Shoulder girdle and arm

Fascia over deltoid

The deep fascia over deltoid is thickest anteriorly and posteriorly, and is adherent in a linear fashion to the external aspect of the superficial fibrous septa of the central (lateral) part of that muscle. It blends with the pectoral fascia in front, and with the thick and strong fascia overlying infraspinatus behind. Above, it blends with the periosteum of the lateral part of the clavicle, the thin periosteum of the acromion, the crest of the scapular spine and the cranial part of the medial border of the scapula, caudal to the spine of the scapula, where it blends with the infraspinatus fascia. Below, it is continuous with the deep brachial fascia, blending with the lateral intermuscular septum ( ).

Pectoral and axillary fascia

The pectoral fascia is thin over pectoralis major. It is attached medially to the sternum and is continuous with the fascia of the rectus sheath caudally. Cranially, it blends with the periosteum of the clavicle and the anterior aspect of the capsule of the sternoclavicular joint. Laterally, it is continuous with the fascia over deltoid; it forms the roof of the infraclavicular fossa between the muscular attachments of pectoralis major and deltoid to the clavicle. The fascia is loosely adherent to the septum between the sternal and clavicular parts of pectoralis major. Inferolaterally, between pectoralis major and latissimus dorsi, the fascia thickens to form the floor of the axilla as the axillary fascia. At the caudal edge of pectoralis major, a deep lamina of fascia ascends to envelop the caudal border of pectoralis minor; it becomes the clavipectoral fascia at the upper edge of pectoralis minor. The hollow of the armpit is produced mainly by the action of this fascia in tethering the axillary skin to the floor of the axilla; it is sometimes referred to as the suspensory ligament of the axilla. The axillary fascia is pierced by the tail of the breast (see Fig. 53.22A ). The pectoral fascia envelops the lateral margin of latissimus dorsi; the deep and superficial layers ensheathe that muscle and are attached behind to the spines of the thoracic and lumbar vertebrae, blending with the thoracolumbar fascia medially and caudally.

Clavipectoral fascia

The clavipectoral fascia is the superior continuation of the deep lamina of the pectoral fascia and the medial continuation of the parietal layer of the subscapular bursal fascia. Laterally, it is continuous with the coraco-acromial ligament above and lateral to the coracoid. It envelops the coracoid blending with the periosteum of the tip of that process, the short head of biceps brachii and coracobrachialis. It covers the interval between those two muscles and pectoralis minor, which it envelops, and then traverses the interval between pectoralis minor and subclavius. It splits around subclavius and is attached to the clavicle anterior and posterior to the groove for subclavius. The posterior layer is contiguous with the deep cervical fascia, a condensation of which forms a tether around the central tendinous part of omohyoid, so indirectly connecting it to the clavicle. The deep aspect of the posterior lamina of the clavipectoral fascia blends with the sheath of the axillary vessels. Medially, it blends with the fascia over the first two intercostal spaces and is attached to the first rib, medial to subclavius. Occasionally, the fascia thickens to form a band between the first rib and coracoid process, the costocoracoid ligament, under which the lateral cord of the brachial plexus is closely applied ( ). The cephalic vein, thoraco-acromial artery and associated veins and lymphatic vessels, and the lateral pectoral nerve all pass through the fascia immediately superior to the upper border of pectoralis minor. This fascia was called the 'clavi-coraco-axillary aponeurosis' by .

Subscapular fascia

The subscapular fascia is thin and attached to the entire circumference of the subscapular fossa. Subscapularis is partly attached to its deep surface medially, which is an example of the extension of the attachment zone of a muscle for more effective action. The fascia extends laterally and blends with the deep layer of the subscapular bursa in front of the tendon of subscapularis and with the external layer of the capsule of the rotator interval of the glenohumeral joint. Inferiorly, it continues as the fascia enveloping teres major.

Infraspinous fascia

The infraspinous fascia covers infraspinatus and is attached to the margins of the infraspinous fossa, except cranially and medially, where there is a loose attachment to the lower aspect of the spine of the scapula. Cranially and laterally, the infraspinatus fascia is continuous with the supraspinatus fascia, creating a single muscular compartment for the two muscles. The deep fibres of deltoid are attached to the infraspinatus fascia over a strip of about 1 cm for the entire length of the spine of the scapula; the examining finger cannot palpate the spine of the scapula from within the space between deltoid and infraspinatus. The fascia is contiguous with the strong lower border of the deltoid fascia along the overlapping posterior border of deltoid.

