Pectoral girdle, shoulder region and axilla

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CHAPTER 46 Pectoral girdle, shoulder region and axilla

SKIN AND SOFT TISSUE

SKIN

Cutaneous vascular supply

The area over the lateral end of the clavicle is supplied by the supraclavicular artery which pierces the deep fascia superior to the clavicle and anterior to trapezius (Fig. 45.4). In the majority of cases this artery arises from the superficial cervical/transverse cervical artery, but it occasionally arises from the suprascapular artery. The area over the deltoid is supplied by the anterior and posterior circumflex humeral arteries. The deltoid branch of the thoraco-acromial axis contributes to the blood supply of the anterior aspect of the shoulder via musculocutaneous perforators through deltoid. For details of the cutaneous supply to the anterior, lateral and posterior chest wall see page 915.

Cutaneous innervation

The segmental supply is described in Chapter 45, page 789. The cutaneous supply to the shoulder region comes from the supraclavicular nerves (p. 436, Fig. 45.15). The floor of the axilla together with part of the upper medial aspect of the arm is supplied by the intercostobrachial nerve (lateral branch of the second intercostal nerve). Occasionally the lateral branch of the third intercostal nerve contributes to the supply of skin in the floor of the axilla. The upper lateral cutaneous nerve of the arm supplies the skin over the inferolateral part of the shoulder.

SOFT TISSUE

Deep fascia

BONE

CLAVICLE

The clavicle lies almost horizontally at the root of the neck and is subcutaneous throughout its whole extent (Fig. 46.1). It acts as a prop which braces back the shoulder and enables the limb to swing clear of the trunk and transmits part of the weight of the limb to the axial skeleton. The lateral or acromial end of the bone is flattened and articulates with the medial side of the acromion, whereas the medial or sternal end is enlarged and articulates with the clavicular notch of the manubrium sterni and first costal cartilage. The shaft is gently curved and in shape resembles the italic letter f, being convex forwards in its medial two-thirds and concave forwards in its lateral third. The inferior aspect of the intermediate third is grooved in its long axis. The clavicle is trabecular internally, with a shell of much thicker compact bone in its shaft. Although elongated, the clavicle is unlike typical long bones in that it usually has no medullary cavity.

The female clavicle is shorter, thinner, less curved and smoother, and its acromial end is carried lower than the sternal in comparison with the male. In males the acromial end is on a level with, or slightly higher than, the sternal end when the arm is pendent. Midshaft circumference is the most reliable single indicator of sex: a combination of this measurement with weight and length yields better results. The clavicle is thicker and more curved in manual workers, and its ridges for muscular attachment are better marked.

Lateral third

The lateral third of the clavicle is flattened and has a superior and an inferior surface, limited by an anterior and a posterior border. The anterior border is concave, thin and roughened and may be marked by a small deltoid tubercle.

The posterior border, also roughened by muscular attachments, is convex backwards. The superior surface is roughened near its margins but is smooth centrally, where it can be felt through the skin. The inferior surface presents two obvious markings. Close to the posterior border, at the junction of the lateral fourth with the rest of the bone, there is a prominent conoid tubercle which gives attachment to the conoid part of the coracoclavicular ligament. A narrow, roughened strip, the trapezoid line, runs forwards and laterally from the lateral side of this tubercle, almost as far as the acromial end (Fig. 46.1B). The trapezoid part of the coracoclavicular ligament is attached to it. A small oval articular facet, for articulation with the medial aspect of the acromion, faces laterally and slightly downwards at the lateral end of the shaft.

Subclavius lies in a groove on the inferior surface (Fig. 46.1B). The clavipectoral fascia is attached to the edges of the groove; the posterior edge of the groove runs to the conoid tubercle, where fascia and conoid ligament merge. Lateral to the groove there is a laterally inclined nutrient foramen. Deltoid (anterior) and trapezius (posterior) are attached to the lateral third of the shaft: both muscles reach the superior surface. The coracoclavicular ligament, which is attached to the conoid tubercle and trapezoid line (Fig. 46.1B), transmits the weight of the upper limb to the clavicle, counteracted by trapezius which supports its lateral part. From the conoid tubercle this weight is transmitted through the medial two-thirds of the shaft to the axial skeleton.

