The acromioclavicular articulation (fig. 530) is a plane joint between the acromial end of the clavicle and the medial margin of the acromion of the scapula. The articular surface of the acromial end of the clavicle is covered with fibrocartilage, and forms a narrow, oval area, which is directed downwards and laterally so as to overlap the corresponding area on the medial border of the acromion. The long axis of the joint lies in an anteroposterior plane. Its ligaments are: Capsular, Coracoclavicular (Trapezoid & Conoid part), and Acromioclavicular
The capsular ligament completely surrounds the articular margins, and is strengthened above by the acromioclavicular ligament.
The acromioclavicular ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the acromial end of the clavicle and the adjoining part of the upper surface of the acromion; it is composed of parallel fibers, which interlace with the aponeuroses of the trapezius and deltoid muscles.
An articular disc is found sometimes in this joint; when present, it generally occupies the upper part of the articulation, and only partially separates the articular surfaces. More rarely, it divides the joint completely into two cavities.
The coracoclavicular ligament (fig. 527) connects the clavicle with the coracoid process of the scapula. It does not properly belong to the acromioclavicular articulation, but is usually described with it since it forms a most efficient means of retaining the clavicle in contact with the acromion. It consists of two parts, viz. trapezoid and conoid, which are usually separated by a bursa.
The trapezoid part, which forms the anterior and lateral fasciculus, is broad, thin, and quadrilateral. It is attached, below, to the upper surface of the coracoid process; above, to the trapezoid line on the under surface of the clavicle. Its anterior border is free; its posterior is joined with the conoid part, the two forming, by their junction, an angle projecting backwards.
The conoid part, which forms the posterior and medial fasciculus, is a dense band of fibers, conical in form, with its base directed upwards. Its apex is attached to a rough impression at the junction of the root with the horizontal portion of the coracoid process, medial to the trapezoid part; its base is fixed to the conoid tubercle on the under surface of the clavicle, and to a line proceeding medially from it for 1.25 cm.
The arteries supplying the joint are derived from the suprascapular (transverse scapular) and acromiothoracic arteries; the nerve is a branch of the suprascapular nerve.
Movements.-The movements of this articulation are of two kinds : (1) a gliding motion of the articular end of the clavicle on the acromion; (2) rotation of the scapula forwards and backwards upon the clavicle. The extent of this rotation is limited by the two portions of the coracoclavicular ligament, the trapezoid limiting rotation forwards, and the conoid rotation backwards.
The acromioclavicular joint has important. functions in the movements of the upper limb. Humphry* pointed out that if there had been no joint between the clavicle amid scapula, the circular movement of the scapula on the ribs (as in throwing the shoulders backwards or forwards) would have been attended with a greater alteration in the direction of the shoulder than is consistent with the free use of the arm in such positions, and it would have been impossible to give a blow straight forwards with the full force of the arm : that is to say, with the combined force of the scapula, arm, and forearm. ” This joint,” as he happily says, ” is so adjusted as to enable either bone to turn in a hinge-like manner upon a vertical axis drawn through the other, and it permits the surfaces of the scapula, like the baskets in a roundabout swing, to look the same way in every position, or nearly so.” Again; when the whole arch formed by the clavicle and scapula rises and falls (in elevation or depression of the shoulder), the joint between these two bones enables the scapula still to maintain its lower part in contact with the ribs.
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