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.

Fig. 46.12 Sternoclavicular joints: anterior aspect; left joint intact and right in coronal section.

Fibrous capsule

The capsule is thickened in front and behind, but above, and especially below, it is little more than loose areolar tissue.

Ligaments

The ligaments are the anterior and posterior sternoclavicular and the costoclavicular on each side, and the midline interclavicular.

Articular disc

The articular disc is flat and almost circular, between the sternal and clavicular surfaces. It is attached above to the posterosuperior border of the articular surface of the clavicle, below to the first costal cartilage near its sternal junction, and by the rest of its circumference to the capsule. It is thicker peripherally, especially in its superoposterior part and a smaller inferomedial one. The capsule around the former is more lax: movements between the clavicle and the disc are more extensive than those between the disc and sternum. The sellar shape of the articular surfaces permits movement in approximately anteroposterior and vertical planes, and some rotation about the long axis of the clavicle. Close-packing probably coincides with maximum posterior rotation associated with full scapular rotation. Some anteroposterior translation also occurs.

Vascular supply

The sternoclavicular joint is supplied by branches from the internal thoracic and suprascapular arteries.

Innervation

The sternoclavicular joint is innervated by branches from the medial supraclavicular nerve and the nerve to subclavius.

Factors maintaining stability

There is relatively little bony articular congruence at the sternoclavicular joint. However, the strength of its associated ligaments, and especially its articular disc, actually make it very stable. These factors, and the usual transmission of forces along the clavicle, make dislocation rare: fracture along the clavicular shaft is far more common.

Articulating surfaces

The articulating surfaces are between the acromial end of the clavicle and the medial acromial margin (Fig. 46.13). The joint is approximately plane, but either surface may be slightly convex, the other being reciprocally concave, and both are covered by fibrocartilage. The clavicular surface is a narrow, oval area which faces inferolaterally and overlaps a corresponding facet on the medial acromial border. The long axis is anteroposterior.

Fibrous capsule

The capsule completely surrounds the articular margins and is strengthened above by the acromioclavicular ligament. The capsule is lined by synovial membrane.

Ligaments

The acromio- and coraco-clavicular ligaments run between the clavicle and the acromion and coracoid processes respectively.

Articular disc

The articular disc often occurs in the upper part of the joint, partially separating the articular surfaces: occasionally it completely divides the joint.

Vascular supply

The acromioclavicular joint receives its arterial supply from branches from the suprascapular and thoracoacromial arteries.

Innervation

The acromioclavicular joint is innervated by branches from the suprascapular and lateral pectoral nerves.

Factors maintaining stability

The coracoclavicular ligament stabilizes the acromioclavicular joint. In acromioclavicular dislocation, the ligament is torn and the scapula falls away from the clavicle, which may be slightly elevated by the unopposed pull of trapezius. Dislocation can occur because of the flatness and orientation of the joint surfaces; once the acromioclavicular joint dislocates it never reduces.

Movements

Movements at the joint are like those of the sternoclavicular joint. These are passive, i.e. no muscle directly moves the joint, but muscles which move the scapula indirectly move the clavicle. Axial rotation of the clavicle is about 30°, the two joints together therefore, permit about 60° of scapular rotation. Angulation with the scapula occurs in any direction.

Articulating surfaces

The articular surfaces are reciprocally curved and are really ovoids. (Here, as in the hip, where ovoid surfaces are almost spherical they are often termed spheroidal.) The area of the humeral convexity exceeds that of the glenoid concavity such that only a small portion opposes the glenoid in any position (Fig. 46.16). The remaining capitular articular surface is in contact with the capsule, so that contact on the glenoid fossa is much more uniformly distributed over its entire articular surface. The radius of curvature of the glenoid fossa in the coronal plane is greater than that of the humeral head, and is deepened by a fibrocartilaginous rim, the glenoid labrum (Fig. 46.13, Fig. 46.17). Both articular surfaces are covered by hyaline cartilage, which is thickest centrally and thinner peripherally over the humerus, and the reverse in the glenoid cavity. In most positions, their curvatures are not fully congruent, and the joint is loose-packed. Close packing (full congruence) is reached with the humerus abducted and laterally rotated (Fig. 5.61).

Fig. 46.16 Coronal sections through the left shoulder joint viewed from the posterior aspect. A, Anteriorly placed coronal section to show tendon of biceps, long head. B, Posteriorly placed coronal section to show subacromial bursa and contents of quadrangular space.

(From Sobotta 2006.)

Fig. 46.17 MRI of shoulder. A, Axial image. B, Median sagittal oblique image at base of coracoid. A: 1. Deltoid. 2. Coracobrachialis. 3. Glenoid. 4. Subscapularis. 5. Infraspinatus. 6. Tendon of the long head of biceps. 7. Head of humerus. 8. Posterior labrum. B: 1. Clavicle. 2. Supraspinatus. 3. Coracoid. 4. Subscapularis. 5. Axillary nerve. 6. Latissimus dorsi. 7. Trapezius. 8. Scapular spine. 9. Infraspinatus. 10. Deltoid. 11. Triceps. Compare A with Fig. 46.23.

Glenoid labrum

The glenoid labrum is a fibrocartilaginous rim around the glenoid fossa. It is triangular in section and varies in size and thickness; its base is attached to the margin of the fossa and its free inner edge projects as a continuation of the curve of the glenoid. It blends above with two fasciculi from the long tendon of biceps. The labrum deepens the cavity, may protect the bone, and probably assists lubrication. Its attachment is sometimes partly deficient anterosuperiorly, in which case synovial membrane may protrude through the gaps.

Fibrous capsule

A fibrous capsule envelops the joint (Fig. 46.14, Fig. 46.16). It is attached medially to the glenoid neck outside the glenoid labrum, and encroaches on the coracoid process to include the attachment of the long head of biceps. The capsule often extends and attaches to the base of the coracoid and to the body of the scapula, forming anterior and posterior recesses. Laterally, it is attached to the anatomical neck of the humerus, i.e. near the articular margin, except inferomedially, where it descends more than 1 cm on the humeral shaft. It is so lax that the bones can be distracted for 2 or 3 cm, which accords with the very wide range of movement possible at the glenohumeral joint. However, such unnatural separation requires relaxation of the upper capsule by abduction.

The fibrous capsule is supported by the tendons of supraspinatus (above), infraspinatus and teres minor (behind), subscapularis (in front) and by the long head of triceps (below). The rotator interval is a medially based triangular area of uncovered capsule between the superior edge of subscapularis and the anterior edge of supraspinatus as these tendons pass on either side of the base of the coracoid: it may represent an area of weakness that increases instability in some shoulders. Triceps is separated from the capsule by the axillary nerve and posterior circumflex humeral vessels as they pass back from the axilla (Fig. 46.14). The capsule is least supported inferiorly, and subjected to the greatest strain in full abduction, when it is stretched tightly across the humeral head. It is strengthened anteriorly by extensions from the tendons of pectoralis major and teres major.