Supraspinous fascia

The supraspinous fascia completes the osseofibrous compartment containing supraspinatus; it is attached to the scapula around the boundaries of the attachment of supraspinatus. It is thick medially, where supraspinatus attaches to its own fascia, but thinner laterally under the coraco-acromial ligament.

Brachial fascia

The deep fascia of the upper arm, the brachial fascia, is continuous with the fasciae covering deltoid and pectoralis major; it forms a thin, loose covering for the anterior muscles of the arm and a more robust covering for the posterior muscles. Medially, just below the middle of the upper arm, it is perforated by the basilic vein, lymphatic vessels and, at various levels, by branches of the brachial cutaneous nerves.

The fascia is thickest distally, where it contains the brachial muscles in distinct compartments anteriorly and posteriorly, and is defined medially and laterally by tough septa. The lateral intermuscular septum is continuous with the fascia overlying the lateral part of deltoid proximally, and has an upward, thinner extension to the lateral crest of the intertubercular sulcus (groove) contiguous with the fascia over the anterior border of deltoid. It is attached to the supracondylar ridge of the lateral epicondyle of the humerus, and is perforated at the level of the junction of the upper three-fifths and lower two-fifths of the humerus by the radial nerve and the radial collateral branch of the profunda brachii artery passing into the anterior compartment from behind. It provides extension for the zone of attachment of the lateral head of triceps posteriorly, and of brachialis, brachioradialis and extensor carpi radialis longus anteriorly. The medial intermuscular septum extends from the medial lip of the intertubercular sulcus, where it is contiguous with the fascia of teres major, next blends with the aponeurosis of attachment of coracobrachialis to the medial aspect of the humerus, and then passes as a thick septum along the medial aspect of the humerus to the medial supracondylar ridge of the medial epicondyle. It gives attachment to the medial head of triceps posteriorly, and brachialis anteriorly. It is perforated by the ulnar nerve at about the same level as the radial nerve laterally, together with the superior ulnar collateral artery and the posterior branch of the inferior ulnar collateral artery. At the elbow, the brachial fascia is attached to the epicondyles of the humerus, so completing the muscular compartments, and the olecranon of the ulna, and is continuous with the antebrachial fascia.

Axillary neurovascular sheath

The axillary neurovascular sheath is closely contiguous with the posterior aspect of the clavipectoral fascia. The second part of the axillary artery lies behind pectoralis minor; most intimal ruptures of the vessel caused by distraction trauma occur in this part of the vessel.

During forceful flexion/adduction/protraction of the upper limb across the trunk (e.g. when the scapula is wrenched from the lateral end of the clavicle in thoracoscapular dissociation), the axillary sheath containing the axial vessels and cords of the brachial plexus is forced into pectoralis minor, which acts as a fulcrum or guillotine, around which the vessels angle sharply, so rupturing the arterial intima. Acute neural injuries occur as a result of direct injury (traction, compression, or both) to the nerve trunks, causing acute sensorimotor deficits, arterial adventitial haematoma or acute aneurysm expanding within the axillary sheath to cause the syndrome of causalgia (rapidly progressive, severe pain in the distribution of the affected nerve trunks with neural deficits). Arterial thrombosis following intimal rupture is associated with late nerve deficits as a result of inadequate neural perfusion, local scarring with distortion of the axillary sheath and its contents, and poor posture of the affected limb.

The neurovascular bundle enclosed within the sheath is separated from the subscapular fascia by a ‘virtual space’ traversed by the subscapular nerves, an arrangement that permits full excursion of subscapularis without distortion of the neurovascular bundle during normal arm movements. The tip of the coracoid can be likened to the tip of the shaft of a steering oar attached to a vessel (the scapula) tethered within the thick clavipectoral fascia: the effect of pectoralis minor acting as an antagonist of trapezius in scapular position and motion control (see below) is optimized by the wide attachment of the clavipectoral fascia and coraco-acromial ligament to the clavicle and scapula (see Fig. 49.18 ).

Spread of infection

When axillary suppuration occurs, the local fascial arrangement affects the spread of pus. Suppuration may be superficial or deep to the clavipectoral fascia. In the former, an abscess would appear at the edge of the anterior axillary fold or in the groove between deltoid and pectoralis major; in the latter, pus would tend to track upwards in the axillary neurovascular sheath and appear at the root of the neck, taking the direction of least resistance. Lymphangitis in the medial aspect of the arm suggests infection deep to the clavipectoral fascia with lymphatic obstruction. When an axillary abscess is incised, the knife should enter the axillary ‘base’, midway between the anterior and posterior margins and near the thoracic side, to avoid the lateral thoracic, subscapular and axillary vessels on the anterior, posterior and lateral walls, respectively.

