The bones

Chapter 7

The upper l i mb

CHAPTER CONTENTS

The bones 247 The joints 254 The muscles 268 The arteries 270 The veins 271

THE BONES OF THE UPPER LIMB

The scapula (Figs 7.1 and 7.2) This flat triangular bone has three processes: • The glenoid process, which is separated from the remainder by the neck of the scapula. The glenoid cavity forms part of the shoulder joint; • The spine, which arises from the posterior surface of the scapula and separates the supraspinous and infraspinous fossae. The spine extends laterally over the shoulder joint as the acromium (see Fig. 7.2); and • The coracoid process, which projects anteriorly from the upper border of the neck of the scapula.

Radiological features of the scapula

Plain radiographs The inferior angle of the scapula lies over the seventh rib or interspace - this is a useful guideline in identifying ribs or thoracic vertebral levels.

The scapula lies over the ribs and obscures some of the lung fields in PA chest radiographs unless the shoulders are rotated forwards. In AP views it is not usually possible to rotate the scapulae off the lung fields. Similarly, in AP views of the scapula the beam is centred over the head of the humerus to project the thoracic cage away from the scapula.

In lateral chest radiographs, the lateral border of the scapula may be confused w i th an oblique fissure. The inferior angle of the scapula may be slightly bulbous and simulate a mass on this view.

Isotope bone scan The inferior angle of the scapula overlying the seventh rib may appear as a 'hot spot'.

Fig. 7.1 The scapula: (a) anterior view; (b) posterior view; (c) lateral view.

Ossification The scapula ossifies in the eighth week of fetal life. An ossification centre appears in the middle of the coracoid process in the first year of life and fuses at 15 years of age. Secondary centres appear in the root of the coracoid process, the medial border and the inferior angle of the scapula between 14 and 20 years, and fuse between 22 and 25 years of age.

The Clavicle (Fig. 7.3; see Fig. 7.2) The clavicle lies almost horizontally between the sternoclavicular and the acromioclavicular joints. It is also attached to the first costal cartilage by the costoclavicular ligament, which arises from the rhomboid fossa on its inferomedial surface. It is connected to the coracoid process by the coracoclavicular ligament at the conoid tubercle and the trapezoid line on its inferolateral surface. The subclavian vessels and the trunks of the brachial plexus pass behind its medial third.

Radiological features of the clavicle

Chest radiograph The clavicle overlies the apices of the lungs in chest radiographs. Apical or lordotic views are used to project the clavicles above the lungs to evaluate this area further. In portable AP chest radiography, if the patient is inclined backwards from a true vertical position the horizontal beam projects the clavicles above the lungs.

Fig. 7.2 Axial radiograph of the shoulder.

1. Medullary cavity of humeral shaft 2. Cortex 3. Head of humerus 4. Lesser tuberosity 5. Tip of acromion process 6. Lateral end of clavicle 7. Acromioclavicular joint 8. Clavicle 9. Glenoid fossa of scapula 10. Coracoid process of scapula 11. Acromion process of scapula

Fig. 7.3 The clavicle: (a) superior view; (b) inferior view.

On a chest radiograph, the distance between the medial end of the clavicle and the spine of the vertebrae is equal on both sides unless the patient is rotated.

The rhomboid fossa is seen in 0.6% of normal chest X-rays and 33% are bilateral.

Ossification

The clavicle begins to ossify before any other bone in the body. It ossifies in membrane from two centres that appear at the fifth and sixth fetal weeks, and fuses in the seventh week. A secondary centre appears at the sternal end at 15 years in females and 17 years in males, and fuses at 25 years of age.

The humerus (Figs 7.4 and 7.5; see Fig. 7.2) The hemispherical head of the humerus is separated from the greater and lesser tubercles by the anatomical neck. Between the tubercles is the bicipital groove for the long head of the biceps. The shaft just below the tubercles is narrow and is called the surgical neck of the humerus.

The shaft is marked by a spiral groove where the radial nerve and the profunda vessels run. The deltoid tuberosity on the lateral aspect of the midshaft is the site of insertion of the deltoid muscle.

The lower end of the humerus is expanded and has medial and lateral epicondyles. The articular surface for the elbow joint has a capitellum for articulation w i th the radial head and a trochlea for the olecranon fossa of the ulna. Above the trochlea are fossae, the coronoid anteriorly and the deeper olecranon fossa posteriorly.

