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5. 11. 2012.

Shoulder joint – part I


The shoulder(glenohumeral) joint is the articulation between the head of the humerus and the glenoid fossa of the scapula. The intrascapular presence of the epiphyseal line between the ventral coracoid and dorsal scapula in the upper part of the glenoid fossa facilitates adjustments of the joint surfaces during growth of the bone. Similar arrangements are also to be found in the hip and the elbow joints.

It is a synovial joint of the ball and socket variety, with the head of the humerus forming the ball and the glenoid fossa of the socket, in which freedom of movement has been developed at the expense of the stability. The mobility of the upper limbs from locomotor activity, and partly due to mobility of the pectoral girdle linking the limb to the trunk. Comparison of the shoulder and hip joints, equivalent joints in the upper and lower limbs, together with their mode of attachment to the axial skeleton, reveals significant and important differences between them even though the basic features of each are similar.
In the frontal plane, the axis of the head and neck of the humerus forms an angle of 135° to 140° with the long axis of the shaft; this is the angle of inclination. Because of this angulation, the centre of the humeral head lies about 1cm medial to the long axis. Although the anatomical and mechanical axes of the humerus do not exactly coincide, unlike the femur they both lie inside the bone(a). Consequently, the action of muscle groups producing movement at the shoulder joint, especially that of medial and lateral rotation, is more easily understood.

As well as being set at an angle to the shaft of the humerus, the axis of the head and neck is rotated backwards against the shaft some 30° to 40°; this is the angle of retroversion(b). The magnitude of the angle of retroversion is said to vary both with age and with race. It is thought that this angle has increased with the attainment of bipedalism, in which there has also been flattening of the thoracic cage anteroposteriorly, and a backward displacement of the scapula, the result being that as the glenoid fossa came to be directed more laterally, the head and neck of the humerus became more twisted in an attempt to maintain maximum joint contact between the two articulating bones. Structurally this adaptation was not entirely successful, which to some extent was fortuitous for in humans the use of the upper limb gradually changed from one of support to one of manipulation.

Articular surfaces

The articular surfaces of the joint are the rounded head of the humerus and the rather shallow glenoid fossa. The adaptation of these two surfaces contributes very little, if at all, to the stability and security of the joint. As in the majority of synovial joints, the two articular surfaces are covered by hyaline cartilage.

Glenoid fossa

The glenoid fossa is situated at the superolateral angle of the scapula and faces laterally, anteriorly and slightly superiorly. It is pear-shaped in outline, with a narrower region superiorly, and concave both vertically and transversely(a). However, the concavity of the joint is irregular and less deep than the convexity of the head of the humerus. In the plane of the axis of the head and neck of the humerus it has been estimated that the curvature of the glenoid fossa, with its larger radius, subtends an angle of some 75°. The articular surface of the fossa is little more than one-third of that of the humeral head. It is deepened to some extent by the presence of the glenoid labrum.

Head of the humerus

The head of the humerus represents two-fifths of a sphere and faces superiorly, medially and posteriorly(b,c). With its smaller radius of curvature, the articular surface subtends an angle of approximately 150° in the plane of the axis of the head and neck. Regardless of the position of the joint, only one-third of the humeral head is in contact with the glenoid fossa at any time. It is essentially the mismatch in coaptation of the articular surfaces which give the joint its mobility.

Joint capsule and synovial membrane

The fibrous capsule of the joint forms a loose cylindrical sleeve between the two bones(d). The majority of the capsular fibres pass horizontally between scapula and humerus, but some oblique and transverse fibres are also present. Although it is thick and strong in parts, particularly anteriorly, because of its laxness the capsule conveys little stability to the joint. On the scapula, the capsule attaches just outside the glenoid labrum anteriorly and inferiorly, and to the labrum superiorly and posteriorly. Recesses formed between the anterior capsular attachment and the glenoid labrum may have pathological significance in shoulder joint trauma.
On the humerus, the capsule attaches to the anatomical neck, around the articular margins of the head, medial to the greater and lesser tubercles, except inferiorly where it joins the medial surface of the shaft about 1cm below the articular margin(d). The dipping down of the capsular attachment medially causes the medial end of the upper epiphyseal line of the humerus to become intracapsular.
The anterior part of the capsule is thickened and strengthened by the presence of the three glenohumeral ligaments, which can only be seen on its inner aspect. The superoposterior part is strengthened near its humeral attachment by the coracohumeral ligament. The tendons of the “rotator cuff” muscles spread out over the capsule; blending with it near their humeral attachments. These short scapular muscles act as extensible ligaments, and are extremely important in maintaining the integrity of the joint.
With the arm hanging in the anatomical position, the lower part of the joint capsule is lax and forms a redundant fold. When the arm is abducted this part of the capsule becomes increasingly taut.
There are two openings in the fibrous capsule; occasionally a third may be present. One is the upper end of the intertubercular groove to allow the long head of biceps to pass into the arm(d). This part of the capsule is thickened, forming the transverse humeral ligament, arching over the tendon as it emerges from the capsule. The second opening is in the front of the capsule, between the superior and middle glenohumeral ligaments, and communicates with the subscapular bursa deep to the tendon of subscapularis(d). A third communication of the joint cavity with the infraspinatus bursa.

Capsular ligaments

The anterior part of the joint capsule is reinforced by three longitudinal bands of fibres called the glenohumeral ligaments(a). They are seldom prominent and when present radiate from the anterior glenoid margin extending downwards from the supraglenoid tubercle.