Lateral two-fifths

The lateral two-fifths of the clavicle are flattened and have superior and inferior surfaces, and anterior and posterior borders. 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, a prominent conoid tubercle 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. 49.6B ). 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 is attached to the groove on the inferior surface (see Fig. 49.6B ). 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. There is a laterally inclined nutrient foramen lateral to the groove. Deltoid (anteriorly) and trapezius (posteriorly) are attached to the lateral two-fifths of the shaft; both muscles are inserted directly into the clavicle and indirectly into the clavicular periosteum. The coracoclavicular ligaments, attached to the conoid tubercle and trapezoid line (see Fig. 49.6B ), transmit the weight of the upper limb to the clavicle, and are counteracted by the cervical part of trapezius, which supports its lateral part (see Fig. 49.18 ).

Medial three-fifths

The medial three-fifths of the shaft of the clavicle are cylindrical or prismoid in form and have four surfaces, although the inferior surface is often reduced to a mere ridge. The anterior surface is roughened over most of its extent but is smooth and rounded laterally, where it forms the upper boundary of the infraclavicular fossa. The upper surface is roughened medially and smooth laterally. The posterior surface is smooth and featureless medially; its lateral half bears a groove in the long axis of the bone. The inferior surface is marked near its 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 constitute an eminence that may form a synovial pseudarthrosis with the upper surface of the first rib; in older subjects, an extra-articular synostosis may form.

The medial three-fifths provide attachment, anteriorly, for the clavicular head of pectoralis major. The clavicular head of sternocleidomastoid is attached to the medial half of the superior surface, but the marking on the bone is not 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, the clavicle arches in front of the trunks and divisions of the brachial plexus and the third part of the subclavian artery. The thyrocervical trunk and its branches, and the suprascapular and transverse cervical vessels are immediately behind and above the upper aspect of this surface. Subclavius is inserted in the subclavian groove on the inferior surface, and the clavipectoral fascia is attached to the edges of the groove. The posterior lip of the groove is continuous with the conoid tubercle laterally and brings the clavipectoral 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.

Sternal end

The sternal end of the clavicle is directed medially, downwards and forwards, and articulates with the clavicular notch of the manubrium sterni. The sternal surface, usually irregular and pitted, is quadrangular (sometimes triangular). Its uppermost part is slightly roughened for attachment of the interclavicular ligament, sternoclavicular capsule and articular disc. Elsewhere, the surface is smooth and articular; it extends on to the inferior surface for a short distance, where it articulates with the first costal cartilage. The sternal end of the clavicle projects upwards beyond the manubrium sterni and can be felt and seen easily, forming the lateral wall of the jugular notch, behind which are the cricoid cartilage, cricothyroid membrane, the lower part of the thyroid cartilage of the larynx and the brachiocephalic vein. The sternal ends of each clavicle thus form a guide to the jugular notch: if the sternal end of the clavicle is displaced backwards (by traumatic dislocation), the landmarks of the notch are lost, the trachea is displaced and there will be difficulty breathing. If the trachea is displaced to one side (for instance, by pneumothorax), the features of the larynx cannot be palpated readily.

The shaft of the clavicle is usually fractured as a result of a violent impact to the side of the shoulder. The fracture typically occurs at the junction of the lateral two-fifths and medial three-fifths, where the transition from antecurve to retrocurve occurs and the bone changes in cross-sectional shape from its flatter lateral part to a tubular medial part. The weight of the arm causes displacement of the lateral fragment downwards, inwards and into forward (ventral) rotation, a posture maintained even in recumbency partly by the action of subclavius. If the fracture heals in this position, the dimensions and shape of the retroclavicular space are altered, particularly during elevation of the arm above shoulder level, and the brachial plexus (and its perfusion) and the subclavian vessels can be distorted (one of the many causes of the syndrome associated with thoracic outlet obstruction).

