The bones

The l o w er l i mb

C H A P T ER C O N T E N TS

The bones 273 The joints 280 The muscles 299 The arteries 302 The veins 305

THE BONES OF THE LOWER LIMB

The femur (Fig. 8.1; see Fig. 6.1) The femur is the longest and the strongest bone of the body. It has a head, neck, shaft and an expanded lower end.

The head is more than half of a sphere and is directed upwards, medially and forwards. It is intra-articular and covered w i th cartilage apart from a central pit called the fovea, where the ligamentum teres is attached. The blood supply of the femoral head is derived from three sources as follows: • Vessels in the cancellous bone from the shaft; • Vessels in the capsule of the hip joint, which reach the head in synovial folds along the neck; • Negligible supply via the fovea from vessels in the ligamentum teres.

The neck of the femur is about 5 cm long and forms an angle of 127° w i th the shaft. It is also anteverted, that is, it is directed anteriorly at an angle of about 10° w i th the sagittal plane. Its junction w i th the shaft is marked superiorly by the greater trochanter and inferiorly and slightly posteriorly by the lesser trochanter. Between these anteriorly is a ridge called the intertrochanteric line, and posteriorly a more prominent intertrochanteric crest. The capsule of the hip joint is attached to the intertrochanteric line anteriorly but at the junction of the medial two-thirds and the lateral third of the neck posteriorly.

The shaft of the femur is inclined medially so that whereas the heads of the femurs are separated by the pelvis, the lower ends at the knees almost touch. It also has a forward convexity. The shaft is cylindrical w i th a prominent ridge posteriorly, the linea aspera. This ridge splits inferiorly into medial and lateral supracondylar lines, w i th the popliteal surface between them. The medial supracondylar line ends in the adductor tubercle.

The lower end of the femur is expanded into two prominent condyles united anteriorly as the patellar surface, but separated posteriorly by a deep intercondylar notch.

Fig. 8.1 Femur: anterior and posterior views.

The most prominent parts of each condyle are called the medial and lateral epicondyles. Above the articular surface on the lateral side is a small depression that marks the origin of the popliteus muscle.

Radiological features of the femur (see Fig. 6.1)

Plain radiographs A line along the upper margin of the neck of the femur transects the femoral head in AP and lateral radiographs. This alignment is changed when the epiphysis is slipped.

A line along the inferior margin of the neck of the femur forms a continuous arc w i th the superior and medial margin of the obturator foramen of the pelvis. This line, known as Shenton's line (see Fig. 8.6), is disrupted in congenital dislocation of the hip.

On an AP radiograph a line between the lowermost part of each condyle, although parallel to the upper tibia, lies at an angle of 81° to the shaft of the femur. This amount of genu valgum is normal.

Radiography The neck of the femur is anteverted approximately 10°. In AP radiography of the neck, therefore, the leg must be internally rotated 10° to position the neck of the femur parallel to the f i l m.

In a lateral view of the neck, a film and grid placed vertically and pressed into the patient's side must be positioned at an angle of 127° to the shaft of the femur because of the angulation of neck to the shaft.

Views of the intercondylar fossa (tunnel views) are taken with the knee flexed at an angle of about 135° w i th a vertical beam and with a beam angled at 70° to the lower leg.

Sesamoid bones

A sesamoid bone called the fabella is frequently seen in the lateral head of the gastrocnemius muscle. On radiographs, this is projected on the posterior aspect of the lateral femoral condyle.

Blood supply of the head of the femur The head of the femur receives its blood supply mainly via the neck, as described above. Thus fracture of the neck, especially subcapital fracture, disrupts this blood supply and results in ischaemic necrosis of the head in 80% of cases.

Ossification of the femur

The primary centre in the shaft appears in the seventh fetal week. A secondary centre is present in the lower femur at birth (this is a reliable indicator that the fetus is f u ll term) and another appears in the head between 6 months and 1 year of age. Secondary centres appear in the greater trochanter at 4 years and in the lesser trochanter at 8 years of age. A ll fuse at 18-20 years of age.

