The Acromio-Clavicular joint is directed from before backwards, and slopes from above, down, and inwards, the outer end of the clavicle projecting to a varying extent above the upper surface of the acromion upon which it rests. An incomplete interarticular cartilage generally exists, and there is a capsular ligament best developed above and below.

The coraco-clavicular attachment Consists of two ligaments, the conoid and trapezoid, which extend from the posterior portion of the upper surface of the coracoid process to the conoid tubercle and trapezoid ridge respectively on the under surface of the clavicle. A synovial bursa frequently exists between these two ligaments. These ligaments chiefly limit the movement of the outer end of the clavicle, which is fixed to and must move with the scapula. When the scapula moves, it carries with it the clavicle, which in turn moves on the sterno-clavicular joint. But for the clavicle, the scapula, in gliding round the chest-wall under the action of the serratus magnus and other muscles, would tend to lie close to that wall, and thus, when the scapula came forward on the chest, its glenoid cavity, instead of being at right angles to the long axis of the arm, would be directed forwards and inwards, so that if the arm were used to strike a blow, the head of the humerus would rest on the posterior part of the capsule, instead of in the glenoid cavity, and thus dislocation would probably occur. This tendency for the scapula to rotate on coming forwards is prevented by the clavicle, and as it does so the acromioclavicular joint comes into play, arid the angle between the scapula and clavicle diminishes. Owing to the shape of this joint, dislocation, when it occurs, is generally upwards and frequently partial. A few cases of downward dislocation are recorded. The displacements are easily reduced, but difficult to retain in position.

The clavicle, owing to its superficial and exposed position, its reception of all shocks communicated to the upper extremity, its slenderness and early ossification, is more frequently fractured than any other bone, the fracture most frequently occurring at the junction of the middle and outer thirds. The fracture occurs here, as this is the most slender part of the bone ; the fixed outer third meets the more movable inner two-thirds, and the two curves meet at this point. The fracture is generally due to indirect violence, is oblique, and runs from above, downwards, and inwards, the inner fragment maintaining its normal position, or having its outer end slightly raised by the clavicular portion of the sterno-mastoid. The outer fragment is (a) depressed by the weight of the limb, aided by the pectorals and latissimus dorsi ; (b) drawn inwards by the same muscles, assisted by the trapezius, rhomboids, levator anguli scapulae, and subclavius ; (c) rotated on a vertical axis, so that the broken end looks inwards and backwards, by the serratus magnus acting on the scapula, assisted by the pectorals.

The rotation is generally difficult to remedy, and shortening frequently results. A recumbent position, with a narrow pillow between the shoulders, helps in reducing the deformity. Fracture due to direct violence may occur at any part of the bone, and is generally transverse. When about the middle third, the displacement is as above ; when between the conoid and trapezoid ligaments, there is no displacement ; when external to these ligaments, the inner end of the outer fragment is raised by the trapezius, while the outer end is drawn forwards by the serratus magnus and pectorals. In fracture at the inner extremity, the inner end of the outer fragment is displaced down and inwards, simulating dislocation. The clavicle has been fractured by muscular violence, generally in the middle third, probably due to the action of the deltoid and clavicular portion of the pectoralis major. It is the most frequent seat of greenstick fracture, owing to its early ossification (the entire shaft being bony at birth), and thick but lax periosteum, half the cases of fractured clavicle occurring before five years of age. The cords of the brachial plexus, the subclavian artery and vein, the acromio-thoracic artery and internal jugular vein, and even the lung may be injured in fractures of the clavicle. The biceps, brachialis anticus, and supinator longus are supplied by the upper cord of the plexus, and may be paralyzed by a blow on the shoulder (which may subsequently fracture the clavicle), or may be affected by the carrying of heavy weights on the shoulder.