Ossification

The clavicle begins to ossify before any other bone in the body. The shaft of the bone undergoes intramembranous ossification in two primary centres, medial and lateral, in stages 18–20 (42–50 postfertilization days). However, the clavicle does not ossify exclusively by intramembranous ossification. Endochondral ossification extends from the shaft to the sternal and acromial ends in stages 20–21. In 14 mm embryos, the clavicle is a band of condensed mesenchyme between the acromion and apex of the first rib, and is continuous with the sternal rudiment. Medial and lateral zones of early cartilage transformation (‘precartilage’) occur within this band; intramembranous centres of ossification appear and soon fuse in the mesenchyme between them. Sternal and acromial zones become true cartilage into which ossification extends from the shaft. Length increases by interstitial growth of these terminal cartilages, which 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. A secondary centre for the sternal end appears in the late teens, or even early twenties, usually 2 years earlier in females ( Fig. 49.7 ). Fusion is probably rapid but reliable data are lacking. An acromial secondary centre sometimes develops at around 18–20 years, but this epiphysis is always small and rudimentary, and rapidly joins the shaft. Growth is usually complete by 23 years in both sexes. Epiphyses are endochondral and probably fuse in the same way as they do in long bones.

Anomalies of clavicular growth and development include defects of ossification, morphology (shape) and length. Defects of ossification in the clavicle and those cranial bones that ossify by intramembranous ossification occasionally coincide, e.g. in cleidocranial dysostosis. Defects of morphology and length occur in the immature clavicle in birth lesions of the brachial plexus; the lateral two-fifths are affected. In a typical case, the acromial end of the clavicle is short and excessively curved ventrally, rotated by up to 90°, while the coracoclavicular ligaments are normal. The coracoid is long and more vertical than usual; the scapula is often small, and the scapular neck region is short with variable glenoid dysplasia. These characteristic defects are predominantly associated with the upper trunk lesion (C5 and C6) that results in a muscular imbalance across the acromioclavicular and glenohumeral joints; the flexor and medial (internal) rotator muscles are relatively short. The short, tight pectoralis minor and coracobrachialis produce a distorting force on the coracoid, which elongates and rotates dorsally. With retropulsion of the humeral head posteriorly, the scapula rotates ventrally, particularly if there is posterior glenoid dysplasia. Deltoid shortening creates a downward distraction on the developing acromion, which lengthens and curves distally. The spine of the scapula is parallel to the body of the scapula rather than dorsally directed, and these deformities cause the acromioclavicular joint to be orientated more horizontally. As a result, the lateral clavicle is excessively curved ventrally. The concept here is that the abnormal shape is driven by asymmetrical muscle paralysis and partial recovery, and that form follows the neural lesion. While poor movement (causing a lack of ‘motivation’ for growth in length ) might explain the clavicular shortening, an equally valid explanation might involve a vascular insufficiency of the lateral ossification centre of the clavicle (with a similar insufficiency of the lateral ossification centres of the scapula for the glenoid): both have a contribution from the suprascapular artery, which is ‘at risk’ during injuries that lead to brachial plexus palsy. Length is a function of perfusion and shortening follows vascular insufficiency ( ).

Costal surface

The costal surface, which is directed medially and forwards when the arm is by the side, is gently concave, especially in its upper part, matching the contour of the chest wall ( Fig. 49.8B ).

Near the lateral border, there is a longitudinal rounded ridge, prominent near the neck but less so below, which is separated from the lateral border by a narrow, grooved area. Subscapularis arises from the subscapular fossa that comprises nearly the whole of the costal surface, including the grooved area immediately adjoining the lateral border, but excluding the area next to the neck of the bone. Small, fibrous intramuscular septa are attached to four or five roughened ridges that subdivide this surface incompletely into a number of smooth areas. The anterior aspect of the neck is separated from subscapularis by a bursal protrusion of the synovial membrane of the shoulder joint (subscapular ‘bursa’). The lower five or six digitations of serratus anterior are attached to an oval area near the inferior angle. The remainder of the muscle is inserted into a narrow strip along the ventral aspect of the medial border, which is wider above, where it receives the large first digitation. The longitudinal thickening of the bone near the lateral border provides a column of the necessary strength to withstand the pull of serratus anterior on the inferior angle during lateral scapular rotation, when the glenoid cavity is turned to face more directly upwards as the arm is raised from the side and carried above the head against gravity.

Dorsal surface

The dorsal surface is divided by the transverse, shelf-like spine of the scapula into two unequal parts: a smaller supraspinous fossa above and an infraspinous fossa below. The fossae are confluent at the spinoglenoid notch between the lateral border of the spine and the dorsal aspect of the neck; their central parts may be very thin ( Fig. 49.8A ). The supraspinous fossa is bounded superiorly and anteriorly by the superior border, from the suprascapular notch and dorsal aspect of the root of the coracoid laterally, to the superior angle medially; and medially, by the upper part of the medial border. The infraspinous fossa is bounded by the medial and lateral borders and the inferior angle.

