Articular disc of acromioclavicular joint

At a glance

English

Articular disc of acromioclavicular joint

Latin (TA2)

Discus articulationis acromioclavicularis

System

Cartilage

Region

Upper limb

Side

Paired, left and right

About the articular disc of acromioclavicular joint

The acromioclavicular joint links the flat lateral end of the clavicle with the medial edge of the acromion, the bony shelf at the top of the shoulder blade. It is a small plane synovial joint, and its two surfaces are covered by fibrocartilage rather than hyaline cartilage. Within it there is often an intra-articular plate of fibrocartilage, the articular disc, which hangs down into the joint from the upper part of the capsule.

The disc is very variable. In many people it is only a wedge-shaped blade projecting partway, sometimes about halfway, into the joint from above; in fewer it forms a complete partition, splitting the joint into two separate cavities. Its upper edge is attached to the superior acromioclavicular ligament, the thickened top of the capsule, and its lower edge may reach the inferior acromioclavicular ligament. It tends to wear and break up from early adult life, and by later middle age it may be little more than fragments.

The disc helps match the two flat, often slightly uneven bone surfaces and spreads load across the joint, which transmits forces from the arm to the clavicle and so to the axial skeleton. The joint itself allows small gliding and rotation of the scapula on the clavicle as the arm is raised. Its stability comes mostly from the capsule and acromioclavicular ligaments, which resist horizontal movement, and from the strong coracoclavicular ligament, made of the conoid and trapezoid parts, which resists the scapula dropping away from the clavicle.

Because the disc degenerates early, the acromioclavicular joint is a common site of osteoarthritis, which causes pain on top of the shoulder that worsens when the arm is reached across the body. A fall onto the point of the shoulder can tear the joint ligaments, producing an acromioclavicular separation in which the clavicle's end rides up; the disc may be torn in the same injury. Painful arthritis here is sometimes treated by removing the end of the clavicle together with the disc remnants.

Attachments and function

Attaches from

Hangs from the superior acromioclavicular ligament at the upper part of the joint capsule.

Attaches to

Projects down between the lateral end of the clavicle and the acromion, sometimes reaching the inferior acromioclavicular ligament to divide the joint completely.

Function

Improves the fit of the two joint surfaces and spreads load between the clavicle and scapula during shoulder movement.

Nerve supply

The joint is supplied by the suprascapular and lateral pectoral nerves.

Blood supply

Branches of the suprascapular artery and the acromial branch of the thoraco-acromial artery supply the capsule.