Radiological features of the humerus

Plain radiographs The lower epiphysis of the humerus lies at a 25° angle to the shaft so that a vertical line down the front of the shaft on a lateral radiograph - the anterior humeral line - bisects the capitellum.

An olecranon foramen may replace the olecranon fossa.

A hook-shaped projection of bone - termed the supracondylar process - occasionally occurs about 5 cm above the medial epicondyle. It varies in length between 2 and 20 mm

and may be continuous w i th a fibrous band, the ligament of Struthers, attached above the epicondyle to form a foramen that transmits the median nerve and the brachial artery.

Avulsion of the medial epicondyle The flexor muscles of the forearm arise from the medial epicondyle of the humerus. Repeated contractions or a single violent contraction of these muscles in a child can result in avulsion of the apophysis (a secondary ossification centre occurring outside a joint) of the medial epicondyle.

Ossification

The primary centre for the humerus appears at the eighth week of fetal life. Secondary centres appear in the head of the humerus at 1 year, the greater tuberosity at 3 years, and the lesser tuberosity at 5 years of age. These fuse w i th one another at 6 years and w i th the shaft at 20 years of age. Secondary centres appear in the capitellum at 1 year, the radial head at 5 years, the internal epicondyle at 5 years, trochlea at 10 years, olecranon at 10 years and external epicondyle at 10 years (CRITOE). These fuse at 17-18 years of age.

The radius and ulna (Figs 7.6 and 7.7; see Fig. 7.5) The radius has a cylindrical head that is separated from the radial tubercle and the remainder of the shaft by the neck. Its lower end is expanded and its most distal part is the radial styloid. The radius is connected by the interosseus membrane to the ulna.

The upper part of the ulna - the olecranon - is hookshaped, w i th the concavity of the hook - the trochlear fossa - anteriorly. A fossa found laterally at the base of the olecranon is for articulation w i th the radial head. The shaft of the ulna is narrow. The styloid process at the distal end is narrower and more proximal than that of the radius.

Fig. 7.5 AP radiograph of the elbow.

1. Shaft of humerus 2. Olecranon fossa 3. Medial epicondyle 4. Lateral epicondyle 5. Olecranon process 6. Capitulum 7. Trochlea

8. Head of radius 9. Neck of radius 10. Coronoid process of ulna 11. Radial tuberosity 12. Shaft of radius 13. Shaft of ulna Fig. 7.6 The radius and ulna: anterior and posterior views

radiograph (see Fig. 7.5). Angulation of the head or a double cortical line are signs of fracture of the radial head.

The triceps muscle is inserted into the tip of the olecranon. Fracture of the olecranon is therefore associated w i th proximal displacement by the action of this muscle.

The ulnar styloid is proximal to the radial styloid, w i th a line joining them on an AP radiograph lying at an angle of 110° w i th the long axis of the radius (see Fig. 7.7). In a lateral radiograph, the articulating surface of the distal radius is angled 10° to a line through the shaft of the radius. Recognition of these normal angles is important in reduction of fractures of the wrist.

The pronator quadratus is a square, flat muscle that arises on the distal ulna and passes to the distal radius. A thin fat pad overlying this muscle is visible as a linear lucency on a lateral radiograph of the wrist. Thickening of the muscle, such as by haematoma in fracture of the underlying bone, can be detected on a radiograph by bowing of the pronator quadratus fat pad.

Ossification of the radius

The primary ossification centre of the radius appears in the eighth week of fetal life. Secondary centres appear distally in the first year and proximally at 5 years of age. These fuse at 20 years and 17 years, respectively.

Radiological features of the radius and ulna

Plain radiographs The head of the radius has a single cortical line on its upper surface and is perpendicular to the neck in the normal Ossification of the ulna

The shaft of the ulna ossifies in the eighth week of fetal life. Secondary centres appear in the distal ulna at 5 years and in the olecranon at 10 years of age. These fuse at 20 and 17 years, respectively.

The carpal bones (Fig. 7.8; see Fig. 7.7) The carpal bones are arranged in two rows of four each. In the proximal row, from lateral to medial, are the scaphoid, lunate and triquetral bones, w i th the pisiform on the anterior surface of the triquetral. The trapezium, trapezoid, capitate and hamate make up the distal row.

