The sternoclavicular joint

The synovial sternoclavicular joint provides the only point of bony connection between the pectoral girdle and upper limb, and the trunk. Although the joint is functionally a ball and socket joint, it does not have the form of such a joint.

Articular surfaces

The medial end of the clavicle articulates with the clavicular notch at the superolateral angle of the sternum and the adjacent upper medial surface of the first costal cartilage. The clavicular articular surface tends to be larger than that on the sternum. Consequently, the medial end of the clavicle projects above the upper margin of the manubrium sterni.

The articular surfaces are reciprocally concavoconvex, although they do not usually have similar radii of curvature. The joint, therefore, is not particularly congruent. Congruence is partly provided by an intraarticular fibrocartilaginous disc. The articular surface on the manubrium sterni is set at approximately 45° to the perpendicular. It is markedly concave from above downwards, and convex from behind forwards, being covered with hyaline cartilage. The clavicular articular surface is convex vertically and flattened or slightly concave horizontally, with the concavity being continued over the inferior surface of the shaft for articulation with the first rib costal cartilage. The greater horizontal articular surface of the clavicle overlaps the sternocostal surface anteriorly and particularly posteriorly, the whole being covered with fibrocartilage rather than hyaline cartilage. 

Joint capsule and synovial membrane

A fibrous capsule surrounds the whole joint like a sleeve attaching to the articular margins of both the clavicle and the sternum, with its inferior part passing between the clavicle and the upper surface of the first costal cartilage. Except for this inferior part, which is weak, the joint capsule is relatively strong, being strengthened anteriorly, posteriorly and superiorly by capsular thickenings known as the anterior and posterior sternoclavicular ligaments and the interclavicular ligament respectively.

Because there are two separate cavities associated with the joint(see below), there are two synovial membranes. A relatively loose lateral membrane lines the capsule, being reflected from the articular margin of the medial end of the clavicle to the margins of the articular disc. Similarly, the medial membrane attaches to the articular margins on the sternum and to the margins of the disc.

Intra-articular structures

A complete, intra-articular, fibrocartilaginous disc separates the joint into two synovial cavities. The disc is flat and round, being thinner centrally, where it may occasionally be perforated and permit communication between the two cavities, than around the periphery. It is attached at its circumference to the joint capsule, particularly in front and behind. More importantly, however, the disc is firmly attached superiorly and posteriorly to the upper border of the medial end of the clavicle, and inferiorly to the first costal cartilage near its sternal end. Consequently, as well as providing some cushioning between the articular surfaces, from forces transmitted from the upper limb, and compensating for incongruity of the joint surfaces, the disc also has an important ligamentous action. Although mainly fibrocartilaginous, it is fibrous or ligamentous at its circumference, and holds the medial end of the clavicle against the sternum. It prevents the clavicle moving upwards and medially along the sloping sternochondral surface under the influence of strong, thrusting forces transmitted from the limb, or when the clavicle is depressed as by a heavy weight carried in the hand.

Ligaments

The joint capsule is strengthened anteriorly, posteriorly and superiorly by the anterior and posterior sternoclavicular ligaments and the interclavicular ligament respectively. In addition, an accessory ligament, the costoclavicular ligament,  binds the clavicle to the first costal cartilage just lateral to the joint.

Anterior sternoclavicular ligament

The anterior sternoclavicular ligament is a strong, broad band of fibres attaching above to the superior and anterior parts of the medial end of the clavicle, passing obliquely downwards and medially to the front of the upper part of the manubrium sterni. It is reinforced by the tendinous origin of sternomastoid.

Posterior sternoclavicular ligament

The posterior sternoclavicular ligament, although not as strong as the anterior ligament, is also a broad band running obliquely downwards and medially. Laterally it attaches to the superior and posterior parts of the medial end  of the clavicle, while medially it is attached to the back of the upper part of the manubrium sterni. The sternal attachment of sternohyoid extends across, and reinforces part of, the posterior ligament.

Interclavicular ligament

The interclavicular ligament strengthens the capsule superiorly, and is formed by fibres attaching to the upper aspect of the sternal end of one clavicle passing across the jugular notch to join similar fibres from the opposite side. Some of these fibres attach to the floor of the jugular notch.

Costoclavicular ligament

The extracapsular costoclavicular ligament is an extremely strong, short, dense band of fibres. It is attached to the upper surface of the first costal cartilage near its lateral end, and to a roughened area on the posterior aspect of the inferior surface of the medial end of the clavicle. The ligament is in two laminae, usually separated by a bursa, which are attached to the anterior and posterior lips of the clavicular rhomboid impression. The anterior fibres run upwards and laterally, while those of the posterior lamina run upwards and medially; thus the fibres have a cruciate arrangement. The direction of fibres in the two laminae is the same as those in the external and internal intercostal muscles respectively.

The costoclavicular ligament essentially limits elevation of the clavicle; however, it is also active in preventing excessive anterior or posterior movements of the medial end of the clavicle. Its position and strength compensate for the weakness of the adjacent inferior part of the joint capsule.

