The sternoclavicular articulation is a double plane joint, the joint-cavity being subdivided by an articular disc. The parts entering into its formation are the sternal end of the clavicle, the clavicular notch of the manubrium sterni, and the cartilage of the first rib. The articular surface of the clavicle is much larger than that of the sternum, and is covered with a layer of cartilage which is considerably thicker than that on the sternum and resembles the fibrocartilage of the articular surfaces of the mandibular joint. The ligaments of this joint are; Capsular, Interclavicular, Anterior sternoclavicular, Costoclavicular, and Posterior sternoclavicular.
The capsular ligament surrounds the articulation; in front and behind, it is of considerable thickness, but above, and especially below-, it is thin and partakes more of the character of areolar than of true fibrous tissue.
The anterior sternoclavicular ligament is a broad band, covering the anterior surface of the joint; it is attached above to the upper and front part of the sternal end of the clavicle, and, passing obliquely downwards and medially, is attached below to the front of the upper part of the manubrium sterni.
The posterior sternoclavicular ligament is a weaker band which covers the posterior aspect of the joint. It is attached to the posterior aspect of the sternal end of the clavicle and passes obliquely downwards and medially to be attached to the back of the upper part of the manubrium sterni.
The costoclavicular ligament is short, flat, strong, and rhomboid in form. Attached below to the upper surface of the cartilage of the first rib, it ascends backwards and laterally, and is fixed above to the impression on the under surface of the medial end of the clavicle.
The articular disc is flat and nearly circular, and is interposed between the articulating surfaces of the sternum and clavicle. It is attached, above, to the upper and posterior border of the articular surface of the clavicle; below, to the cartilage of the first rib, near its junction with the sternum; and by the rest of its circumference to the capsular ligament. It is thicker at the circumference than at the center, and divides the joint into two cavities; each of its surfaces is clothed with synovial membrane.
The arteries supplying the joint are derived from the internal mammary and suprascapular (transverse scapular) arteries; the nerves, from the medial (anterior) supraclavicular nerves and the nerve to the subclavius muscle.
Movements.–The sternoclavicular joint is the only point of articulation of the shoulder-girdle with the trunk. It is the center on which the movements of the clavicle take place, and it must be remembered that, in nearly all its movements the clavicle carries the scapula with it. The clavicle may be moved forwards, backwards, upwards and downwards; it may also be circumducted. The movements attendant on elevation and depression of the shoulder take place between the clavicle and the articular disc. When the shoulder is moved forwards and backwards, the articular dise moves on the manubrium sterni. Elevation of the shoulder is limited principally by the costoclavicu,lar ligament. When the clavicle is forcibly depressed, as in lifting a heavy weight, it is supported by the first rib, and the upward movement of its sternal end is checked by the sternoclavicular and interclavicular ligaments, and the articular disc.
Applied Anatomy.-The strength of this joint depends upon its ligaments, and especially on the articular disc. It is owing to these, and to the fact that the force of the blow is usually transmitted along the long axis of the clavicle, that dislocation rarely occurs, and that the clavicle is broken rather than displaced. Dislocation may be either forwards, backwards or upwards. Should the clavicle be displaced backwards it may cause pressure on the trachea and great vessels of the neck. Owing to the shape of the articular surfaces, and the fact that the strength of the joint mainly depends upon the ligaments, the displacement when reduced is very liable to recur.
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