The clavicle is often fractured, commonly by indirect forces, as a result of a violent impact to the hand or shoulder. The break is usually at the junction of the lateral and intermediate thirds, where the curvature changes, for this is the weakest part of the bone. A fracture medial to the conoid tubercle interrupts weight transmission from the arm to the axial skeleton. The resulting deformity is caused by the weight of the arm, which acts on the lateral fragment through the coracoclavicular ligament and draws it downwards. The medial fragment, as a rule, is a little displaced.

Medial two-thirds

The medial two-thirds of the shaft of the clavicle is cylindrical or prismoid in form and possesses four surfaces, but the inferior surface is often reduced to a mere ridge. The anterior surface is roughened over most of its extent but it is smooth and rounded at its lateral end, where it forms the upper boundary of the infraclavicular fossa. The upper surface is roughened in its medial part and smooth at its lateral end. The posterior surface is smooth and featureless. The inferior surface is marked, near the sternal end, by a roughened oval impression, which is often depressed below the surface. Its margins give attachment to the costoclavicular ligament, which connects the clavicle to the upper surface of the first rib and its cartilage. Rarely, this area is smooth or raised to form an eminence which may articulate with the upper surface of the first rib by means of a synovial joint. There is a groove in the long axis of the bone in the lateral half of the posterior surface.

The medial two-thirds provide attachment, anteriorly, for the clavicular head of pectoralis major: the area is usually clearly indicated on the bone. The clavicular head of sternocleidomastoid is attached to the medial half of the superior surface, but the marking on the bone is not very conspicuous. The smooth, posterior surface is devoid of muscular attachments except at its lower part immediately adjoining the sternal end, where the lateral fibres of sternohyoid are attached. Medially, this surface is related to the lower end of the internal jugular vein (from which it is separated by sternohyoid), the termination of the subclavian vein, and the start of the brachiocephalic vein. More laterally, it arches in front of the trunks of the brachial plexus and the third part of the subclavian artery. The suprascapular vessels are related to the upper part of this surface. The inferior surface gives insertion to subclavius in the subclavian groove: the clavipectoral fascia, which encloses subclavius, is attached to the edges of the groove. The posterior lip of the groove runs into the conoid tubercle and carries the fascia into continuity with the conoid ligament. A nutrient foramen is found in the lateral end of the groove, running in a lateral direction: the nutrient artery is derived from the suprascapular artery. The impression for the costoclavicular ligament is very variable in its character.

Ossification

The clavicle begins to ossify before any other bone in the body, and is ossified from three centres. The shaft of the bone is ossified in condensed mesenchyme from two primary centres, medial and lateral, which appear between the fifth and sixth weeks of intrauterine life, and fuse about the 45th day. Cartilage then develops at both ends of the clavicle. The medial cartilaginous mass contributes more to growth in length than does the lateral mass: the two centres of ossification meet between the middle and lateral thirds of the clavicle. A secondary centre for the sternal end appears in late teens, or even early twenties, usually 2 years earlier in females (Fig. 46.2). Fusion is probably rapid but reliable data are lacking. An acromial secondary centre sometimes develops at around 18 to 20 years, but this epiphysis is always small and rudimentary and rapidly joins the shaft.

The clavicle does not ossify exclusively by intramembranous ossification. In 14 mm embryos the clavicle is a band of condensed mesenchyme between the acromion and apex of the first rib, which is continuous with the sternal rudiment. Medial and lateral zones of early cartilage transformation (‘precartilage’) occur within this band, and intramembranous centres of ossification appear, and soon fuse, in the mesenchyme between them. Sternal and acromial zones soon become true cartilage into which ossification extends from the shaft. Length increases by interstitial growth of these terminal cartilages; the latter develop zones of hypertrophy, calcification and advancing endochondral ossification like other growth cartilages. Diameter increases by subperichondral deposition in the extremities and subperiosteal deposition in the shaft. Epiphyses are endochondral and probably fuse in the same way as they do in long bones. Defects of ossification in the clavicle and those cranial bones which ossify by intramembranous ossification occasionally coincide, e.g. in cleidocranial dysostosis.