There are usually two or three openings in the capsule: below the coracoid process, connecting the joint to a bursa behind the tendon of subscapularis (anterior); between the humeral tubercles, transmitting the long tendon of biceps and its synovial sheath; connecting the joint to a bursa under the tendon of infraspinatus (posterior and inconstant).

Ligaments

The ligaments associated with the glenohumeral joint are the glenohumeral (superior, middle and inferior), coracohumeral and transverse humeral.

Glenohumeral ligaments

Three glenohumeral ligaments, only visible from within the joint, reinforce the capsule anteriorly and inferiorly (Fig. 46.18). They do not act as traditional ligaments (which carry a pure tensile force along their length), but become taut at varying positions of abduction and humeral rotation, acting as ‘check-reins’. Moreover, they do not have the strength characteristics of the ligaments at the knee. The superior glenohumeral ligament passes from the supraglenoid tubercle, just anterior to the origin of the long head of biceps, to the humerus near the proximal tip of the lesser tubercle on the medial ridge of the intertuberculous groove, the fovea capitis. It forms an anterior cover around the long head of biceps, and is part of the rotator interval. Together with the coracohumeral ligament it is an important stabilizer in the inferior direction, helping to keep the humeral head suspended (the coracohumeral ligament is more robust than the superior glenohumeral ligament). The middle glenohumeral ligament arises from a wide attachment below the superior glenohumeral ligament, along the anterior glenoid margin as far as the inferior third of the rim, and passes obliquely inferolaterally, enlarging as it does, to attach to the lesser tubercle deep to the tendon of subscapularis, with which it blends. The width and thickness of this ligament may be as much as 2 cm and 4 mm respectively. It provides anterior stability at 45° and 60° abduction. The thicker and longer inferior glenohumeral ligament complex is a hammock-like structure with anchor points on the anterior and posterior sides of the glenoid. It arises from the anterior, middle and posterior margins of the glenoid labrum, below the epiphysial line, and passes anteroinferiorly to the inferior and medial aspects of the neck of the humerus. The anterior, superior edge of the inferior ligament is thickened as the superior band, and the diffuse thickening of the anterior part of the capsule to which it is attached is known as the axillary pouch. The anterior band of the inferior glenohumeral ligament is thought to be the primary static anterior stabilizer of the abducted and externally rotated glenohumeral joint. (For further details consult Burkart & Debski 2002.)

Fig. 46.18 Sagittal magnetic resonance arthrogram image following distension of the glenohumeral joint by intra-articular contrast injection in a 28-year-old male. Note the clearly outlined glenohumeral ligaments.

Synovial membrane

The synovial membrane lines the capsule and covers parts of the anatomical neck. The long tendon of biceps traverses the joint in a synovial sheath which continues into the intertubercular sulcus as far as the surgical neck of the humerus (Fig. 46.14, Fig. 46.16).

Bursae Many bursae adjoin the shoulder joint.

They are usually found between the tendon of subscapularis and the capsule, communicating with the joint between the superior and middle glenohumeral ligaments; on the superior acromial aspect; between the coracoid process and capsule; between teres major and the long head of triceps: anterior and posterior to the tendon of latissimus dorsi. The subacromial bursa, between deltoid and the capsule, does not communicate with the joint cavity but is prolonged under the acromion and coracoacromial ligament, and between them and supraspinatus: it appears to be attached, together with the subdeltoid fascia, to the acromion. Bursae sometimes occur behind coracobrachialis and between the tendon of infraspinatus and the capsule, occasionally opening into the joint.

Vascular supply

The glenohumeral joint is supplied by branches from the anterior and posterior circumflex humeral, suprascapular and circumflex scapular vessels.

Innervation

The glenohumeral joint is innervated mainly from the posterior cord of the brachial plexus. The capsule is supplied by the suprascapular nerve (posterior and superior parts), axillary nerve (anteroinferior) and the lateral pectoral nerve (anterosuperior).

Factors maintaining stability

The articulation between the relatively large humeral head and the shallow glenoid fossa allow a wide range of movement at the expense of providing an unstable bony complex. The anterior joint capsule is strong but lax. A variety of additional factors help to increase the stability of the joint: the glenoid labrum deepens the concavity of the articulating glenoid, the glenohumeral ligaments act as static stabilizers in certain positions, and there is a negative pressure within the joint. The coracoacromial arch (coracoid, acromion and coracoacromial ligament) prevents upward dislocation of the humerus. The tendons of subscapularis, supraspinatus, infraspinatus and teres minor fuse with the lateral part of the joint capsule to form the ‘rotator cuff’. These short muscles collectively produce a compressive force during active glenohumeral movements which maintains congruent contact between the head of the humerus and the glenoid fossa, helps to resist skid, and checks excessive translation. The rotator cuff also provides strong lateral stability and prevents this part of the lax capsule from being nipped during joint movements. The long head of biceps offers additional superior support. The long head of triceps offers inferior support which is particularly important when the shoulder is abducted. However, the glenohumeral joint is least stabile inferiorly when the shoulder is fully abducted.

Movements at the shoulder (glenohumeral) joint

The shoulder is capable of any combination of swing and spin over a very wide range. Laxity of the capsule, and a humeral head which is large relative to a shallow glenoid fossa, afford a wider range of movement than at any other joint. Flexion–extension, abduction–adduction, circumduction and medial and lateral rotation all occur at the shoulder. Although the majority of the movement of the shoulder occurs at the glenohumeral joint, there is a varying contribution from the scapulothoracic articulation in most directions, most significantly in abduction, and excluding lateral rotation.

In analysis of shoulder movements it is preferable to refer humeral movement to the scapula, rather than to conventional anatomical planes (Fig. 46.19). When the arm hangs at rest the glenoid fossa faces almost equally forwards and laterally, and the humeral capitular and scapular (topographical) axes correspond, although the humerus, relative to the anatomical position, is medially rotated. Flexion carries the arm anteromedially on an axis through the humeral head orthogonal to the glenoid fossa at its centre. Abduction and adduction occur in a vertical plane orthogonal to that of flexion–extension; the axis is horizontal, through the humeral head, parallel with the glenoid plane. Pure abduction raises the arm anterolaterally in the plane of the scapula. However, when referred to the trunk, flexion and extension occur in the paramedian plane, and abduction and adduction in the coronal plane. In this sense, raising the arm vertically from flexion or raising it from abduction are both accompanied by humeral rotation in opposite directions. Whether ‘scapular’ or any other plane of abduction is described, these are selections from an infinite series. In scapular abduction points on the humeral surface pursue vertical cords but in rotation they are horizontal. In ‘pure’ flexion–extension, in a plane orthogonal to the scapula the axis of movement, and the notional ‘mechanical axis’, are regarded as projected from the centre of the glenoid cavity.

Fig. 46.19 Left deltopectoral groove showing clavipectoral fascia and thoracoacromial axis.

(From Sobotta 2006.)

