Overview

The small, oval articular facets are covered by fibrocartilage, sometimes with a wedge-shaped incomplete articular disc. The joint permits gliding and rotation of the scapula on the clavicle (≈20–30°) during arm elevation. The weak capsule is reinforced superiorly by the acromioclavicular ligament (horizontal stability). Vertical stability depends on the strong extrinsic coracoclavicular ligament, with its medial conoid and lateral trapezoid parts, which suspends the scapula and upper limb from the clavicle and transmits forces from the arm to the clavicle.

The AC joint lies at the top of the shoulder and is easily palpated as a small step lateral to the end of the clavicle.

Location: Top of the shoulder, between the lateral clavicle and the acromion.

Anatomy

Innervation: Suprascapular, lateral pectoral and axillary nerves.
Articulations: Acromial end of the clavicle ↔ acromion of the scapula.
Ligaments: Acromioclavicular ligament (capsular, horizontal stability); coracoclavicular ligament — conoid and trapezoid parts (vertical stability, main suspensory ligament of the upper limb).

Function

Allows the scapula to rotate and glide on the clavicle during arm movements
Transmits forces from the upper limb to the clavicle

Clinical significance

AC joint separation ("shoulder separation")

A fall onto the point of the shoulder (rugby, cycling) drives the acromion downward. Graded (Rockwood I–VI): AC ligament sprain only (I), AC rupture with intact coracoclavicular ligament (II), and rupture of both (III and above) — the lateral clavicle appears prominent ("step deformity") because the shoulder drops. Most low grades are treated non-operatively.

AC joint osteoarthritis

Common in older adults and weightlifters; pain localised to the top of the shoulder on cross-body adduction; inferior osteophytes may contribute to rotator cuff impingement.

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.