SCAPULA

The scapula is a large, flat, triangular bone which lies on the posterolateral aspect of the chest wall, covering parts of the second to seventh ribs (Fig. 46.3, Fig. 46.4). It has costal and dorsal surfaces, superior, lateral and medial borders, inferior, superior and lateral angles, and three processes, the spine, its continuation the acromion and the coracoid process. The lateral angle is truncated and bears the glenoid cavity for articulation with the head of the humerus. This part of the bone may be regarded as the head, and it is connected to the plate-like body by an inconspicuous neck. The long axis of the scapula is nearly vertical and the relatively featureless costal surface can easily be distinguished from the dorsal surface, which is interrupted by the shelf-like projection of the spine (Fig. 46.3A). The bone is very much thickened in the immediate neighbourhood of the lateral border, which runs from the inferior angle below, to the glenoid cavity above. The main processes, and thicker parts of the scapula, contain trabecular bone; the rest consists of a thin layer of compact bone. The central supraspinous fossa and the greater part of the infraspinous fossa are thin and even translucent; occasionally the bone in them is deficient, and the gaps are filled by fibrous tissue.

The inferior angle lies over the seventh rib, or over the seventh intercostal space. It can be felt through the skin and the muscles which cover it and, when the arm is raised above the head, it can be seen to pass forwards round the chest wall. The superior angle is placed at the junction of the superior and medial borders, and is obscured by the muscles which cover it. The lateral angle is truncated and broadened. It constitutes the head of the bone. On its free surface it bears the glenoid cavity for articulation with the head of the humerus in the shoulder joint. Very gently hollowed out, the glenoid forms a poor socket for the humeral head. It is narrow above and wider below, and is pear-shaped in outline. Immediately above the glenoid cavity a small, roughened area encroaches on the root of the coracoid process and is termed the supraglenoid tubercle. The neck of the scapula is the constriction immediately adjoining the head. It can be identified most easily on its inferior and dorsal aspects. Ventrally, it can be regarded as extending between the infraglenoid tubercle and the anterior margin of the suprascapular notch.

Superior border

The superior border, thin and sharp, is the shortest. At its anterolateral end it is separated from the root of the coracoid process by the suprascapular notch (Fig. 46.3B). Near the suprascapular notch it gives origin to the inferior belly of omohyoid. The notch is bridged by the superior transverse ligament which is attached laterally to the root of the coracoid process and medially to the limit of the notch. The ligament is sometimes ossified. The foramen, thus completed, transmits the suprascapular nerve to the supraspinous fossa, whereas the suprascapular vessels pass backwards above the ligament.

Lateral border

The lateral border of the scapula forms a clearly defined, sharp, roughened ridge, which runs sinuously from the inferior angle to the glenoid cavity. At its upper end it widens into a rough, somewhat triangular area, which is termed the infraglenoid tubercle (Fig. 46.4B). The lateral border separates the attachments of subscapularis and teres minor and major. These muscles project beyond the bone and, with latissimus dorsi, cover it so completely that it cannot be felt through the skin. The long head of triceps is attached to the infraglenoid tubercle.

The grooved part of the costal surface, the narrow flat lateral strip of the dorsal surface and the adjacent thickened ridge (Fig. 46.4B), are often included in the ‘lateral border’ during clinical examination.