Glenohumeral abduction is approximately 90°, but angles up to 120° have been reported. Some 60° further abduction occurs at the sterno- and acromioclavicular articulations. Contralateral vertebral flexion also aids in bringing the arm to the vertical. During active elevation, movements at the glenohumeral and acromioclavicular joints are simultaneous, except in the initial few degrees, when most, often all, movement is glenohumeral. For every 15° of elevation, glenohumeral movement is said to be 10° and scapular movement 5°. During the initial stages of abduction, subscapularis, infraspinatus and teres minor counteract the strong upward component of pull of deltoid, which would otherwise cause the humeral head to slide up; the additive turning moments exerted by deltoid and supraspinatus about the shoulder joint can then abduct the arm.

In flexion, the humerus swings at right angles to the scapular plane and scapular rotation cannot increase the elevation (120°) obtainable in full flexion. If the fully flexed humerus is also abducted, elevation increases pro rata until, when the humerus reaches the scapular plane, i.e. when true abduction is reached, 180° of elevation becomes possible. In medial or lateral rotation, the humerus revolves about one-quarter of a circle around a vertical axis; the range is greatest when the arm is pendent, and least when it is vertical. When assessing the rotational range at the glenohumeral joint, the forearm should be flexed to a right angle at the elbow: this will prevent the effects of superadded pronation or supination in the pendent limb. In circumduction, which is a succession of the foregoing movements, the distal end of the humerus describes the base of a cone, its apex at the humeral head. This glenohumeral movement can be much increased by scapular movements, e.g. in acts of slinging objects with force.

The peculiar relation of the long head of biceps to the shoulder joint may serve several purposes. By its connection with both the shoulder and elbow, the muscle harmonizes their actions as an elastic ligament during all their movements. It helps to prevent the humeral head impinging on the acromion when deltoid contracts and to steady it in movements of the arm. In paralysis of supraspinatus it may also help initiate abduction of the arm, particularly when the humerus is laterally rotated.

Muscles producing movements

The muscles which produce movements at the glenohumeral joint are principally deltoid, pectoralis major, latissimus dorsi and teres major. These long muscles all converge on the humerus, acting at mechanical advantage on a joint which, as a result of glenoid shallowness and capsular laxity, is relatively unstable. The long muscles are counteracted by the rotator cuff, a group of short muscles (subscapularis, supraspinatus, infraspinatus and teres minor) which are attached nearer to the joint, and which centre the head of the humerus in the glenoid fossa through the midrange of motion, when the capsuloligamentous structures are lax.

Rotator cuff disease

The subacromial space is defined inferiorly by the humeral head, and superiorly by the anterior edge and inferior surface of the anterior third of the acromion, coracoacromial ligament and acromioclavicular joint, forming the coracoacromial arch. It is occupied by the supraspinatus tendon, subacromial bursa, tendon of the long head of biceps brachii, and the capsule of the shoulder joint. Rotator cuff disease is a painful condition with a multifactorial aetiology in which severe or chronic impingement of the rotator cuff tendons on the undersurface of the coracacromial arch is often a significant factor. The cuff normally impinges against the coracoacromial arch when the humerus is abducted, flexed and internally rotated. This is known as the impingement position. The supraspinatus tendon is anatomically affected most by the impingement, which interestingly also coincides with an area of reduced vascularity in this tendon. Severe impingement can be caused by thickening of the coracoacromial arch, by inflammation of the cuff from disorders such as rheumatoid arthritis, or as a result of prolonged overuse in the impingement position, e.g. in cleaning windows. When associated with a tendinopathy from age-related degenerative changes within the tendon, impingement may be associated with partial or complete tears of the cuff. Clinically, this condition causes tenderness over the anterior portion of the acromion, and pain which typically occurs on abducting the shoulder between 60° and 120° (the painful arc).

Glenohumeral joint dislocations

The glenohumeral joint is the most frequently dislocated joint in the body. It is most unstable anteroinferiorly, which explains why the vast majority of dislocations are anterior, and occur when the arm is forced backwards when it is in abduction, external rotation and extension. Clinically, a dislocated shoulder loses its normal contour, and the acromion process, rather than the greater tubercle, becomes the most lateral bony structure. The axillary nerve and artery may be injured during dislocation, and this can lead to inability to abduct the shoulder as a result of paralysis of deltoid together with an area of anaesthesia over the distal part of the muscle (sometimes referred to as the ‘badge area’ of skin), as well as ischaemic changes in the limb. Posterior dislocation is rare and typically occurs when violent movements produce marked internal rotation and adduction, e.g. in epileptic seizures or electric shock.

Attachments

Pectoralis major (Fig. 46.19) is a thick, fan-shaped muscle. It arises from the anterior surface of the sternal half of the clavicle; half the breadth of the anterior surface of the sternum down to the level of the sixth or seventh costal cartilage; the first to the seventh costal cartilages (first and seventh often omitted); the sternal end of the sixth rib; and the aponeurosis of external oblique. The clavicular fibres are usually separated from the sternal fibres by a slight cleft. The muscle converges to a flat tendon, approximately 5 cm across, which is attached to the lateral lip of the intertubercular sulcus of the humerus. The tendon is bilaminar. The thicker anterior lamina is formed by fibres from the manubrium, which are joined superficially by clavicular fibres and deeply by fibres from the sternal margin and the second to fifth costal cartilages. Clavicular fibres may be prolonged into the deltoid tendon. The posterior lamina receives fibres from the sixth (and often seventh) costal cartilages, sixth rib, sternum, and aponeurosis of the external oblique. Costal fibres join the lamina without twisting. Fibres from the sternum and aponeurosis curve around the lower border, turning successively behind those above them, which means that this part of the muscle is twisted so that the fibres that are lowest at their medial origin are highest at their insertion on the humerus. The posterior lamina reaches higher on the humerus than the anterior, and gives off an expansion which covers the intertubercular sulcus and blends with the capsular ligament of the shoulder joint. An expansion from the deepest part of the lamina lines the intertubercular sulcus at its linear insertion; from its lower border another expansion descends into the deep fascia of the upper arm.

The rounded lower border of the muscle forms the anterior axillary fold, and becomes conspicuous in abduction against resistance.

Variants

The abdominal slip from the aponeurosis of the external oblique is sometimes absent. The number of costal attachments and the extent to which the clavicular and costal parts are separated vary. Right and left muscles may decussate across the sternum. A superficial vertical slip, or slips, may ascend from the lower costal cartilages and rectus sheath to blend with sternocleidomastoid or to attach to the upper sternum or costal cartilages. This is sternalis (rectus sternalis). The muscle may be partially or completely absent.

Relations

Skin, superficial fascia, platysma, medial and intermediate supraclavicular nerves, breast, and deep fascia are all anterior. The sternum, ribs and costal cartilages, clavipectoral fascia, subclavius, pectoralis minor, serratus anterior, external intercostal muscles and membranes are all posterior. Pectoralis major forms the superficial layer of the anterior axillary wall, and hence lies anterior to the axillary vessels and nerves and the upper parts of biceps and coracobrachialis. Its upper border is separated from deltoid by the infraclavicular fossa, which contains the cephalic vein and deltoid branch of the thoraco-acromial artery. Its lower border forms the anterior axillary fold. Pectoralis major is separated from latissimus dorsi on the medial axillary wall, but the two muscles converge as they approach the lateral axillary wall: the floor of the intertuberculous sulcus lies between their attachments.

