Why does AC joint injury cause shoulder pain?

The acromioclavicular, or AC, joint connects the collarbone to the shoulder blade. It helps the shoulder girdle transfer force when you raise, push or carry with the arm. An injury may stretch the supporting ligaments or bruise the joint surfaces. The resulting inflammation can make the upper shoulder tender, especially directly over the joint.

Pain may be sharper when reaching across the body, lifting the arm above shoulder level, fastening clothing or sleeping on the affected side. A small bump or swelling over the joint can occur. Shoulder pain has several possible causes, so the pattern and examination matter more than the location alone.

Reduce activities that reproduce the pain while keeping the elbow, wrist and fingers moving. Cold wrapped in a cloth may ease swelling for short periods. Support the arm if it feels heavy, and avoid repeatedly testing the painful movement. A clinician or physiotherapist can assess the joint, check shoulder motion and guide a gradual return to lifting. Medicines for pain or inflammation may not suit everyone, particularly with stomach, kidney, bleeding or pregnancy concerns, so ask a pharmacist or clinician before using them.

Arrange an assessment if pain began after a fall or blow, if the shoulder looks misshapen, or if arm movement remains very limited. Seek urgent care for a cold or pale hand, new numbness or weakness, severe swelling, an open wound, or pain that is rapidly worsening. These features can indicate a more significant injury requiring prompt examination.

Ask whether the tenderness is centred on the AC joint, whether imaging is needed, which movements should be limited, and when strengthening can begin. You can also ask how to sleep comfortably and what changes should prompt an earlier review.