Scapular angles

The inferior angle overlies the seventh rib or intercostal space. Palpable through the skin and covering muscles, it is also visible as it advances round the thoracic wall when the arm is raised. It is covered on its dorsal aspect by the upper border of latissimus dorsi, a small slip from which is frequently attached to the inferior angle. The superior angle, at the junction of the superior and medial borders, is obscured by the upper part of trapezius. The lateral angle, truncated and broad, bears the glenoid cavity which articulates with the head of the humerus at the glenohumeral joint. It provides a shallow, and limited, socket for the humeral head. Its outline is piriform, narrower above (Fig. 46.4B). The glenohumeral ligaments are attached to its anterior margin. When the arm is by the side, the cavity is directed forwards, laterally and slightly upwards. When the arm is raised above the head it is directed almost straight upwards. Just above it a small rough supraglenoid tubercle encroaches on the root of the coracoid process. The anatomical neck, the constriction adjoining the rim of the glenoid cavity, is most distinct at its inferior and dorsal aspects. Anteriorly and posteriorly it extends between the infraglenoid and supraglenoid tubercles, passing lateral to the root of the coracoid process. The long head of biceps brachii is attached to the supraglenoid tubercle, and the long head of triceps brachii is attached to the infraglenoid tubercle.

Spine of the scapula

The spine of the scapula forms a shelf-like projection on the upper part of the dorsal surface of the bone, and is triangular in shape (Fig. 46.3A). Its lateral border is free, thick and rounded and helps to bound the spinoglenoid notch, which lies between it and the dorsal surface of the neck of the bone. Its anterior border joins the dorsal surface of the scapula along a line which runs laterally and slightly upwards from the junction of the upper and middle thirds of the medial border. The plate-like body of the bone is bent along this line, which accounts for the concavity of the upper part of the costal surface. The dorsal border is the crest of the spine, and is subcutaneous throughout nearly its whole extent. At its medial end the crest expands into a smooth, triangular area. Elsewhere the upper and lower edges and the surface of the crest are roughened for muscular attachments. The upper surface of the spine widens as it is traced laterally, and is slightly hollowed out. Together with the upper area of the dorsal surface of the bone, the upper surface of the spine forms the supraspinous fossa. The lower surface is overhung by the crest at its medial, narrow end, but is gently convex in its wider, lateral portion. Together with the lower area of the dorsal surface of the bone, the lower surface of the spine forms the infraspinous fossa, which communicates with the supraspinous fossa through the spinoglenoid notch.

Supra- and infraspinatus are attached to the upper and lower surfaces of the spine of the scapula, respectively. The flattened triangular area at its root lies opposite the spine of the third thoracic vertebra and is covered by the tendon of trapezius; a bursa intervenes to enable the tendon to play over this part of the bone. The posterior fibres of deltoid are attached to the lower border of the crest. The middle fibres of trapezius are attached to the upper border of the crest, and the lowest fibres of trapezius terminate in a flat triangular tendon which glides over the smooth area at the base of the spine and inserts into a rough prominence, erroneously called the deltoid tubercle, on the dorsal or subcutaneous aspect of the spine near its medial end.

Acromion

The acromion projects forwards, almost at right angles, from the lateral end of the spine, with which it is continuous. The lower border of the crest of the spine becomes continuous with the lateral border of the acromion at the acromial angle, which forms a subcutaneous, bony landmark. The medial border of the acromion is short and is marked anteriorly by a small, oval facet, directed upwards and medially, for articulation with the lateral end of the clavicle. The lateral border, tip and upper surface of the acromion can all be felt through the skin without difficulty. There may be an accessory articular facet on the inferior surface of the acromion.

The acromion is subcutaneous over its dorsal surface, being covered only by the skin and superficial fascia. The lateral border, which is thick and irregular, and the tip of the process, as far round as the clavicular facet, give origin to the middle fibres of deltoid. The medial aspect of the tip gives attachment, below deltoid, to the lateral end of the coraco-acromial ligament. The articular capsule of the acromioclavicular joint is attached around the margins of the clavicular facet. Behind the facet, the medial border of the acromion gives insertion to the horizontal fibres of trapezius. The inferior aspect of the acromion is relatively smooth, and together with the coraco-acromial ligament and the coracoid process forms a protective arch over the shoulder joint. The tendon of supraspinatus passes below the overhanging acromion and is separated from it and from deltoid by the subacromial bursa.