Vascular supply

Pectoralis major is supplied by one dominant vascular pedicle from the pectoral branch of the thoraco-acromial axis, supplemented by several smaller secondary segmental vessels from the deltoid and clavicular branches of the thoraco-acromial axis, and perforating branches of the internal thoracic arteries and superior and lateral thoracic arteries.

The presence of a dominant vascular pedicle (together with its musculocutaneous perforators) means that a musculocutaneous flap in this region can be surgically raised solely on the pectoral branch (the pectoralis major musculocutaneous flap). This flap can be used to reconstruct areas of missing tissue following head and neck cancer resections.

Innervation

Pectoralis major is supplied through the medial and lateral pectoral nerves. Fibres for the clavicular part are from C5 and 6; those for the sternocostal part are from C6, 7, 8, and T1.

Action

The two parts of the muscle can act separately or together. The whole muscle assists adduction and medial rotation of the humerus against resistance. It swings the extended arm forwards and medially, its clavicular part acting with the anterior fibres of deltoid and coracobrachialis: the sternocostal part is relaxed. The opposite movement is usually aided by gravity. When it is resisted, the sternocostal part acts together with latissimus dorsi, teres major and the posterior fibres of deltoid: the clavicular part is relaxed. With the raised arms fixed, e.g. gripping a branch, the same combination of muscles draws the trunk up and forwards. Pectoralis major is active in deep inspiration. Electromyography suggests that the clavicular part acts alone in medial rotation.

Clinical anatomy: testing

To test the clavicular head, the abducted arm is flexed against resistance. To test the sternocostal head, the arm is adducted against resistance.

Poland syndrome

Poland syndrome is a rare congenital anomaly where there is hypoplasia of the thoracic chest wall muscles and ipsilateral hypoplasia of the arm and hand. It occurs in approximately 1 : 50,000 live births. The major clinical features are that the sternocostal head of pectoralis major and all of pectoralis minor are absent. In addition, there may be hypoplasia of latissimus dorsi, serratus anterior, external oblique, supraspinatus, infraspinatus, deltoid and the intercostal muscles, and hypoplasia of the hemithorax and ribs. There may be ipsilateral breast hypoplasia and absent nipple-areolar-complex (Ch. 55).

Hypoplasia affecting the arm ranges from syndactyly to symbrachydactyly and ectrodactyly. The second, third and fourth fingers are the most affected; the wrist, forearm, upper arm and scapula are variably involved.

The aetiology is unclear. It has been suggested that the condition is caused by disruption of the arterial blood supply to the subclavian vessels during the sixth and seventh week of embryonic life, or by disruption of lateral plate mesoderm 2–4 weeks after fertilization.

Attachments

Pectoralis minor is a thin, triangular muscle lying posterior (deep) to pectoralis major (Fig. 46.20). It arises from the upper margins and outer surfaces of the third to fifth ribs (frequently second to fourth), near their cartilages, and from the fascia over the adjoining external intercostal muscles. Its fibres ascend laterally under cover of pectoralis major, converging in a flat tendon which is attached to the medial border and upper surface of the coracoid process of the scapula. Part or all of the tendon may cross the coracoid process into the coraco-acromial ligament, or even beyond to the coracohumeral ligament, thereby gaining attachment to the humerus.

Fig. 46.20 Deep muscles of the front of the chest and left arm. Roman numerals refer to costal cartilages.

(From Sobotta 2006.)

Variants

Slips of the muscle are sometimes separated and vary in number and level. In rare cases, one passes from the first rib to the coracoid (pectoralis minimus). The costal attachments can be 2nd to 5th ribs; 3rd to 5th; 2nd to 4th; 3rd to 4th. The muscle can be present or absent when the pectoralis major is absent.

Relations

Pectoralis major, the lateral pectoral nerve and pectoral branches of the thoraco-acromial artery are anterior. The ribs, external intercostals, serratus anterior, the axilla, axillary vessels, lymphatics and brachial plexus are all posterior. The upper border of pectoralis minor is separated from the clavicle by a triangular gap which is filled by the clavipectoral fascia, behind which are the axillary vessels, lymphatics and nerves. The lateral thoracic artery follows the lower border of the muscle. The medial pectoral nerves pierce and partly supply the muscle.

Vascular supply

Pectoralis minor is supplied by pectoral and deltoid branches of the thoraco-acromial and superior and lateral thoracic arteries.

Innervation

Pectoralis minor is innervated by branches of the medial and lateral pectoral nerves, C5, 6, 7, 8 and T1.

Action

Pectoralis minor assists serratus anterior in drawing the scapula forwards around the chest wall. With levator scapulae and the rhomboids it rotates the scapula, depressing the point of the shoulder. Both pectoral muscles are electromyographically quiescent in normal inspiration, but are active in forced inspiration.

Attachments

Subclavius is a small, triangular muscle tucked between the clavicle and the first rib (Fig. 46.20). It arises from the junction of the first rib and its costal cartilage by a thick tendon, prolonged at its inferior margin and anterior to the costoclavicular ligament. It passes upwards and laterally to a groove on the under surface of the middle third of the clavicle, where it is attached by muscular fibres.

Subclavius may be attached to the coracoid process or the upper border of the scapula as well as, or instead of, the clavicle.

Relations

Posteriorly subclavius is separated from the first rib by the subclavian vessels and brachial plexus, and anteriorly it is separated from pectoralis major by the anterior lamina of the clavipectoral fascia.

Vascular supply

Subclavius is supplied by the clavicular branch of the thoraco-acromial artery and the suprascapular artery.

Innervation

Subclavius is supplied by the subclavian branch of the brachial plexus, which contains fibres from C5 and 6.

Action

Subclavius probably pulls the point of the shoulder down and forwards and braces the clavicle against the articular disc of the sternoclavicular joint: however it is inaccessible to palpation, and difficult to investigate by electromyography. It ‘protects’ the subclavian vessels in fractures of the clavicle, which rarely involve these vessels.

Attachments

Trapezius is a flat, triangular muscle which extends over the back of the neck and upper thorax (Fig. 54.15). The paired trapezius muscles form a diamond shape, from which the name is derived: the lateral angles occur at the shoulder tips, the superior angle at the occipital protuberance and superior nuchal lines, and the inferior angle at the spine of the twelfth thoracic vertebra. On either side, the muscle is attached to the medial third of the superior nuchal line, external occipital protuberance, ligamentum nuchae, and apices of the spinous processes and their supraspinous ligaments from C7 to T12. Superior fibres descend, inferior fibres ascend, and the fibres between them proceed horizontally: all converge laterally on the shoulder. The superior fibres are attached to the posterior border of the lateral third of the clavicle; the middle fibres to the medial acromial margin and superior lip of the crest of the scapular spine; and the inferior fibres pass into an aponeurosis which glides over a smooth triangular surface at the medial end of the scapular spine and is attached to a tubercle at its lateral apex. The occipital attachment is by a fibrous lamina, which is also adherent to the skin. The spinal attachment is by a broad triangular aponeurosis from the sixth cervical to the third thoracic vertebrae, and by short tendinous fibres below this.

