not from the upper epiphysis.

Figure 3.12 Stages in ossification of the femur.

Palpation

The femur is almost completely surrounded by muscles and is only palpable in limited areas. At its upper end, the greater trochanter is an obvious landmark, projecting just a little more laterally than the iliac crest, being easily located by running the hands down from the middle of the crest some 7–10 cm. The greater trochanter is perhaps easier to feel if the fingers are brought forward from the hollows on the sides of the buttocks in the region of the back pocket. Its posterior border can be palpated for about 5 cm, running down towards the shaft, while its upper border is an important landmark to locate the level of the hip joint.

At its lower end, the femur is well covered with muscle until just above the knee joint. As the fingers pass down the medial side of the thigh the medial condyle can be palpated. This is marked just behind its centre by the medial epicondyle, above which the adductor tubercle can be palpated with the tendinous part of adductor magnus attaching to it. On the lateral side of the knee, the lateral condyle can be palpated with the lateral epicondyle projecting from its outer surface. At the lower edge of each of these condyles, the knee joint line can be palpated particularly as it passes forwards.

If the knee is fully flexed, the patella is seen to move downwards revealing on the front of the knee the two femoral condyles, covered by the lower part of quadriceps femoris and its retinacula.

The patella

A triangular sesamoid bone (Fig. 3.13) formed in the tendon of quadriceps femoris, with its apex pointing inferiorly and its base uppermost. It is flattened from front to back, having anterior and posterior surfaces and superior, lateral and medial borders.

Figure 3.13 The patella: (A) anterior surface, (B) posterior surface.

The anterior surface of the patella is marked by a series of roughened vertical ridges produced by the fibres of quadriceps which pass over it. It is slightly convex forwards and its shape varies according to the pull of the muscle.

The posterior surface has a large, smooth oval facet covered with hyaline cartilage for articulation with the patellar surface of the femur. It is divided by a broad vertical ridge into a smaller medial and a larger lateral facet. The cartilage on each of these facets is marked by two horizontal lines dividing each surface into upper, middle and lower sections. Below, there is a roughened area on the posterior aspect of the apex for the upper attachment of the ligamentum patellae.

The base of the patella is roughened for the attachment of rectus femoris and vastus intermedius. The medial and lateral borders are rounded but also roughened, receiving attachments of vastus medialis and lateralis.

Ossification

At birth the patella is cartilaginous, ossifying from a single centre or several centres between 3 years and puberty. Occasionally the patella may be absent.

Palpation

As the patella lies subcutaneously, the whole margin as well as its anterior surface can be palpated.

The tibia

A long bone (Fig. 3.14), which transmits body weight from the medial and lateral condyles of the femur to the foot. It is the larger of the two bones of the leg, being situated medial to the fibula. It consists of a shaft and two extremities, the upper extremity being much larger than the lower.