Coracoid process

The coracoid process arises from the upper border of the head of the scapula and is bent sharply so as to project forwards and slightly laterally (Fig. 46.3B, Fig. 46.4A). When the arm is by the side, the coracoid process points almost straight forwards and its slightly enlarged tip can be felt through the skin, although it is covered by the anterior fibres of deltoid. The supraglenoid tubercle marks the root of the coracoid process where it adjoins the upper part of the glenoid cavity. There is another impression on the dorsal aspect of the coracoid process at the point where it changes direction: it gives attachment to the conoid part of the coracoclavicular ligament.

The coracoid process lies about 2.5 cm below the clavicle at the junction of the lateral fourth with the rest of the bone and is connected to its under surface by the coracoclavicular ligament. It is covered by the anterior fibres of deltoid and can be identified only on deep pressure through the lateral border of the infraclavicular fossa. The attachment of the conoid part of the ligament has already been considered: the trapezoid part is attached to the upper aspect of the horizontal part of the process. Pectoralis minor is attached to the superior aspect of the coracoid process. The wider, medial end of the coraco-acromial ligament and, below that, the coracohumeral ligament, are attached to the lateral border. Coracobrachialis is attached to the medial side of the tip of the process, and the short head of biceps is attached to the lateral side of the tip. The inferior aspect of the process is smooth and helps to complete the coraco-acromial arch.

Ligaments

The main scapular ligaments are the coracoacromial and superior transverse scapular; there may also be a weaker, variable inferior transverse scapular (spinoglenoid) ligament (see below Fig. 46.13, see below Fig. 46.14).

Ossification

The cartilaginous scapula is ossified from eight or more centres: one in the body, two each in the coracoid process and the acromion, one each in the medial border, inferior angle and lower part of the rim of the glenoid cavity (Fig. 46.5). The centre for the body appears in the eighth intrauterine week. Ossification begins in the middle of the coracoid process in the first year or in a small proportion of individuals before birth and the process joins the rest of the bone about the 15th year. At or soon after puberty centres of ossification occur in the rest of the coracoid process (subcoracoid centre), in the rim of the lower part of the glenoid cavity, frequently at the tip of the coracoid process, in the acromion, in the inferior angle and contiguous part of the medial border and in the medial border. A variable area of the upper part of the glenoid cavity, usually the upper third, is ossified from the subcoracoid centre; it unites with the rest of the bone in the 14th year in the female and the 17th year in the male. A horse-shoe shaped epiphysis appears for the rim of the lower part of the glenoid cavity; thicker at its peripheral than at its central margin, it converts the flat glenoid cavity of the child into the gently concave fossa of the adult. The base of the acromion is formed by an extension from the spine; the rest of the acromion is ossified from two centres which unite and then join the extension from the spine. The various epiphyses of the scapula have all joined the bone by about the 20th year.

HUMERUS

The humerus, the longest and largest bone in the upper limb, has expanded ends and a shaft (Fig. 46.6, Fig. 46.7, Fig. 46.8). The rounded head occupies the proximal and medial part of the upper end of the bone and forms an enarthrodial articulation with the glenoid cavity of the scapula. The lesser tubercle projects from the front of the shaft, close to the head, and is limited on its lateral side by a well-marked groove. The distal end, loosely termed ‘condylar’, is adapted to the forearm bones at the elbow joint.

The capsular ligament of the elbow joint is attached anteriorly to the upper limits of the radial and coronoid fossae, so that both these bony depressions are intracapsular and therefore lined with synovial membrane. Medially it is attached to the medial non-articular aspect of the projecting lip of the trochlea and to the root of the medial epicondyle. Posteriorly it ascends to, or almost to, the upper margin of the olecranon fossa, which is therefore intracapsular and covered with synovial membrane. Laterally it skirts the lateral borders of the trochlea and capitulum, lying medial to the lateral epicondyle.