Variants

The clavicular attachment of trapezius varies in extent, sometimes reaching mid-clavicle, and occasionally blending with sternocleidomastoid. The vertebral attachment sometimes stops at the eighth thoracic spine. The occipital attachment may be absent. Cervical and dorsal parts are occasionally separate.

Vascular supply

The upper third of trapezius is supplied by a transverse muscular branch which arises from the occipital artery at the level of the mastoid process. It enters the muscle on its deep surface and gives off several musculocutaneous perforators to the overlying skin.

The middle portion of trapezius, together with an area of overlying skin, is supplied by the superficial cervical artery or by a superficial branch of the transverse cervical artery, via musculocutaneous perforators.

The lower third of trapezius is supplied by a muscular branch from the dorsal scapular artery, passing medial to the medial border of the scapula. It reaches the deep surface of the muscle either by piercing the rhomboids or by passing between rhomboid major and minor at the level of the base of the spine of the scapula. It anastomoses with the medial and lateral perforating branches of the posterior intercostal arteries.

The presence of these three discrete vascular pedicles means that musculocutaneous flaps can be surgically raised based on these individual arteries and their accompanying venae comitantes. These flaps can be used for reconstructing areas in the posterior head and neck.

Innervation

Trapezius is innervated by the spinal part of the accessory nerve. Sensory (proprioceptive) branches are derived from the ventral rami of C3 and C4.

Action

Trapezius cooperates with other muscles in steadying the scapula, controlling it during movements of the arm, and maintaining the level and poise of the shoulder. Electromyographic activity is minimal in the unloaded arm, and heavy loads can be suspended with a small contribution from the upper part. Acting with levator scapulae, the upper fibres elevate the scapula and with it the point of the shoulder; acting with serratus anterior, trapezius rotates the scapula forward (upwards) so that the arm can be raised above the head; and acting with the rhomboids, it retracts the scapula, bracing back the shoulder. With the shoulder fixed, trapezius may bend the head and neck backwards and laterally. Trapezius, levator scapulae, rhomboids and serratus anterior combine in producing a variety of scapular rotations (p. 803).

Clinical anatomy: testing

Trapezius is palpated while the shoulder is shrugged against resistance.

Attachments

Deltoid is a thick, curved triangle of muscle. It arises from the anterior border and superior surface of the lateral third of the clavicle, the lateral margin and superior surface of the acromion, and the lower edge of the crest of the scapular spine (other than its smooth medial triangular surface) (Fig. 46.19). The fibres converge inferiorly to a short, substantial tendon which is attached to the deltoid tubercle on the lateral aspect of the midshaft of the humerus. Anterior and posterior fibres converge directly to this tendon. The intermediate part is multipennate: four intramuscular septa descend from the acromion to interdigitate with three septa ascending from the deltoid tubercle. The septa are connected by short muscle fibres, which provide powerful traction. The fasciculi are large, producing a coarse longitudinal striation. The muscle surrounds the glenohumeral articulation on all sides except inferomedially, lending the shoulder its rounded profile. In contraction its borders are easily seen and felt. The tendon gives off an expansion into the brachial deep fascia which may reach the forearm.

Variants

Deltoid may fuse with pectoralis major or may receive additional slips from trapezius, the infraspinous fascia or the lateral scapular border.

Relations

The skin, superficial and deep fasciae, platysma, lateral supraclavicular and upper lateral brachial cutaneous nerves are all superficial. The coracoid process, coraco-acromial ligament, subacromial bursa, tendons of pectoralis minor, coracobrachialis, both heads of biceps, pectoralis major, subscapularis, supraspinatus, infraspinatus, teres minor, long and lateral heads of triceps, the circumflex humeral vessels, axillary nerve, and the surgical neck and upper shaft of the humerus, including both tubercles, all lie deep to deltoid. The anterior border of the muscle is separated proximally from pectoralis major by the infraclavicular fossa, which contains the cephalic vein and deltoid branches of the thoraco-acromial artery. Distally, the muscles are in contact, and their tendons are usually united. The posterior border overlies infraspinatus and triceps.

Vascular supply

Deltoid is supplied by acromial and deltoid branches of the thoraco-acromial artery; the anterior and posterior circumflex humeral arteries; subscapular artery; and the deltoid branch of profunda brachii.

Innervation

Deltoid is innervated by the axillary nerve, C5 and 6.

Action

Different parts of the muscle can act independently as well as together. Anterior fibres assist pectoralis major in drawing the arm forwards and rotating it medially. Posterior fibres act with latissimus dorsi and teres major in drawing the arm backwards and rotating it laterally. The multipennate acromial part of deltoid is a strong abductor: aided by supraspinatus it abducts the arm until the joint capsule is tense below. Movement takes place in the plane of the body of the scapula, which is the only way that scapular rotation can be fully effective in raising the arm above the head. In true abduction (Fig. 46.19), acromial fibres contract strongly, while clavicular and posterior fibres prevent departure from the plane of motion. In the early stages of abduction, traction by the deltoid is upward, but the humeral head is prevented from translating upward by the synergistic downward pull of subscapularis, infraspinatus and teres minor. Electromyography suggests that deltoid contributes little to medial or lateral rotation but confirms that it takes part in most other shoulder movements. It may also aid supraspinatus in resisting the downward drag of a loaded arm. A common action of the deltoid is arm-swinging while walking.

Clinical anatomy: testing

Deltoid can be seen and felt to contract when the arm is abducted against resistance.

Attachments

Levator scapulae is a slender muscle attached by tendinous slips to the transverse processes of the atlas and axis, and to the posterior tubercles of the transverse processes of the third and fourth cervical vertebrae (Fig. 28.6, see below Fig. 46.21). It descends diagonally to approach the medial scapular border between its superior angle and the triangular smooth surface at the medial end of the scapular spine.

Fig. 46.21 Rhomboid muscles.

Variants

Levator scapulae varies considerably in its vertebral attachments and the extent to which it separates into slips. There may be accessory attachments to the mastoid process, occipital bone, first or second rib, scaleni, trapezius, and serratus muscles.

Vascular supply

Levator scapulae receives its arterial supply mainly from the transverse cervical and ascending cervical arteries. The vertebral extremity of the muscle is supplied by branches from the vertebral artery.

Innervation

Levator scapulae is innervated directly by branches of the third and fourth cervical spinal nerves, and from the fifth cervical nerve via the dorsal scapular nerve.

Action

The levator scapulae and the rhomboids assist other scapular muscles in controlling the position and movement of the scapula. Acting with trapezius, rhomboids retract the scapula, bracing back the shoulder; with levator scapulae and pectoralis minor they rotate the scapula, depressing the point of the shoulder. With the cervical vertebral column fixed, levator scapulae acts with trapezius to elevate the scapula or to sustain a weight carried on the shoulder; with the shoulder fixed, the muscle inclines the neck to the same side.

