Articular surfaces and ligaments
The AC joint is a plane synovial joint between the small oval facet on the lateral end of the clavicle and a matching facet on the medial margin of the acromion. Both surfaces are covered by fibrocartilage rather than hyaline cartilage, because the clavicle forms by intramembranous ossification. A wedge-shaped articular disc often hangs down from the upper capsule into the joint, and it commonly degenerates from middle age.
Acromioclavicular ligament
The capsule is thin and is thickened superiorly and inferiorly as the acromioclavicular ligament. Its superior part is the stronger, and it is reinforced by fibres from the attachments of deltoid and trapezius. It controls horizontal (anteroposterior) stability.
Coracoclavicular ligament
The coracoclavicular ligament lies medial to the joint and is not part of its capsule, but it is its main stabiliser. It runs from the coracoid process up to the undersurface of the clavicle in two parts:
| Part | Shape and position | Clavicular attachment |
|---|---|---|
| Conoid ligament | Inverted cone, medial and posterior | Conoid tubercle |
| Trapezoid ligament | Flat quadrilateral sheet, lateral and anterior | Trapezoid line |
The coracoclavicular ligament gives vertical stability. It suspends the scapula from the clavicle and transmits weight from the upper limb to the clavicle, and from there through the sternoclavicular joint to the axial skeleton.
Movements
The AC joint allows small gliding and rotational movements that let the scapula move on the clavicle. These accompany protraction, retraction, elevation, depression and upward rotation of the scapula. As the arm is raised overhead, the clavicle rotates on its long axis and the AC joint accommodates the change in angle between clavicle and scapula. No muscle acts directly on the joint; it moves passively with the muscles that move the scapula.
Relations, blood supply and innervation
The AC joint lies directly under the skin at the top of the shoulder, which makes it easy to palpate and inject. Deltoid attaches in front and lateral to it, and trapezius behind and medial to it. The coracoacromial ligament runs from the coracoid process to the acromion just in front of the joint, forming part of the arch above supraspinatus.
The joint is supplied by the suprascapular artery and the acromial branch of the thoracoacromial artery. Its nerves come from the suprascapular, lateral pectoral and axillary nerves, which explains why AC joint pain is felt around the top of the shoulder.
Clinical relevance
AC joint injury, often called shoulder separation, follows a fall or tackle onto the point of the shoulder that drives the acromion downwards away from the clavicle. The injury progresses from ligament sprain to complete disruption, and the Rockwood classification grades it I to VI.
- Grade I: AC ligament sprain, no displacement.
- Grade II: AC ligament torn, coracoclavicular ligament intact, partial displacement.
- Grade III: both ligament complexes torn, with an obvious step between clavicle and acromion.
- Grades IV to VI: complete injuries with posterior displacement through trapezius (IV), gross superior displacement with deltotrapezial fascia stripped (V), or inferior displacement below the coracoid (VI).
Lower grades are managed without surgery. Grades IV to VI are usually reconstructed, and grade III is debated. The joint is also a common site of osteoarthritis, which causes pain on cross-body adduction of the arm. Osteolysis of the distal clavicle is seen in weightlifters. On radiographs a Zanca view, angled upwards, shows the joint clear of the spine of the scapula.