With the arm by the side, the medial epicondyle lies on a plane which is posterior to that of the lateral epicondyle, so that the humerus appears to be rotated medially. In this position the head of the humerus is directed almost equally backwards and medially, and the posterior surface of the shaft faces posterolaterally. Since the glenoid fossa of the scapula faces anterolaterally, the humerus is not rotated medially relative to the scapula in this position of rest, but it is so rotated relative to the conventional anatomical position. This position of the bone must be remembered when movements of the arm and forearm are considered.

Proximal end

The proximal end of the humerus consists of the head, anatomical neck and the greater and lesser tubercles. It joins the shaft at an ill-defined ‘surgical neck’, which is closely related on its medial side to the axillary nerve and posterior humeral circumflex artery (Fig. 46.7).

Head

The head of the humerus forms rather less than half a spheroid; in sectional profile it is spheroidal (strictly ovoidal) (Fig. 46.7). Its smooth articular surface is covered with hyaline cartilage, which is thicker centrally. When the arm is at rest by the side, it is directed medially, backwards and upwards to articulate with the glenoid cavity of the scapula. The humeral articular surface is much more extensive than the glenoid cavity, and only a portion of it is in contact with the cavity in any one position of the arm.

Shaft

The shaft of the humerus is almost cylindrical in its proximal half but is triangular on section in its distal half, which is compressed in an anteroposterior direction. It can be identified when the arm is grasped firmly, but its outline is obscured by the strong muscles which surround it. It has three surfaces and three borders – which are not everywhere equally obvious.

Surfaces

The anterolateral surface is bounded by the anterior and lateral borders and is smooth and featureless in its upper part, which is covered by deltoid. About, or a little above, the middle of this surface, deltoid is inserted into the deltoid tubercle; further distally the surface gives origin to the lateral fibres of brachialis, which extend upwards into the floor of the lower end of the groove for the radial nerve (Fig. 46.6). Brachioradialis is attached to the proximal two-thirds of the roughened anterior aspect of the lateral supracondylar ridge, and extensor carpi radialis longus is attached to its distal third. Behind these muscles, the ridge gives attachment to the lateral intermuscular septum of the arm.

The anteromedial surface is bounded by the anterior and medial borders. Rather less than its upper third forms the rough floor of the intertubercular sulcus; the rest of the surface is smooth. Distal to the intertubercular sulcus a small area of the anteromedial surface is devoid of muscular attachment, but its lower half is occupied by the medial part of brachialis (Fig. 46.6A). Coracobrachialis is attached to a roughened strip on the middle of the medial border. The humeral head of pronator teres is attached to a narrow area close to the lowest part of the medial supracondylar ridge, and the ridge itself gives attachment to the medial intermuscular septum of the arm.

A little below its midpoint, the nutrient foramen, which is directed downwards, opens close to the medial border. A hook-shaped process of bone, the supracondylar process, from 2 to 20 mm in length, occasionally projects from the anteromedial surface of the shaft, approximately 5 cm proximal to the medial epicondyle. It is curved downwards and forwards, and its pointed apex is connected to the medial border, just above the epicondyle, by a fibrous band to which part of pronator teres is attached. The foramen completed by this fibrous band usually transmits the median nerve and brachial artery, but sometimes encloses only the nerve, or the nerve plus the ulnar artery (in cases of high division of the brachial artery). A groove which lodges the artery and nerve usually exists behind the process, and may protect the nerve and artery from compression by muscles.

The posterior surface, between the medial and lateral borders, is the most extensive surface and is occupied mostly by the medial head of triceps. A ridge, sometimes rough, descends obliquely and laterally across its proximal third, and gives attachment to the lateral head of triceps. Above triceps, the axillary nerve and the posterior circumflex humeral vessels wind round this aspect of the bone under cover of deltoid. Below and medial to the attachment of the lateral head of triceps, a shallow groove which contains the radial nerve and the profunda brachii vessels, runs downwards and laterally to gain the anterolateral surface of the shaft. The area for the origin of the fleshy medial head of triceps includes a very large part of the posterior surface of the bone. It covers an elongated triangular area, the apex of which is placed on the medial part of the posterior surface above the level of the lower limit of the insertion of teres major. The area widens below and covers the whole surface almost down to the lower end of the bone.