Trabecular markings in the neck of femur and bone density Additional biomechanical integrity of the neck of the femur is provided by an ordered internal trabecular network with a combination of primary compressive trabeculae, primary tensile trabeculae and secondary compressive trabeculae. These trabeculae, readily identified on conventional radiographs, surround a triangular area where there is a relative paucity of trabecular markings, termed Ward's triangle. The integrity of these described trabeculae reflects relative osteoblastic activity and stress. In osteoporosis, the disappearance of secondary compressive trabeculae prior to the disappearance of primary tensile trabeculae prior to subsequent disappearance of primary compressive trabeculae forms the basis of the Singh index.

It is important to be aware that only 30% of bone density in the femoral neck is cancellous or metabolically active, in contrast to 50% in the vertebral body. This factor accounts for the detection on DEXA scanning of osteoporotic changes in the spinal vertebral body before the femoral neck in most cases.

The patella

This is a sesamoid bone in the quadriceps tendon that continues at its apex as the ligamentum patellae. The upper two-thirds of the posterior surface is covered with articular cartilage and is entirely within the knee joint, and its anterior surface is covered by the prepatellar bursa. The lateral articular surface is usually larger than the medial surface.

Radiological features of the patella

Plain radiographs The outer surface of the patella as seen on tangential (skyline) views is irregular owing to the entry of nutrient vessels here.

Occasionally, the upper outer segment of the patella is separate from the remainder of the bone. Such a patella is called a bipartite patella and must be recognized as normal and not fractured.

Dislocation of the patella Lateral dislocation of the patella is more common than medial dislocation and occurs following valgus injury with associated imposed bowstringing of the extensor mechanism over the knee joint. Anatomical structures have evolved to prevent dislocation, including relative hypertrophy of the vastus medialis muscle and overgrowth of the lateral femoral condyle.

Wiberg describes three shapes of patella: type 1, in which the medial and lateral articular facets of the patella are equal in size and dimension, type 2 in which the lateral facet is slightly larger than the medial facet, and type 3 in which the lateral facet is dominant and the medial facet is atretic and redundant. The type 3 configuration is more commonly associated w i th tracking disorders and transient subluxation.

Evaluation of patellofemoral alignment is routinely achieved using the skyline position w i th the beam centred on the patellofemoral joint from below. Patellofemoral tracking is best achieved using the Merchant views, w i th the beam directed from above down to a cassette held over the tibia. The Merchant views can therefore be acquired in weightbearing and in varying degrees of flexion and extension.

Transverse fracture of the patella is associated w i th separation of the fragments by the p u ll of the quadriceps muscle. Comminuted fractures as a result of direct trauma, on the other hand, usually leave the extensor expansion intact. In association w i th disruption of the articular cartilage, these fractures are treated by patellectomy rather than K-wire stabilization.

Ossification This begins at 3 years and is complete by puberty.

The tibia (Fig. 8.2) The upper end of the tibia is expanded as the tibial plateau. This has an articular surface w i th a large medial and a smaller lateral condyle, which articulate w i th the condyles of the femur. Between the condyles is the intercondylar eminence or the tibial spine, which has medial and lateral projections - the medial and lateral intercondylar tubercles.

Anteriorly, at the upper end of the shaft of the tibia is the tibial tubercle into which the ligamentum patellae is inserted. The anteromedial surface of the shaft of the tibia is subcutaneous. The posterior surface of the shaft has a prominent oblique ridge - the soleal line.

The lower end of the tibia has the medial malleolus medially and the fibular notch for the inferior tibiofibular joint laterally. Its inferior surface is flattened and articulates w i th the talus in the ankle joint.

The fibula (see Fig. 8.2) Apart from its role in the ankle joint, the fibula is mainly a site of origin of muscles and has no weightbearing function. It has a head w i th a styloid process into which the biceps femoris is inserted, a neck, a narrow shaft and a lower end expanded as the lateral malleolus. Proximal and distal tibiofibular joints unite it w i th the tibia and it articulates w i th the talus in the ankle joint.

The lateral malleolus is more distal than the medial malleolus. The calcaneofibular ligament is attached to its tip. This is often damaged in inversion injuries.