Superior glenohumeral ligament. The slender superior glenohumeral ligament arises from the upper part of the glenoid margin and adjacent labrum immediately anterior to the attachment of the tendon of the long head of biceps. It runs laterally parallel to the biceps tendon to the upper surface of the lesser tubercle.

Middle glenohumeral ligament. The middle glenohumeral ligament arises below the superior and attaches to the humerus on the front of the lesser tubercle below the insertion of subscapularis.

Inferior glenohumeral ligament. The inferior glenohumeral ligament is usually the best developed of the three ligaments, although it is tend to increase the tension in some or all of the glenohumeral ligaments. Lateral rotation of the humerus will put all three ligaments under tension, whereas medial rotation relaxes them.
In abduction, only the middle and inferior ligaments become taut, while the superior becomes relaxed.

Transverse humeral ligament. At the upper end of the intertubercular groove, the transverse humeral ligament bridges the gap between the greater and lesser tubercles(b). It is formed by some of the transverse fibres of the capsule and serves to hold the biceps tendon in the intertubercular groove as it leaves the joint.

Synovial membrane

The synovial membrane of the joint lines the capsule, and thus also extends downwards as a pouch when the arm is hanging by the side(a). It attaches to the articular margins of both bones, and is therefore reflected upwards on the medial side of the humeral shaft to its attachment. Consequently, although the medial part of the epiphyseal line is intracapsular, it is extrasynovial(a). The membrane extends through the anterior opening of the capsule forming the subscapularis bursa, which may be limited in its extent to the posterior surface of the tendon of subscapularis. However, it may be sufficiently large to extend above the upper border of the tendon and so come to lie below the coracoid process. This upward extension may be replaced by a separate subcoracoid bursa. The posterior extension of the membrane through the joint capsule forms the infraspinatus bursa.
The intracapsular part of the long head of biceps is enclosed within a double-layered tubular sheath of synovial membrane which is continuous with that of the joint at its glenoid attachment(a). This sheath surrounds the biceps tendon as it passes beneath the transverse humeral ligament into the intertubercular groove, and extends some 2cm into the arm(b).
An important, but non-communicating, bursa associated with the shoulder joint is the subacromial bursa. It lies between, and separates, the coracoacromial arch and deltoid from the superolateral part of the shoulder joint. That part of the bursa which extends laterally under deltoid is usually referred to as the subdeltoid bursa.
Some of these bursae are of clinical significance as adhesions may form preventing free gliding movements. This is particularly true for bicipital sheath and the subdeltoid bursa. Indeed, the subdeltoid bursa may become inflamed(bursitis) and affect the underlying tendon of supraspinatus, leading to its rupture in a small number of cases.

Intra-articular structures

Glenoid labrum

As in the hip joint, the glenoid fossa is deepened by the presence of a fibrocartilaginous rim, the glenoid labrum(c). It is triangular in cross-section, with a thin free edge and is about 4mm deep. The base of the labrum attaches to the margin of the glenoid fossa; the outer surface gives attachment to the joint capsule posteriorly and superiorly, while the inner(joint) surface is in contact with the head of the humerus and is lined by cartilage continuous with that of the glenoid fossa. The upper part of the labrum may not be completely fixed to the bone so that its inner edge may project into the joint like a meniscus.
The superior outer margin of the labrum gives attachment to the tendon of the long head of biceps, while inferiorly the tendon of the long head of triceps partly arises from it.

Long head of biceps

The tendon of the long head of biceps  runs intracapsularly from its attachment to the supraglenoid tubercle and adjacent superior margin of the glenoid labrum until it emerges from the joint deep to the transverse humeral ligament. During its intracapsular course, and for some 2cm beyond, the tendon is ensheathed in a synovial sleeve.

Accessory ligaments

Apart from the capsular ligaments, there are two further ligaments associated with the shoulder joint. Both are considered to be accessory ligaments, although one, the coracohumeral ligament, blends with the joint capsule.
The other, the coracoacromial ligament, completes a fibro-osseous arch above the joint.

Coracohumeral ligament

The coracohumeral ligament is a fairly strong, broad band arising from the lateral border of the coracoid process near its root. It becomes flattened as it passes laterally with its two margins diverging above the intertubercular groove to attach to the upper part of the anatomical neck in the region of the greater and lesser tubercles and to the intervening transverse humeral ligament.

The anterior border of the medial part of the ligament is free, but as it passes laterally it fuses with the tendon of subscapularis as it blends with the capsule prior to its insertion on the lesser tubercle. The posterior part of the ligament blends with the tendon of supraspinatus as it attaches to the superior facet on the greater tubercle.

Coracoacromial ligament

The coracoacromial ligament is not directly associated with the joint but forms, with the coracoid and acromion process, a fibro-osseous arch above the head of the humerus. It is a strong, triangular ligament, whose anterior and posterior borders tend to be thicker than the intermediate part. Occasionally, the pectoralis minor tendon is prolonged and pierces the base of the ligaments to become continuous with the coracohumeral ligament. The coracoacromial ligament is attached by a broad base to the lateral border of the horizontal part of the coracoid process, with the blunt apex fixed to the apex of the acromion in front of the acromioclavicular joint. Superiorly are the clavicle and deltoid, while inferiorly it is separated from the tendon of the supraspinatus and the shoulder joint by the subacromial bursa.
The arch so formed by the ligament and bony processes serves to increase the surface upon which the head of the humerus is supported when force is transmitted upwards along the humerus.

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