Distal end

The distal end of the humerus is a modified condyle: it is wider transversely and has articular and non-articular parts (Fig. 46.8, Fig. 46.9). The articular part is curved forwards, so that its anterior and posterior surfaces lie in front of the corresponding surfaces of the shaft. It articulates with the radius and the ulna at the elbow joint, and is divided by a faint groove into a lateral capitulum, and a medial trochlea.

The capitulum is a rounded, convex projection, considerably less than half a sphere, which covers the anterior and inferior surfaces of the lateral part of the condyle of the humerus but does not extend onto its posterior surface. It articulates with the discoid head of the radius, which lies in contact with its inferior surface in full extension of the elbow but slides onto its anterior surface during flexion. The groove of the trochlea winds backwards and laterally as it is traced from the anterior to the posterior surface of the bone, and it is wider, deeper and more symmetrical posteriorly. Anteriorly, the medial flange of the pulley is much longer than the lateral, and the surface adjoining its projecting medial margin is convex to accommodate itself to the medial part of the upper surface of the coronoid process of the ulna. These asymmetries entail varying angulation between the humeral and ulnar axes, together with some conjunct rotation. The non-articular part of the condyle includes the medial and lateral epicondyles, olecranon and coronoid and radial fossae.

Ossification

The humerus is ossified from eight centres, in the shaft, head, greater and lesser tubercles, capitulum with the lateral part of the trochlea, the medial part of the trochlea, and one for each epicondyle (Fig. 46.10). The centre for the shaft appears near its middle in the eighth week of intrauterine life, and gradually extends towards the ends. Before birth (20%), or in the first six months afterwards, ossification begins in the head, during the first year in females and second year in males in the greater tubercle, and about the fifth in the lesser tubercle. The existence of a centre in the lesser tubercle is often questioned, perhaps because it is often obscured in the usual anteroposterior radiological views (Fig. 46.11).

By the sixth year the centres for the head and tubercles have joined to form a single large epiphysis, which is hollowed out on its inferior surface to adapt it to the somewhat conical upper end of the metaphysis. It fuses with the shaft of the humerus about the twentieth year in males, two years earlier in females. The lower end is ossified as follows. During the first year ossification begins in the capitulum and extends medially to form the chief part of the articular surface; the centre for the medial part of the trochlea appears in the ninth year in females and tenth year in males.

Ossification begins in the medial epicondyle in the fourth year in females, sixth in males, and in the lateral epicondyle about the 12th year. The centres for the lateral epicondyle, capitulum and trochlea fuse around puberty and the composite epiphysis unites with the shaft in the fourteenth year in females, sixteenth in males. The centre for the medial epicondyle forms a separate epiphysis, which is entirely extracapsular and is placed on the posteromedial aspect of the epicondyle. It is separated from the rest of the lower epiphysis by a downgrowth from the shaft, with which it unites about the 20th year.

JOINTS

STERNOCLAVICULAR JOINT

The sternoclavicular joint is a synovial sellar joint and represents the only skeletal articulation between the upper limb and the axial skeleton.

Articulating surfaces

The articulating surfaces are the sternal end of the clavicle and the clavicular notch of the sternum, together with the adjacent superior surface of the first costal cartilage (Fig. 46.12). The larger clavicular articular surface is covered by fibrocartilage, which is thicker than the fibrocartilaginous lamina on the sternum. The joint is convex vertically but slightly concave anteroposteriorly, and is therefore sellar; the clavicular notch of the sternum is reciprocally curved, but the two surfaces are not fully congruent. An articular disc completely divides the joint.

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