Supraspinatus is attached to the medial two-thirds of the supraspinous fossa on the dorsal surface; the fascia that covers the muscle is attached to the margins of the fossa. A fat pad lies between the supraspinatus fascia and the under-surface of trapezius. Teres minor is attached to the upper two-thirds of a flattened strip that adjoins the lateral border. The strip is grooved near its upper end by the circumflex scapular vessels passing between teres minor and the bone as they enter the infraspinous fossa. The lower limit of the attachment of teres minor is indicated by an oblique ridge, which runs from the lateral border to the neighbourhood of the inferior angle and cuts off a somewhat oval area where teres major is attached. The dorsal aspect of the inferior angle may give origin to a small slip that joins the deep surface of latissimus dorsi. The infraspinous fossa is hollowed out laterally but is convex medially. Infraspinatus is attached to the infraspinous fossa, with the exception of an area near the neck of the bone. The strong infraspinatus fascia passes on to teres minor and teres major, and sends fascial partitions between them that reach the bone along the ridges marking the limits of their attachments.

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 (see Fig. 49.8B ), which can vary in shape and size. Medial to the suprascapular notch, the superior border gives origin to the inferior belly of omohyoid. The notch is bridged by the superior transverse ligament (or suprascapular 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 has a triangular or rhomboidal cross-section. It forms a clearly defined, sharp, roughened ridge that runs sinuously from the inferior angle to the glenoid cavity. At its upper end, it widens into a rough, somewhat triangular, area: the infraglenoid tubercle ( Fig. 49.9B ). The lateral border separates the attachments of subscapularis and teres minor and major. These muscles project beyond the bone and, with latissimus dorsi below, 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 (see Fig. 49.9B ) are often included in the ‘lateral column’ in surgical practice. By definition, the remainder of the scapular body (costal and dorsal surfaces) and the medial border are called the ‘medial column’.

Medial border

The medial border extends from the inferior to the superior angle. It is slightly thickened and flat, and is further thickened at the medial end of the spine. In its lower two-thirds, this border can easily be felt through the skin, but its upper third is more deeply placed and more difficult to palpate. It is thin and often angled opposite the root of the spine. Levator scapulae is attached to a narrow strip, extending from the superior angle to the root of the spine, and rhomboid minor is attached below this, opposite the root of the spine. Rhomboid major is attached to the remainder of the border.

Scapular angles

The inferior angle lies over the seventh rib, or over the seventh intercostal space. It can be felt through the skin and the muscles that cover it, and, when the arm is raised above the head, it can be seen to rotate forwards (protract) around the chest wall; when the arm is lowered, the scapula should restitute (retract) to its original position without a disturbance of rhythm. 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. It lies over the dorsal surface of the second rib and can be palpated deeply above and behind the clavicle.

The lateral angle, truncated and broad, comprises the glenoid fossa (cavity) and scapular neck, with the coracoid projecting forwards. When the arm is by the side, the fossa is directed forwards, laterally and slightly upwards. When the arm is raised above the head, it is directed almost straight upwards, i.e. the plane of the fossa is horizontal. The supraglenoid tubercle is a small, rough, sloping area at the cranial margin of the fossa; it often encroaches on the root of the coracoid process, and is the site of attachment for the tendon of the long head of biceps brachii. The infraglenoid tubercle is a larger, rough area at the caudal margin of the glenoid fossa, at the upper end of the lateral margin, and is the site of attachment for the tendon of the long head of the triceps brachii. The anatomical neck is the region of the scapula extending between the infraglenoid and supraglenoid tubercles anteriorly and posteriorly, lateral to the root of the coracoid process; anteriorly, there is no landmark to distinguish the anatomical neck from the costal surface of the body of the scapula. Posteriorly, the anatomical neck is limited by the spinoglenoid notch at the root of the spine of the scapula. The bone here has an asymmetric, triangular cross-sectional shape; the plane of the anterior surface of the neck makes an acute angle of about 60° with the plane of the glenoid fossa, but the dimension and shape of the posterior surface are more variable.

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 (see Fig. 49.8A ). Its lateral border is free, thick and rounded, and bounds the spinoglenoid notch, which lies between it and the dorsal surface of the neck of the bone. Its anterior aspect joins the dorsal surface of the scapula along a line that 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. The crest expands into a smooth, triangular area at its medial end. 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.

