|Function:||Movement of the shoulder girdle|
|Arterial supply:||suprascapular artery, thoracoacromial artery|
|Innervation:||suprascapular nerve, lateral pectoral nerve|
|Search for Acromioclavicular joint in Gray's.|
The acromioclavicular joint (AC joint) is a atypical synovial plane joint between the lateral end of the clavicle and the medial surface of the acromion of the scapula. It helps maintain the distance between scapula and trunk, and is involved in the movements of the shoulder girdle.
The acromion can be felt as the most lateral portion of the scapular spine, lying immediately above the swelling of the deltoid muscle. The anterior surface of the clavicle is easily felt bilaterally at the anterior portion of the base of the neck. The acromioclavicular joint lies between the two, palpable as a slight depression at the lateral end of the clavicle.
The acromioclavicular joint is an articulation between the flat and inferiorly angulated lateral end of the clavicle, oval in shape, and the medial surface of the acromion, which bears a similarly shaped facet. The articular cartilages are made of fibrocartilage, making it an atypical synovial joint. The fibrous capsule completely surrounds the joint, and is strengthened superiorly by the acromioclavicular ligament. An articular disc occurs in the upper part of the joint only, slightly separating the articular surfaces.
Stability is provided mainly by the coracoclavicular ligament, the scapula hanging suspended from the clavicle in the anatomical position from the coracoacromial ligament (mainly the conoid portion).
The joint is a gliding joint, but functionally acts as a pivot, meaning that the clavicle can be a strut facilitating scapular movement. Movements of the joint relate to scapular movements:
- In protraction and retraction of the scapula, the acromion glides horizontally across the tip of the clavicle
- In abduction of the arm, the coracoacromial ligament becomes taut after about 20 degrees of scapular rotation. The clavicle then is able to rotate another 40 degrees to provide aditional movement. Rotation is limited by the fibrous capsule of the joint.
- In elevation and depression there is minimal movement at the joint.
Muscles acting at the joint
All movements of the acromioclavicular joint are passive, the relvant muscles inserting into the body of the scapula to cause it to move. See shoulder girdle for details.
The articular disc of the joint is variably present, ranging from a small disc separating the surfaces to a disc completely dividing the joint into two cavities.
Either the clavicular or acromial joint surface may be concave, and if this is the case the other will be reciprocally convex.