Attachments

Rhomboid major is a quadrilateral sheet of muscle which arises by tendinous fibres from the spines and supraspinous ligaments of the second to fifth thoracic vertebrae, and descends laterally to the medial border of the scapula between the root of the spine and the inferior angle (Fig. 46.21). Most of its fibres usually end in a tendinous band between these two points, joined to the medial border by a thin membrane. Occasionally this is incomplete, in which case some muscular fibres are attached directly into the scapula. The attachments of rhomboid major – and also those of rhomboid minor, levator scapulae and serratus anterior – can be more extensive, with ‘folds’ or extensions passing to both dorsal and costal aspects of the scapula adjacent to its medial margin.

Vascular supply, innervation, action and clinical anatomy: testing

All are described with rhomboid minor.

Attachments

Rhomboid minor is a small, cylindrical muscle. It runs from the lower ligamentum nuchae and the spines of the seventh cervical and first thoracic vertebrae to the base of a smooth triangular surface at the medial end of the spine of the scapula, where dorsal and ventral layers enclose the inferior border of levator scapulae (Fig. 46.21). The dorsal layer of rhomboid minor is attached to the rim of the triangular surface, dorsolateral to and below levator scapulae. The ventral layer is strong and wide, extending 2–3 cm medial to and below levator scapulae; here the fasciae of rhomboid minor and serratus anterior are tightly fused. Rhomboid minor is usually separate from rhomboid major, but the muscles overlap and are occasionally united.

Variants

There is some variability in the vertebral and scapular attachments of rhomboids major and minor. A slip of muscle may extend from the upper border of rhomboid minor to reach the occipital bone (rhomboid occipitalis).

Vascular supply of the rhomboids

Rhomboids major and minor are supplied by the dorsal scapular artery or deep branch of the transverse cervical artery and by dorsal perforating branches from the upper five or six posterior intercostal arteries.

Innervation of the rhomboids

Rhomboids major and minor are innervated by a branch of the dorsal scapular nerve, C4, 5.

Action of the rhomboids

Both rhomboid major and minor retract the medial border of the scapula superiorly and medially. They are used in squaring the shoulders.

Clinical anatomy: testing of the rhomboids

Rhomboids major and minor can be palpated deep to trapezius on bracing the shoulder back against resistance.

Attachments

Latissimus dorsi is a large, flat, triangular muscle that sweeps over the lumbar region and lower thorax and converges to a narrow tendon (Fig. 46.22, Fig. 54.15). It arises by tendinous fibres from the spines of the lower six thoracic vertebrae anterior to trapezius; from the posterior layer of thoracolumbar fascia, by which it is attached to the spines and supraspinous ligaments of the lumbar and sacral vertebrae; and from the posterior part of the iliac crest. It also springs by muscular fibres from the posterior part (outer lip) of the iliac crest lateral to erector spinae, and by fleshy slips from the three or four lower ribs, interdigitating with external oblique (Fig. 61.9). From this extensive attachment, fibres pass laterally with different degrees of obliquity (the upper fibres are nearly horizontal, the middle oblique, and the lower almost vertical) to form a sheet approximately 12 or 13 mm thick that overlaps the inferior scapular angle. The muscle curves around the inferolateral border of teres major to gain its anterior surface. Here it ends as a flattened tendon, approximately 7 cm long, in front of the tendon of teres major. It is attached to the floor of the intertubercular sulcus of the humerus, with an expansion to the deep fascia. The attachment extends higher on the humerus than that of teres major. As the muscle curves round teres major, the fasciculi rotate around each other, so that fibres that originate lowest at the midline insert highest on the humerus, and fibres that originate highest at the midline insert lowest on the humerus. A bursa sometimes occurs between the muscle and the inferior scapular angle. The tendons of latissimus dorsi and teres major are united at their lower borders: they are separated by a bursa near their humeral attachments.

Fig. 46.22 Deep muscles of the left shoulder girdle. A, Anterior aspect after removal of several superficial muscles. B, Posterior aspect.

(From Sobotta 2006.)

Latissimus dorsi and teres major together form the posterior axillary fold. When the arm is adducted against resistance, this fold is accentuated, and the whole inferolateral border of latissimus dorsi can then be traced to its attachment to the iliac crest.

The lower, lateral margin of the muscle is usually separated from the posterior border of external oblique by the lumbar triangle; the base of this small triangle is the iliac crest and its floor the internal oblique. It should not be confused with the triangle of auscultation, which is medial to the scapula, and is bounded above by the trapezius, below by latissimus dorsi and laterally by the medial border of the scapula; part of rhomboid major is exposed in the triangle. If the scapulae are drawn forwards, by folding the arms across the chest, and the trunk is bent forwards, parts of the sixth and seventh ribs and the interspace between them (overlying the apex of the lower pulmonary lobe) become subcutaneous.

Variants

Latissimus dorsi commonly receives some additional fibres from the scapula as it crosses the inferior scapular angle. A muscular axillary arch, 7–10 cm in length and 5–15 mm in breadth, may sometimes be present: it crosses from the edge of latissimus dorsi, midway in the posterior fold, over the front of the axillary vessels and nerves to join the tendons of pectoralis major, coracobrachialis or the fascia over the biceps. The vertebral and costal attachments of latissimus dorsi may be reduced or, in rare cases, increased. A fibrous slip usually passes from the tendon, near its humeral insertion, to the long head of triceps.

Vascular supply

Latissimus dorsi is supplied by a single dominant vascular pedicle, the thoracodorsal artery, itself a continuation of the subscapular artery (p. 817). The thoracodorsal artery (and its accompanying venae comitantes) enters the muscle at a single neurovascular hilum on its costal surface, 6–12 cm from the subscapular artery and 1–4 cm medial to the lateral border of the muscle. The artery gives off one to three large branches to serratus anterior before dividing at, or even before, the hilum to supply latissimus dorsi itself. The basic pattern of branching is a bifurcation into lateral and medial branches. The larger lateral branch follows a course parallel to, and 1–4 cm from, the upper border of the muscle; the smaller branch diverges at an angle of 45° and travels medially. In a small number of cases the artery trifurcates: this usually yields a small recurrent branch which returns to supply the proximal part of the muscle, but in some cases it provides a third major branch which supplies the distal part of the muscle. Occasionally the lateral branch gives off a further collateral to serratus anterior. The major branches give off 5–9 longitudinal branches which travel distally, parallel with the muscle fibres. Musculocutaneous perforators arising from these vessels supply the overlying skin.

In addition to the dominant vascular pedicle, latissimus dorsi is supplied inferiorly by several smaller secondary segmental vascular pedicles. These are dorsal perforating arteries derived from the 9th, 10th and 11th posterior intercostal and 1st, 2nd and 3rd lumbar arteries, and they all enter the muscle on its deep surface. They anastomose within the muscle with branches of the thoracodorsal artery.

The blood supply of latissimus dorsi is particularly important because of its applications in plastic and reconstructive surgery. The presence of the dominant vascular pedicle provides the anatomical basis for raising the muscle either alone, or together with an overlying paddle of skin in the form of a musculocutaneous flap based solely on the thoracodorsal artery and its accompanying venae comitantes. These flaps can be used in a pedicle fashion, e.g. in reconstructing a breast following a mastectomy, or used as free tissue transfer to cover large areas of tissue loss anywhere in the body.