Supraspinatus is attached to the upper surface of the spine of the scapula; the infraspinatus, by contrast, is not attached to the lower surface of the spine. 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 and, to a variable extent, to the medial border by attachment to the infraspinatus fascia. The middle fibres of trapezius are attached to the upper border of the crest. The lowest fibres of trapezius terminate in a flat, triangular tendon that 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 reliable, 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 (middle) fibres of trapezius. The inferior aspect of the acromion is relatively smooth and forms a protective arch over the shoulder joint, together with the coraco-acromial ligament and the coracoid process. 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 neck of the scapula and its body is bent sharply so as to project forwards and slightly laterally (see Figs 49.8 – 49.9 ). When the arm is by the side, the coracoid process points almost straight forwards. Its tip can be felt through the skin and is covered by the anterior fibres of deltoid, about 2.5 cm below the clavicle at the junction of the lateral fifth with the rest of the bone, at the lateral border of the infraclavicular fossa. It forms a short, curved cantilever fixed to the scapula at its base and is displaced by the actions of the muscles attached to its body and tip. Pectoralis minor pulls the coracoid (and, therefore, the scapula) forwards and medially around the chest wall, balanced by trapezius, its antagonist; this action forms the foundation on which coracobrachialis can help support the long lever arm of the humerus during humeral motion, together with its antagonist, the deltoid. During forceful forward elevation of the protracted shoulder girdle, subclavius and pectoralis minor may contribute to deceleration of the limb.

The infraclavicular brachial plexus and the axillary vessels lie below and medial to the coracoid. The acromial branch of the thoraco-acromial artery passes above it, and the sensory branch of the lateral pectoral nerve to the rotator interval capsule, together with accompanying vessels, lies immediately below.

On the dorsal aspect of the coracoid process, at the point where it changes direction, a rough impression forms part of the region of attachment of the conoid portion of the coracoclavicular ligament. The trapezoid portion of the coracoclavicular ligament is attached to the upper aspect of the horizontal part of the process, anterior to the conoid part. Pectoralis minor is attached to the superior and medial aspects of the coracoid process. The wider, medial, end of the coraco-acromial ligament is attached to the lateral border and is continuous inferiorly with the lateral aponeurotic part of the tendon of the short head of biceps brachii. The coracohumeral ligament is attached to the root of the coracoid at its lateral border. The interval between the anterior aspect of the scapular neck immediately medial to the glenoid fossa and the deep surface of the coracoid is often bridged by the glenocoracoid ligament. The inferior aspect of the coracoid process is otherwise smooth and saddle-shaped. When the arm is elevated, the upper border of subscapularis is apposed to, and runs under, this surface, which forms a pulley for the muscle, increasing the power generated during forceful medial (internal) rotation of the shoulder in elevation, such as during a serve in tennis. Coracobrachialis is attached to the deep aspect of the lower part of the medial side and the deep surface of the tip of the process, and the short head of biceps is attached to the lateral side and superficial aspect of the tip.

Scapular movements

Scapular movement is a product of interconnected suspension, motion and articulation mechanisms. The suspension mechanism consists of the scapular articulation with the clavicle and the suspension muscles of the scapula and clavicle, notably trapezius, attached to the lateral clavicle, the acromion and the spine of scapula, and its antagonists pectoralis minor and subclavius. The articulation with the clavicle comprises two linked systems: ‘clavicle – acromioclavicular joint – acromion – spine of scapula – lateral scapular angle’ and ‘clavicle – coracoclavicular ligament – coracoid – lateral scapular angle – spine of scapula’. This mechanism therefore subserves the function of scapular rotation or tilt limited by the ellipsoid range of motion of the lateral clavicle. The motion mechanism consists of the agonist–antagonist force couple of serratus anterior passing to the chest wall anterolaterally, and levator scapulae, rhomboid major, rhomboid minor and serratus posterior (when present), which all pass to the vertebral column medially. This mechanism therefore subserves the function of scapular protraction and retraction around the elliptic paraboloid of the chest wall. The articulation mechanism consists of the scapular neck region and the glenoid fossa, which articulates with the humeral head. The muscles that subserve the function of holding the humeral head on the glenoid through the wide range of motion of the glenohumeral joint are the rotator cuff muscles, which take their attachment from the body of the scapula. This mechanism therefore concerns the relationship between the position and orientation of the glenoid fossa and the scapular body.

Ossification

The cartilaginous scapula is ossified from eight or more centres: one in the body, two each in the coracoid process and the acromion, and one each in the medial border, inferior angle and lower part of the rim of the glenoid cavity ( Fig. 49.10 ). The medial part originates from dermomyotomes of somites 17–24 (level of fifth cervical vertebra – level of fifth thoracic vertebra) and the lateral part from somatopleuric mesenchyme. The primary ossification centre for the body appears in stage 23 (53–58 postfertilization days). Ossification begins in the middle of the coracoid process in the first year or, in a small proportion of individuals, before birth; the process joins the rest of the bone about the fifteenth 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 fourteenth year in the female and the seventeenth year in the male. A horseshoe-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 that unite and then join the extension from the spine. The various epiphyses of the scapula have all joined the bone by about the twentieth year.

