The shoulder girdle is formed by the clavicle and scapula, the clavicle alone being articulated to the trunk at the sterno-clavicular articulation. A few cases of avulsion of the entire upper limb have been recorded, the only joint involved being the small sterno-clavicular one. The skin over the clavicle is freely movable, and hence frequently escapes wounding and penetration from fracture, while injury is frequently accompanied by severe pain from implication of the supraclavicular nerves, which may also rarely be caught in callus after fracture, and so cause persistent pain. On the other hand, pain over the clavicle may be caused by disease of the upper cervical spine, the pain being referred along the supraclavicular branches of the third and fourth cervical nerves. In addition to the skin, fascia, and superficial nerves, the anterior surface of the clavicle is covered by platysma fibres, and occasionally is crossed by the cephalic vein, or a branch from it to the external jugular. The innominate vein lies behind the sternal end of the clavicle, and also the bifurcation of the innominate artery on the right, and the common carotid on the left. These structures are separated from the bone by the sterno-hyoid and thyroid muscles. The subclavian vessels (with the vein most internal) and cords of the brachial plexus lie behind the inner or greater curve of the clavicle, frorn which they are separated by the subclavius muscle and axillary sheąth. Owing to its position and the weakness of its wall, the vein is most readily pressed on by tumour or fracture, the subclavius muscle, however, saving it in many instances. In addition to the structures mentioned, the suprascapular and internal mammary arteries, the external jugular veins, the phrenic and posterior thoracic nerve (to serratus magnus), omo-hyoid and scalene muscles, and apex of lung lie behind the clavicle.
The surfaces of both clavicle and sternum are covered with fibro-cartilage at the sterno-clavicular articulation, and a disc of fibro-cartilage intervenes, which is attached above to the clavicle, and below to the sternum, and generally divides the joint into two distinct synovial compartments.
The capsular ligament is attached both to the clavicle and sternum, and the margin of the plate of fibro-cartilage, and is well defined arteriorly and posteriorly.
A strong ligamentous band-the interclavicular ligament- extends from the upper border of the articular facet of one clavicle to the other, over the sternal notch, into which many fibres are inserted ; and the rhomboid ligament consists of fibres directed upwards and outwards from the first costal cartilage to the under surface of the clavicle. To these two latter accessory ligaments the strength of the articulation is mainly due. The joint permits of limited movement in nearly all directions, and, when diseased, abduction of the arm is particularly painful, as the joint surfaces are then brought most nearly into apposition. Forward movement is checked by the posterior ligament, assisted by the anterior ; backward movement by the anterior, assisted by the posterior and the rhomboid ; upward by the rhomboid, interclavicular, and interarticular cartilage.
Dislocation of the sternal end of the clavicle is not common, that forwards being the most frequent, and followed by those backwards and upwards, the latter being very uncommon. In the forward variety the head of the bone lies in front of the manubrium, and carries the sterno-mastoid forwards. In the backward dislocation the rhomboid ligament is torn in addition to the capsule, and the head lies between the manubrium and the sterno-hyoid and thyroid muscles, sometimes pressing on the trachea or oesophagus, causing dyspnoea or dysphagia, or on the subclavian artery or innominate vein. In the upward variety the head lies on the manubrium between the sternomastoid and sterno-hyoid muscles. The dislocations are generally easily reduced, but difficult to retain in position. This joint is said to be more frequently involved in pycernia than any other. The pus generally presents anteriorly, though it may burst posteriorly, and so enter the mediastinum, and the condition is not followed by anchylosis, owing to the character of the joint and the constant movement. Not merely does the clavicle supply the only articulation between the upper limb and the trunk, it also plays an important part in regulating the position of the shoulder in relation to the chest-wall, and gives attachment to important muscles both of the neck and of the upper extremity. Before discussing its regulating power, it will be necessary to consider the connections between the clavicle and scapula. These consist of two sets - acromioclavicular articulation and coraco-clavicular ligaments.