The fibula is proportionately thicker in children than in adults.

Radiological features of the tibia and fibula

Plain radiographs The tibial tuberosity is very variable in appearance, particularly during the growth period. Asymmetry and irregularity on radiographs may be quite normal.

Some irregularity of the tibia at the upper part of the interosseous border may simulate a periosteal reaction here.

Ossification of the tibia

The primary ossification centre for the shaft of the tibia appears in the seventh fetal week. A secondary ossification centre is present in the upper end at birth and in the lower

Fig. 8.2 Tibia and fibula: (a) anterior view; (b) posterior view.

end at 2 years. The upper centre fuses w i th the shaft at 20 years, the lower sooner at 18 years.

Ossification of the fibula

Ossification of the primary centre in the shaft begins in the eighth fetal week, in the lower secondary centre in the first year and in the upper at 3 years. The lower epiphysis fuses w i th the shaft at 16 years and the upper at 18 years. The tibia in adulthood In adulthood, the tibia, like all other long bones is characterized by thick compact bony cortex in the tubulated diaphysis, in contrast to cortical thinning in the flared metaphysis. Biomechanical integrity of the tibial metaphysis, in contrast to the diaphysis, is provided by an additional internal ordered trabecular network, cancellous bony matrix.

At MRI, following injury, bruising is most marked at sites where there is an internal trabecular network of cancellous bone, and is therefore more marked in the metaphyses than in the diaphysis.

Reflecting a lack of cancellous matrix and vascular supply, healing of distal tibial shaft fractures is often delayed and may require stabilization, internal fixation and bone grafting.

The bones of the foot (Figs 8.3 and 8.4) In addition to metatarsals and phalanges, there are seven tarsal bones in the foot. These are the talus, calcaneus, navicular, cuboid and three cuneiform bones. Of these, the talus and calcaneus are most important radiologically.

The talus

The long axis of the talus points forwards and medially so that its anterior end is medial to the calcaneus. The talus has a:

Body between the malleoli, w i th a superior articular surface called the trochlear surface; • Neck grooved inferiorly as the sulcus tali, which, w i th the sulcus calcanei, forms the sinus tarsi; • Head anteriorly that articulates w i th the navicular; and • Posterior process, which is sometimes separate as the os trigonum.

The inferior surface of the talus (see also the subtalar joint) has a large facet posteriorly for articulation w i th the calcaneus. Separated from this by the sinus tarsi are three facets, separated by ridges, as follows: • The middle talocalcaneal facet for the sustentaculum tali of the calcaneus; • A facet for the plantar ligament; and • The anterior talocalcaneal articular surface.

Fig. 8.3 Bones of the foot: dorsal view.

These, in turn, are continuous w i th the talonavicular facet on the head of the talus. The talus has no muscular attachments.

The calcaneus

This is the largest tarsal bone. It lies under the talus w i th its long axis pointing forward and laterally. It is irregularly cuboidal in shape w i th a shelf-like process anteromedially to support the talus - known as the sustentaculum tali. Its upper surface has three facets for the talus (the middle one is on the superior surface of the sustentaculum tali), which correspond w i th the facets under the talus.

The plantar surface has a large calcaneal tuberosity posteriorly which has medial and lateral tubercles. There is a small peroneal tubercle on the lateral surface.

Fig. 8.4 Oblique radiograph of the foot

1. Fibula 2. Tibia 3. Talus 4. Neck of talus 5. Head of talus

6. Sinus tarsi 7. Posterior process of talus 8. Calcaneum 9. Sustentaculum tali 10. Navicular 11. Cuboid 12. Medial cuneiform (superimposed) 13. Intermediate cuneiform (superimposed) 14. Lateral cuneiform (superimposed) 15. Styloid process of fifth metatarsal 16. Shaft of first metatarsal 17. Head of first metatarsal 18. Proximal phalanx, first toe 19. Distal phalanx, fifth toe 20. Base of fourth metatarsal

The arches of the foot The longitudinal arch is more pronounced medially. It has two components: • The medial arch, formed by the calcaneus, the talus, the medial three cuneiforms and the first three metatarsal bones; and • The lateral arch, formed by the calcaneus, the cuboid and the fourth and fifth metatarsals.

