A-C joint arthritis is a pain on the outer aspect of the shoulder. It results from wear and tear changes in the joint.
Dislocation of the A-C joint occurs frequently after a fall onto the point of the shoulder such as being dumped in the surf or tackled at football. The force of impact drives the scapula (shoulder blade) into the clavicle which pops up and ruptures the supporting ligaments under the clavicle.
This is usually associated with significant pain at the moment of injury and there may be an associated bump on the tip of the shoulder due to the upward movement of the clavicle.
A-C joint dislocations are easy to see clinically and are confirmed with a plain x-ray.
Generally, it is treated non-operatively and settles well. Sometimes there is mild instability and as a result the joint wears out and becomes very painful. When the pain is a problem it is relieved by removing the outer 8 mm of the clavicle. While this seems excessive, it is generally very effective and in fact makes the shoulder look more normal and symmetrical. There is no prolonged rehabilitation after this surgery and pain relief is reliable.