Innervation

Latissimus dorsi is supplied by the thoracodorsal nerve, from the posterior cord of the brachial plexus, C6, 7 and 8. This nerve runs in the neurovascular pedicle and divides approximately 1.3 cm proximal to the point of bifurcation or trifurcation of the thoracodorsal artery. The pattern of branching follows that of the artery closely, and the neural and vascular branches travel together.

Action

Latissimus dorsi is active in adduction, extension and especially in medial rotation of the humerus. Humeral adduction and extension are most powerful when the initial position of the arm is one of partial abduction, flexion or a combination of the two. With the sternocostal part of pectoralis major and teres major it adducts the raised arm against resistance. It assists backward swinging of the arm, as in walking and many athletic pursuits. When the arms are raised above the head, as in climbing, it pulls the trunk upwards and forwards. It takes part in all violent expiratory efforts, such as coughing or sneezing: this is readily confirmed by palpation. Electromyography suggests that it aids deep inspiration, but it is also active towards the end of forcible expiration, e.g. when blowing a sustained note on a musical instrument. When the arm is elevated, the stretched fibres of latissimus dorsi press on the inferior scapular angle, keeping it in contact with the chest wall. Despite this range of actions, surgical transposition of the muscle does not produce any serious restriction of normal activity.

Clinical anatomy: testing

Latissimus dorsi is palpated when the abducted arm is adducted against resistance and can be felt to contract during coughing.

Attachments

Serratus anterior is a large muscular sheet which curves around the thorax. It arises from an extensive costal attachment and inserts on the scapula (Fig. 46.19, Fig. 46.20). Fleshy digitations spring anteriorly from the outer surfaces and superior borders of the upper eight, nine or even ten ribs, and from fasciae which cover the intervening intercostals. They lie on a long, slightly curved, line which passes inferolaterally across the thorax. The first digitations spring from the first and second ribs and intercostal fascia, the others from a single rib, and the lower four interdigitate with the upper five slips of external oblique. The muscle follows the contour of the chest wall closely. It passes ventral to the scapula and reaches the medial border of the scapula in the following way. The first digitation encloses, and is attached to, a triangular area of both the costal and dorsal surfaces of the superior scapular angle. The next two or three digitations form a triangular sheet which is attached to the costal surface along almost its entire medial border. The lower four or five digitations converge to be attached by musculotendinous fibres to a triangular impression on the costal surface of the inferior angle: they enclose the inferior angle and are also attached to a smaller triangular part of its dorsal surface near its tip.

Variants

Digitations may be absent, particularly the first and eighth, and sometimes also the intermediate part. Serratus anterior may be partly fused with levator scapulae, adjacent external intercostals or external oblique.

Vascular supply

Serratus anterior is supplied by superior and lateral thoracic arteries, and branches from the thoracodorsal artery before (occasionally after) it divides in latissimus dorsi.

Innervation

Serratus anterior is innervated by the long thoracic nerve, C5, 6 and 7, which descends on the external surface of the muscle.

Action

With pectoralis minor, serratus anterior protracts (draws forward) the scapula, as a prime mover in all reaching and pushing movements. The upper part, with levator scapulae and upper fibres of trapezius, suspends the scapula, but slight activity is sufficient to support the unloaded arm. The heavier insertion lower down pulls the inferior scapular angle forwards around the thorax, assisting trapezius in upward rotation of the bone, an action that is essential to raising the arm above the head. In the initial stages of abduction, serratus anterior helps other muscles to fix the scapula, so that deltoid acts effectively on the humerus, and not the scapula. While deltoid is raising the arm to a right angle with the scapula, serratus anterior and trapezius are simultaneously rotating the scapula: the combination allows the arm to be raised to the vertical. To effect this upward rotation of the scapula, forward pull on the inferior angle by the lower digitations of serratus anterior is coupled with an upward and medial pull on the lateral end of the clavicle and acromion by the upper fibres of trapezius, and a downward pull on the base of the scapular spine by the lower fibres of trapezius. Conversely, slow downward scapular rotation assisted by gravity is achieved by controlled lengthening of these muscles. More powerful downward rotation requires balanced contraction of the upper fibres of serratus anterior, levator scapulae, rhomboids, pectoralis minor and the middle part of trapezius. When weights are carried in front of the body, serratus anterior prevents backward rotation of the scapula. Electromyography shows that serratus anterior is not active in normal human respiration. This may not apply to laboured respiration, e.g. when asthmatics and athletes may be observed to fix the scapula by grasping a rail or other support.

Clinical anatomy: testing

The muscular digitations of serratus anterior can be seen and felt when the outstretched hand pushes against resistance.

When serratus anterior is paralysed, the medial border of the scapula, and especially its lower angle, stand out prominently. The arm cannot be raised fully. Pushing is ineffective, indeed attempts to do so produce further projection, known as ‘winging’ of the scapula.

Attachments

Supraspinatus arises from the medial two-thirds of the supraspinous fossa and from the supraspinous fascia (Fig. 46.22). The fibres converge, under the acromion, into a tendon which crosses above the shoulder joint and is attached to the highest facet of the greater tubercle of the humerus. The tendon blends into the articular capsule and may give a slip to the tendon of pectoralis major. Fibrocartilage has been described at the tendinous insertion, as in other tendons attached to epiphysial bone.

Vascular supply

Supraspinatus is supplied by the suprascapular and dorsal scapular arteries.

Innervation

Supraspinatus is innervated by the suprascapular nerve, C5 and 6.

Action

The conventional view is that supraspinatus initiates abduction of the shoulder and assists deltoid in abduction thereafter. However, there is evidence that both supraspinatus and deltoid are involved throughout the range of abduction, including initiation of the movement. As part of the rotator cuff, supraspinatus helps to stabilize the head of the humerus in the glenoid fossa during movements of the glenohumeral joint. With the arm dependent, even when moderately loaded, supraspinatus and tension in the upper capsule prevent downward displacement of the humerus.

Clinical anatomy: testing

Supraspinatus can be palpated deep to trapezius when the arm is abducted at the shoulder joint against resistance.

The tendon of supraspinatus is separated from the coraco-acromial ligament, acromion and deltoid by the large subacromial bursa; when this is inflamed, abduction of the shoulder joint is painful. The tendon is the most frequently torn element of the musculotendinous cuff around the shoulder joint.

Attachments

Infraspinatus is a thick triangular muscle which occupies most of the infraspinous fossa (Fig. 46.22). It arises by muscular fibres from the medial two-thirds of the fossa, by tendinous fibres from ridges on its surface and from the deep surface of the infraspinous fascia, which separates it from teres major and minor. Its fibres converge to a tendon which glides under the lateral border of the spine of the scapula, and then passes across the posterior aspect of the capsule of the shoulder joint to be attached to the middle facet on the greater tubercle of the humerus. The tendon is sometimes separated from the capsule by a bursa, which may communicate with the joint cavity. Infraspinatus is sometimes fused with teres minor.

Vascular supply

Infraspinatus is supplied by the suprascapular and circumflex scapular arteries.

Innervation

Infraspinatus is innervated by the suprascapular nerve, C5 and 6.

