This is a synovial joint between the overhanging lateral end of the clavicle and the under-hanging medial border of the acromion. The articulating surfaces are covered (like those of the sternoclavicular joint) by fibrocartilage (so it is an atypical synovial joint).
Ligaments of acromioclavicular joint:
A sleeve like capsule surrounds the articular surfaces; it is not strong, but on top there is a thickening of fibres which constitutes the acromioclavicular ligament. An incomplete disc of fibrocartilage hangs down into the upper part of the joint cavity.
The coracoclavicular ligament, extremely strong, is the principal factor in providing stability to the joint. It consists of two parts, conoid and trapezoid. Examine the clavicle and scapula. The conoid ligament, an inverted cone, extends upwards from the knuckle of the coracoid to a wider attachment around the conoid tubercle, on the under surface of the clavicle. The trapezoid ligament is attached to the ridge of the same name on the upper surface of the coracoid process and extends laterally, in an almost horizontal plane, to the trapezoid ridge on the under surface of the clavicle.
Nerve supply of acromioclavicular joint:
Lateral supraclavicular nerves (C4) from the cervical plexus.
Movements of acromioclavicular joint:
These are passive; no muscle connects the bones to move the joint. Muscles which move the scapula cause it to move on the clavicle. Scapular movements on the chest wall fall into three groups:
- Protraction and retraction around the chest wall
- Elevation or depression.
These basic movements can be combined in varying proportions, and each of these transmits, through ligaments, corresponding movements to the clavicle. All movements of the scapula involve movements in the joint at either end of the clavicle.
Horizontally, in protraction and retraction of the tip of the shoulder, the scapula hugs the thoracic wall, held to it by serratus anterior and pectoralis minor. The scapula moves in a circle of a shorter radius (i.e. the radius of the upper thorax) than the length of the clavicle. Hence movement takes place between the acromion and the fibrocartilage. The axis of this movement is vertical and passes through the conoid ligament. The acromion glides to and fro on the tip of the clavicle.
In abduction of the arm the scapula does not retain its position relative to the clavicle but rotates around the conoid ligament as it swings forwards on the chest wall, and movement takes place between the fibrocartilage and the clavicle. The axis of scapular rotation passes through the conoid ligament and the acromioclavicular joint; the scapula swings to and fro like a pendulum below these two fixed points on the clavicle. The total range of scapular rotation on the chest wall is about 60°, but only 20° of this occurs between the scapula and the clavicle. The coracoclavicular ligaments are then taut, and transmit the rotating force to the clavicle, whose rotation then accounts for the remainder of scapular rotation on the chest wall. In both movements at this joint the fulcrum around which the scapula swings is the coracoclavicular ligament.
Elevation (shrugging the shoulders) is produced by the upper fibers of trapezius together with levator scapulae and the rhomboids, mutually neutralizing their rotatory effects. Depression of the scapula is produced by the lower fibres of trapezius and the lateral fibers of latissimus dorsi. Elevation and depression move the medial end of the clavicle (see above), but they scarcely move the acromioclavicular joint.
Stability of acromioclavicular joint:
The stability of the joint is provided by the coracoclavicular ligament. The scapula and upper limb hang suspended from the clavicle by the conoid ligament (assisted by the deltoid, biceps and triceps muscles). Forces transmitted medially from the upper limb to the glenoid cavity are transmitted from scapula to clavicle by the trapezoid ligament and from clavicle to first rib by the costoclavicular ligament. Thus a fall on outstretched hand or elbow puts no strain on either end of the clavicle at the joints. If the clavicle fractures as a result, it always does so between these ligaments. Falls on the shoulder may dislocate the acromioclavicular joint, forcing the acromion under the clavicle and tearing the coracoclavicular ligament.