The primordial scapula migrates caudally to its usual position at a level between the second and seventh ribs by the end of the embryonic period (53–58 postfertilization days). With further development, the scapula alters shape. Initially, the horizontal diameter exceeds the vertical but the ratio gradually decreases until mature dimensions are reached, an adaptation thought to provide upright hominids with increased range, and therefore freedom of use, of the upper limb. Avian gene-deletion studies have shed light on the development of the scapula. Scapular body development appears to be controlled by the Emx2 gene, also expressed in the developing mesonephros. The development of the acromion and the spine of the scapula are controlled by the Pax1 gene, and glenoid and coracoid development is controlled by the Hoxc6 gene ( ), one of the homeobox genes responsible for somite differentiation and development; development of the glenoid and coracoid appears likely to be more intimately related to the development of the upper limb bud as a whole. The development of the scapular body appears to be independent of that of the processes and glenoid region. Adult scapulae may show a line of fusion across the body and spine of the scapula, suggesting that the primordia of the processes (including the glenoid) and the primordium of the body migrate towards each other; since the trapezius is derived from branchial arch mesoderm it would appear that the development of the spine of the scapula and acromion and trapezius are intimately related, but separate from the body. This hypothesis is supported by the observation that; in the condition of congenital undescended scapular syndrome (Sprengel deformity), the development of the scapular body is altered, and there is intramuscular ossification of variable parts of the medial scapulospinal muscles; the development of the processes (including the glenoid) is unaffected ( ; ). Since the medial border of the scapula develops from the dermomyotomes of somites 17–24 (see above) any vertebral anomalies at this level may result in tethering of scapular structures to the cervical spine, sometimes by an abnormal omovertebral tether (fibrous, chondral, or osseous), that prevents the upper border of the scapula descending to its usual position.

Ligaments

The intrinsic scapular ligaments are strengthening ligaments (coraco-acromial ligament) and bounding ligaments (superior transverse scapular or suprascapular ligament, inferior transverse scapular or spinoglenoid ligament, and the variable glenocoracoid ligament).

Head

The head of the humerus forms rather less than half a spheroid; in sectional profile, it is spheroidal (strictly, ovoidal) (see Fig. 49.12 ). Its smooth articular surface is covered with hyaline cartilage that 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, which means that only a portion of it is in contact with the cavity in any one position of the arm.

Anatomical neck

The anatomical neck of the humerus immediately adjoins the margin of the head, forming a slight constriction that is least obvious next to the greater tubercle. A slight roughness indicates the line of capsular attachment of the shoulder joint, other than at the intertubercular sulcus, where the long tendon of biceps brachii emerges. Medially, the capsular attachment diverges from the anatomical neck and descends 1 cm or more on to the shaft.

Lesser tubercle

The lesser tubercle is anterior to and just distal to the anatomical neck. It is palpable through the thickness of deltoid about 3 cm below the anterior edge of the acromion. It can be distinguished from the coracoid by rotation of the humerus under the palpating finger; the lesser tubercle slips away from the examining finger, the tip of the coracoid does not. The lateral edge of the lesser tubercle is sharp and forms the medial border of the intertubercular sulcus. Subscapularis is attached to the lesser tubercle (see Fig. 49.11A ). The transverse ligament of the shoulder (transverse humeral ligament) is attached to the upper aspect of the lateral margin of the tubercle.

Greater tubercle

The greater tubercle is the most lateral part of the proximal end of the humerus and projects beyond the lateral border of the acromion. Its posterosuperior aspect, near the anatomical neck, bears three smooth, flattened impressions for the attachment of supraspinatus (uppermost), infraspinatus (middle) and teres minor (lowest and placed on the posterior surface of the tubercle) (see Fig. 49.11 ). The attachments of subscapularis and teres minor are not confined to their respective tubercles, but extend for varying distances on to the adjacent metaphysis. The projecting lateral surface of the tubercle presents numerous vascular foramina and is covered by deltoid, producing the normal rounded contour of the shoulder. A part of the subacromial bursa may cover the upper part of this area and separate it from deltoid. The intertubercular sulcus (bicipital groove) lies between the tubercles. It contains the long tendon of biceps, its synovial sheath, and an ascending branch from the anterior circumflex humeral artery. The rough lateral lip of the groove is marked by the bilaminar tendon of pectoralis major, and its medial lip by the tendon and muscular insertion of teres major. The floor of the groove provides attachment for a frequent upward extension of the tendon of pectoralis major, and for the tendon of latissimus dorsi more caudally ( ).