Together the carpal bones form an arch, w i th its concavity situated anteriorly. The flexor retinaculum is attached laterally to the scaphoid and the ridge of the trapezium, and medially to the pisiform and the hook of the hamate. It converts the arch of bones into a tunnel, the carpal tunnel, which conveys the superficial and deep flexor tendons of the fingers and the thumb (except flexor carpi ulnaris and palmaris longus tendons) and the median nerve. The extensor retinaculum on the dorsum of the wrist attaches to the pisiform and triquetrum medially and the radius laterally Six separate synovial sheaths run beneath i t.

Radiological features of the carpal bones

Radiography These are radiographed in the anteroposterior, lateral and oblique positions (see Fig. 7.7). Carpal tunnel views are

Fig. 7.7 AP radiograph of the wrist and hand.

1. Distal radius 2. Styloid process of radius 3. Distal ulna 4. Styloid process of ulna 5. Distal radioulnar joint 6. Radiocarpal joint 7. Scaphoid 8. Lunate 9. Triquetral 10. Pisiform 11. Hamate 12. Hook of hamate 13. Capitate 14. Trapezoid 15. Trapezium 16. First metacarpophalangeal joint 17. Base of fourth metacarpal 18. Shaft of fourth metacarpal 19. Head of fourth metacarpal 20. Fourth metacarpophalangeal joint 2 1. Shaft of proximal phalanx, ring finger 22. Proximal interphalangeal joint, little finger 23. Middle phalanx, middle finger 24. Distal interphalangeal joint, index finger 25. Distal phalanx, thumb 26. Sesamoid bone 27. Soft tissues overlying distal phalanx of middle finger

Fig. 7.8 Bones of the hand.

obtained by extending the wrist and taking an inferosuperior view that is centred over the anterior part of the wrist.

Supernumerary bones These may be found in the wrist and include the os centrale found between the scaphoid, trapezoid and capitate, which may represent the tubercle of the scaphoid that has not fused w i th its upper pole, and the os radiale externum, which is found on the lateral side of the scaphoid distal to the radial styloid.

Nutrient arteries of the scaphoid In 13% of subjects these enter the scaphoid exclusively in its distal half. If such a bone fractures across its midportion, the blood supply to the proximal portion is cut off and ischaemic necrosis is inevitable. This occurs in 50% of patients w i th displaced scaphoid fractures.

Ossification of the carpal bones These ossify from a single centre each. The capitate ossifies first and the pisiform last, but the order and timing of the ossification of the other bones is variable. Excluding the pisiform, they ossify in a clockwise direction from capitate to trapezoid as follows: the capitate at 4 months; the hamate at 4 months; the triquetral at 3 years; the lunate bone at 5 years; and the scaphoid, trapezium and trapezoid at 6 years. The pisiform ossifies at 11 years of age.

The metacarpals and phalanges

The five metacarpals are numbered from the lateral to the medial side. Each has a base proximally that articulates w i th that of the other metacarpals, except in the case of the first metacarpal, which is as a result more mobile and less likely to fracture. The third metacarpal has a styloid process extending from its base on the dorsal aspect. Each metacarpal has a rounded head distally, which articulates w i th the proximal phalanx.

The phalanges are 14 in number, three for each finger and two for the thumb. Like the metacarpals, each has a head, a shaft and a base. The distal part of the distal phalanx is expanded as the tuft of the distal phalanx.

Radiological features of the metacarpals and phalanges

Bone age A radiograph of the left hand is used in the determination of bone age. Standards of age determined by epiphyseal appearance and fusion have been compiled for the left hand and wrist by Greulich and Pyle, and by Tanner and Whitehouse (TW2 method).

The metacarpal sign A line tangential to the heads of the fourth and fifth metacarpals does not cross the head of the third metacarpal in 90% of normal hands - this is called the metacarpal sign. This line does, however, cross the third metacarpal head in gonadal dysgenesis.

The carpal angle This is formed by lines tangential to the proximal ends of the scaphoid and lunate bones. In normal hands the average angle is 138°. It is reduced to an average 108° in gonadal dysgenesis.

The metacarpal index This is calculated by measuring the lengths of the second, third, fourth and fifth metacarpals and dividing by their breadths taken at their exact midpoint. The sum of these divided by four is the metacarpal index, which has a normal range of 5.4-7.9. An index greater than 8.4 suggests the diagnosis of arachnodactyly.

Sesamoid bones

Two sesamoid bones are found related to the anterior surface of the metacarpophalangeal joint of the thumb in the normal radiograph. A single sesamoid bone in relation to this joint in the little finger is seen in 83% of radiographs,