The clavicle articulates with the acromian part of the scapula to form the acromio-clavicular joint. This joint is held congruence by the acromio-clavicular ligament and the coracoclavicular ligament, which rises from the coracoid part of the scapula. Injuries to this joint usually arise from a fall onto the shoulder, resulting in the clavicle separating from the acromian. The levels of pain and functional limitation depend on where both ligaments are involved and whether the ligaments themselves are torn, or just stretched. The degree of separation can be measured with a simple X-ray of the shoulder joint, which is often required anyway to check for associated fractures of the clavicle. Ultrasound scans can also be used here. In cases of mild lesions, there is usually only minor swelling of the joint and only partial separation of the two joint surfaces. To measure severity, the injury is graded using the Rockwood scale. This scale ranges from Grade I through Grade VI, increasing in severity with each increment. Typically, most AC injuries are between Grade I and Grade III.

Grade I:
The most common type of AC joint injury is a slight displacement of the joint. The AC ligament may be stretched or partially torn. Patients may experience mild tenderness when the joint is touched, and minor pain with arm movement.

Grade II:
The joint has become partially dislocated, and the AC ligament is completely torn. Pain in the joint is moderate to severe at this level. A small bump on the top of the shoulder may also be visible as the collarbone is no longer properly secured in place.

Grade III:
A complete displacement is evident, as the shoulder falls under the weight of the arm. This forces the clavicle higher, causing a noticeable bump or deformity on the shoulder. Additionally, the AC ligament, coracoclavicular (CC) ligament, and the capsule that surrounds the joint are all torn. Pain is felt upon any arm movement, and is also experienced around the joint and in the area of the CC ligaments.

Injuries with Grades IV to VI are commonly associated with extreme an incident such as a motor vehicle accident.

Treatment of these mild injuries consists of tight strapping of the joint to ensure the ligament fibres remain in place to aid their healing. A period of rehabilitation after this immobilization is important to relieve any stiffness which may have formed in the joint as efficient movement of the shoulder requires a fully functioning acromioclavicular joint. In more complex injuries where the ligaments have completely torn, the clavicle completely separates from the acromian. In these cases, the pain can be particularly intense and surgical intervention is required to restore the continuity of the joint.