Surfaces

The posterior surface, between the medial and lateral borders, is broad, flat and convex distally. The medial head of triceps is attached to the posterior surface over an elongated triangular area, the apex of which is placed on the medial part of the bone above the level of the lower limit of insertion of teres major. The area widens below and covers the whole surface almost down to the lower end of the bone. The lateral head of triceps is attached to a ridge, sometimes rough, that descends obliquely and laterally above the attachment for the medial head. Above triceps, the axillary nerve and the posterior circumflex humeral vessels wind around the bone on the deep surface of deltoid. Below the attachment of the lateral head of triceps, but above the attachment of the medial head, a shallow groove runs from the upper medial aspect of the surface downwards and laterally towards the lateral border; it contains the radial nerve and its branches, and the profunda brachii vessels.

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 attached to 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 (see Fig. 49.11 ). The nerve perforates the lateral intermuscular septum close to the bone at the level of the junction of the upper three-fifths and lower two-fifths of the humerus, and enters the anterior compartment of the arm close to the anterolateral surface, between brachialis medially and brachioradialis laterally.

The anteromedial surface is bounded by the anterior and medial borders. Below the intertubercular sulcus, the rest of the upper half of the surface is smooth and devoid of muscular attachment; its lower half is occupied by the medial part of brachialis (see Fig. 49.11A ). Coracobrachialis is attached to a long rough strip in the middle third of the medial border, posterior to the brachial neurovascular sheath, which lies in close relation to the humerus in the middle third of this surface. The humeral head of pronator teres is attached to a narrow area close to the lowest part of the medial supracondylar ridge; 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, ranging 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 that lodges the artery and nerve is usually found posterior to the process.

Anterior border

The anterior border starts on the front of the greater tubercle and runs downwards almost to the lower end of the bone. Its proximal third forms the lateral lip of the intertubercular sulcus and is roughened for muscular attachments. The succeeding portion is also roughened and forms the anterior limit of the deltoid tubercle; the lower half of the border is smooth and rounded.

Lateral border

The lateral border is most conspicuous at the lower end of the bone, where it is thickened to form the lateral supracondylar ridge, and its sharp edge is roughened along its anterior aspect. In its middle and upper thirds, the border is barely discernible, but in a well-marked bone it can be traced upwards to the posterior surface of the greater tubercle. A little above its middle, it is marked by a V-shaped, roughened area: the deltoid tubercle. The limbs of the V are broad; the groove for the radial nerve runs downwards and laterally behind the posterior limb and fades away on the lower part of the anterolateral surface. The lateral intermuscular septum is attached to the lateral border, and is a condensation of the fascia over the lower part of deltoid and the neighbouring brachialis, forming a septum between the anterior and posterior muscular compartments. The septum is most obvious in the lower three-fifths of the arm, and is perforated by the radial nerve and accompanying vessels.

Medial border

The medial border, although rounded, can be identified without difficulty in the lower half of the shaft, where it becomes the medial supracondylar ridge. In its proximal third, the medial border is indistinct until it broadens out to form a triangular area. The lateral border of this area forms the medial lip of the intertubercular sulcus, and the medial border runs upwards as the calcar humerale to the anatomical neck, where it is roughened, with vascular apertures, and forms the area of attachment of the inferior part of the shoulder capsule. In its middle third, the medial border is interrupted by a wide, shallow groove, the radial (spiral) groove that crosses the bone obliquely, passing downwards and forwards from its posterior to its anterior surface.

Fractures of the humeral shaft

Humeral shaft fractures are common; the pattern of the fracture and the displacement of the fragments depend on the force of injury and on the level at which the bone is broken. If there is wide displacement at the time of fracture, closely associated nerves and vessels are at risk of direct injury. Nerves that are ‘fixed’ in relation to the bone and attached septa are at further risk of indirect injury by stretching or, later, by the callus of bone healing. (For a discussion of radial nerve injury and fractures of the humeral shaft, see .)

Trapezius

Trapezius is derived from head paraxial mesenchyme with neural crest connective tissue, rather than from the dermomyotomes of lower somites, and is therefore unique among shoulder girdle muscles. It is attached to the spine of the scapula and acromion, segments of the scapula for which development is encoded by a single gene ( ).