A series of transverse arches are formed and are most marked at the distal part of the tarsal bones and the proximal end of the metatarsals. Each foot has one half of the f u ll transverse arch.

The arches are maintained by the shape of the bones of the foot, the ligaments and muscles, particularly of the plantar surface.

Radiological features of the bones of the foot

(Fig- 8.5)

Plain radiographs Boehler's critical angle of the calcaneus (see Fig. 8.5a) is the angle between a line drawn from the posterior end to the anterior end of its superior articular facet and a second line from the latter point to the posterosuperior border of the calcaneus. It is normally 30-35°, w i th an angle less than 28° occurring when there is significant structural damage to the bone.

Heel-pad thickness (see Fig. 8.5a) is measured on a lateral radiograph of the calcaneus between the calcaneal tuberosity posteroinferiorly and the skin surface. Normal thicknesses are 21 mm in the female and 23 mm in the male.

On a lateral radiograph of the foot in children over 5 years old the long axis of the talus points along the shaft of the first metatarsal. In the younger child the talus is more vertical and its long axis points below the first metatarsal (see Fig. 8.5b, c).

The peroneus brevis tendon is attached to the styloid process of the base of the f i f th metatarsal. The oblique epiphyseal line here should not be confused w i th a fracture of the styloid process, which is usually transverse (see Fig. 8.5d, e).

On a lateral radiograph, medial longitudinal arch angle is measured between a line along the inferior border of the os calcis and a line along the inferior border of the first metatarsal. This angle normally measures 115-125°. An angle greater than 125° is a marker of pes cavus and posterior column neurological insult. An angle less than 115° is termed pes planus, and in the acquired form is usually a marker of disruption of the aponeurosis or the tibialis posterior tendon.

Fig. 8.5 Radiological features of the bones of the foot: (a) Boehler's calcaneal angle and heel pad thickness; (b and c) orientation of the talus in a child; (d and e) styloid process of the fifth metatarsal: epiphyseal line and fracture; (f and g) sesamoid bones of the foot.

Sesamoid bones

The commonest sesamoid bones seen in foot radiographs (see Fig. 8.5f, g) are: • Two sesamoids are found in the tendon of flexor hallucis brevis at the base of the metatarsophalangeal joint of the hallux. These may be bipartite. The sum of the two parts in a bipartite patella is larger than the size of the associated unipartite sesamoid, in contrast to a fracture where the sum of the two parts equals the size of the associated intact sesamoid; • Sesamoid bones are often found at other metatarsophalangeal joints or at the interphalangeal joints of the first and second toes; • Os trigonum posterior to the talus; • Os vesalianum at the base of the fifth metatarsal; • Os peroneum between the cuboid and the base of the fifth metatarsal within the tendon of the peroneus brevis muscle; and • Os tibiale externum medial to the tuberosity of the navicular within the tendon of the tibialis posterior muscle. Ossification of the bones of the foot

Except for the calcaneus, the tarsal bones ossify from one centre each - the calcaneum and talus ossify in the sixth fetal month and the cuboid is ossified at birth; the cuneiforms and navicular ossify between 1 and 3 years of age.

The secondary centre of the calcaneus ossifies in the posterior aspect of the bone at 5 years and its density may be very irregular in the normal foot. It fuses at puberty.

The navicular may ossify from many ossification centres. This should not be confused w i th fragmentation of osteochondrosis or w i th fracture. Similarly, the epiphysis at the base of the proximal phalanx of the hallux may be bipartite in the normal foot. Coalition of the bones of the foot Tarsal coalition represents abnormal fusion between two or more tarsal bones and is a frequent cause of foot and ankle pain. Coalition results from abnormal differentiation and segmentation of primitive mesenchyme preventing the development of a normal joint. Approximately 90% of tarsal coalitions involve the talocalcaneal or calcaneonavicular joints w i th either fibrous, cartilaginous or osseous bridging. Pain