Blood and nerve supply

The arterial supply of the sternoclavicular joint is from branches of the internal thoracic artery, the superior thoracic branch of the axillary artery, the clavicular branch of the thoracoacromial trunk, and the suprascapular artery. Venous drainage is to the axillary and external jugular veins. Lymphatics from the joint pass to the lower deep cervical group of nodes, sometimes called the supraclavicular nodes, and thence to the jugular trunk. A few lymphatics may pass to the apical group of axillary nodes.

The nerve supply of the joint is by twigs from the medial supraclavicular nerve(C3, 4) and the nerve to subclavius(C5 and 6).

Relations

Overlying the joint anteriorly is the tendinous attachment of the sternal head of sternomastoid. Posteriorly, the sternoclavicular joint is separated from the brachiocephalic vein and common carotid artery on the left and the brachiocephalic trunk, sternohyoid and sternothyroid muscles on the right. The superior vena cava is formed on the right hand side, by the union of the two brachiocephalic veins, just below the joint at the lower border of the first costal cartilage.

On the right hand side, both the phrenic and vagus nerves lie lateral to the sternoclavicular joint as they enter the thorax from the neck. However, on the left hand side, the vagus may pass behind the joint as it descends between the common carotid and subclavian arteries.

Stability

The shape of the articular surfaces and the surrounding musculature provide only a limited amount of security for the joint. The stability of the sternoclavicular joint is primarily dependent on the strength and integrity of its ligaments, particularly the costoclavicular ligament. Unfortunately, when dislocation of the joint takes place it is liable to occur.

Movements

Although the articular surfaces do not conform at those of a ball and socket joint, the sternoclavicular joint nevertheless has three degrees of freedom of movement, that is elevation and depression, protraction and retraction, and axial rotation. The fulcrum of these movements, except axial rotation, is not at the joint centre but through the costoclavicular ligament. Consequently, the movements of elevation and depression, and protraction and retraction involve a gliding between the clavicle and the intra-articular disc, and between the disc and sternum.

Elevation and depression

The axis of rotation for elevation and depression runs horizontally and slightly obliquely, anterolaterally through the costoclavicular ligament. Some authorities have suggested that two axes of rotation can be identified for elevation and depression, one for the gliding of the clavicle with respect to the disc, and the other for gliding of the disc against the sternum. Nevertheless, functionally the combined axis of movement runs through the costoclavicular ligament.

Because the axis of movement is somewhat removed from the joint centre, as the lateral end of the clavicle moves, so its medial end moves in the opposite direction. Consequently, elevation of the lateral end of the clavicle causes the medial end to move downwards and laterally. The range of movement of the lateral end of the clavicle is approximately 10cm of elevation and 3cm of depression, giving a total angular range of movement of some 60°. Elevation is limited by tension in the costoclavicular ligament and by tone in the subclavius muscle. Depression of the clavicle, in which the medial end moves upwards and medially, is limited by tension in the interclavicular ligament and by the intra-articular disc. If these two mechanisms fail, then movement is eventually limited by contact between the clavicle and upper surface of the first rib.

Protraction and retraction

The axis of movement for protraction and retraction lies in a vertical plane running obliquely, inferolaterally through the middle part of the costoclavicular ligament. Again the two ends of the clavicle move in opposite directions because of the position of the fulcrum about which movement takes place, so that in protraction of the lateral end, the medial end moves back and vice versa. In these movements, the medial end of the clavicle and the intra-articular disc tend to move as one unit against the sternum. The range of movement of the lateral end of the clavicle is approximately 5cm of protraction(anterior movement) and 2cm of retraction(posterior movement), giving a total angular range of movement of about 35°. Movement anteriorly is limited by tension in the anterior sternoclavicular and costoclavicular ligaments, while posterior movement is limited by the posterior sternoclavicular and costoclavicular ligaments.

Axial rotation

Whereas elevation, depression, protraction and retraction of the clavicle are active movements brought about by direct muscle action, axial rotation is entirely passive, being produced by rotation of the scapula and transmitted to the clavicle by the coracoclavicular ligament. Pure axial rotation of the clavicle is not possible in the living subject; it always accompanies movements in other planes. The axis about which rotation occurs passes through the centre of the articular surfaces.

The range of movement is small when the clavicle is in the frontal plane, but increases considerably when the lateral end of the clavicle is carried backwards. The degree of axial rotation possible is between 20° and 40° depending on the position of the clavicle.

That there should be any axial rotation possible at the sternoclavicular joint is due to the relative incongruity of the articular surfaces, the presence of an intra-articular disc and the relative laxness of the capsular thickenings.

Accessory movements

With the subject lying supine, a downward pressure by the thumb on the medial end of the clavicle produces a posterior gliding of the clavicle against the sternum.

Palpation

The line of the sternoclavicular joint can be easily identified through the skin and subcutaneous tissues at the medial end of the clavicle. The projection of the medial end of the clavicle above the sternum can also be palpated.