Action

Infraspinatus is a lateral rotator of the humerus. Together with supraspinatus, subscapularis and teres minor, it helps to stabilize the head of the humerus in the glenoid fossa during shoulder movements (p. 806).

Attachments

Subscapularis is a bulky, triangular muscle which fills the subscapular fossa (Fig. 46.20). In its medial two-thirds, the fibres are attached to the periosteum of the costal surface of the scapula. Other fibres arise from tendinous intramuscular septa, which are attached to ridges on the bone, and from the aponeurosis which covers the muscle and separates it from teres major and the long head of triceps. The fibres converge laterally into a broad tendon which is attached to the lesser tubercle of the humerus and the front of the articular capsule. The tendon is separated from the neck of the scapula by the large subscapular bursa, which communicates with the shoulder joint.

Variation is unusual. A separate slip may pass from the medial border of the scapula to the glenohumeral capsule or to the periosteum medial to the intertuberculous sulcus of the humerus.

Relations

Subscapularis forms much of the posterior axillary wall. Its anterior surface is apposed inferomedially to serratus anterior, and superolaterally to coracobrachialis and biceps, the axillary vessels, brachial plexus and subscapular vessels and nerves. Its posterior surface is attached to the scapula and glenohumeral capsule. Its lower border contacts teres major and latissimus dorsi.

Vascular supply

Subscapularis is supplied by small branches from the suprascapular, axillary and subscapular arteries.

Innervation

Subscapularis is innervated by the upper and lower subscapular nerves, C5, 6.

Action

Subscapularis is a medial rotator of the humerus. Together with supraspinatus, infraspinatus and teres minor, it helps to stabilize the head of the humerus in the glenoid fossa during shoulder movements (p. 806).

Attachments

Teres major is a thick, flat muscle which arises from the oval area on the dorsal surface of the inferior scapular angle, and from the fibrous septa interposed between the muscle and teres minor and infraspinatus (Fig. 46.22). Its fibres ascend laterally and end in a flat tendon, approximately 5 cm long, which is attached to the medial lip of the intertubercular sulcus of the humerus. The tendon lies behind that of latissimus dorsi, from which it is separated by a bursa; the tendons are united along their lower borders for a short distance.

Variants

Teres major may be fused with the scapular part of latissimus dorsi, and it may send a slip to join the long head of triceps or the brachial fascia.

Vascular supply

Teres major is supplied by the thoracodorsal branch of the subscapular artery on its way to latissimus dorsi and by the posterior circumflex humeral artery.

Innervation

Teres major is innervated by the lower subscapular nerve, C5, 6 and 7.

Action

Despite its name, teres major has a nerve supply and action that is distinct from teres minor. Teres major draws the humerus backwards and rotates it medially. Electromyographic studies are equivocal about its major role in movement, but its involvement as a contributor to static posture and arm-swinging is not contested.

Clinical anatomy: testing

Teres major can be palpated posterior to the posterior axillary fold during adduction of the humerus against resistance.

Attachments

Teres minor is a narrow elongated muscle which arises from the upper two-thirds of a flattened strip on the dorsal surface of the scapula adjoining its lateral border, and from two aponeurotic laminae which separate it from infraspinatus and teres major (Fig. 46.22). It runs upwards and laterally. The upper fibres end in a tendon attached to the lowest facet on the greater tubercle of the humerus. The lower fibres are attached directly into the humerus distal to this facet and above the origin of the lateral head of triceps. The tendon passes across, and blends with, the lower posterior surface of the capsule of the shoulder joint.

Teres minor may be fused with infraspinatus.

Vascular supply

Teres minor is supplied by the circumflex scapular artery, which pierces the origin of the muscle as it turns upward in the infraspinous fossa, and by the posterior circumflex humeral artery.

Innervation

Teres minor is innervated by the axillary nerve, C5 and 6.

Action

Teres minor acts as a lateral rotator and weak adductor of the humerus. Together with supraspinatus, infraspinatus and subscapularis, it helps stabilize the head of the humerus in the glenoid fossa during shoulder movements (p. 803).

Variants

About a third of the superficial cervical and dorsal scapular arteries arise in common from the thyrocervical trunk as a transverse cervical artery, together with a superficial and a deep branch (superficial cervical and dorsal scapular arteries respectively). In this case the dorsal scapular artery arises near the superior border of the scapula: it passes laterally, anterior to the brachial plexus and then posterior to levator scapulae (Fig. 46.26).

Fig. 46.26 A–F, Variations in origin and branches of the thyrocervical trunk. The superficial cervical artery is the distinct vessel arising from the thyrocervical trunk (C and E) or from the suprascapular artery (D), which supplies the territory of the superficial branch of the (absent) transverse cervical artery.

(By permission from Cormack GC, Lamberty BGH 1994 The Arterial Anatomy of Skin Flaps, 2nd edn. Edinburgh: Churchill Livingstone.)

Pectoral branch

The pectoral branch is the largest branch. It descends between the pectoral muscles, gives a branch to pectoralis minor, and then continues on the deep surface of pectoralis major. It enters the muscle and anastomoses with the intercostal branches of the internal thoracic and lateral thoracic arteries. It gives off perforating branches to the breast, and musculocutaneous perforators to the skin over pectoralis major.

Acromial branch

The acromial branch crosses the coracoid process under deltoid, which it supplies, pierces the muscle and ends on the acromion. It anastomoses with branches of the suprascapular artery, the deltoid branch of the thoraco-acromial artery and the posterior circumflex humeral arteries.

Clavicular branch

The clavicular branch ascends medially between the clavicular part of pectoralis major and the clavipectoral fascia. It supplies the sternoclavicular joint and subclavius.

Deltoid branch

The deltoid branch often arises with the acromial branch. It crosses pectoralis minor to accompany the cephalic vein between pectoralis major and deltoid, and supplies both muscles.

Lesions of the suprascapular nerve

The commonest cause involving the suprascapular nerve is neuralgic amyotrophy. An entrapment neuropathy may occur in the scapular notch, often related to a degenerate cyst originating from the posterior glenoid labrum, or the nerve may be damaged by trauma to the scapula and shoulder. There is pain in the shoulder and wasting and weakness of supraspinatus and infraspinatus.

Lesions of the axillary nerve

The commonest causes of axillary nerve lesions are trauma (dislocation of the shoulder, fracture of the surgical neck of the humerus), and neuralgic amyotrophy. There is wasting and weakness of deltoid, which is usually clinically evident, and a patch of sensory loss on the outer aspect of the arm. This can be differentiated from a C5 root lesion by finding normal function in the distribution of the suprascapular nerve.

Lesions of the musculocutaneous nerve

An isolated lesion of the musculocutaneous nerve is rare, but may occur in injuries to the upper arm and shoulder, e.g. fracture of the humerus, and in patients with neuralgic amyotrophy. There is marked weakness of elbow flexion, because biceps brachii and much of brachialis are paralysed, and sensory impairment on the extensor aspect of the forearm in the distribution of the lateral cutaneous nerve of the forearm. Pain and paraesthesiae may be aggravated by elbow extension.