An important cause of pain involving the acromioclavicular joint is distal clavicular osteolysis, particularly post trauma or repetitive microtrauma associated with sports like weight lifting. It may also be asymptomatic.
On radiographs it is seen as an area of osteopenia in the distal end of the clavicle, usually in the distal 2cm of the clavicle. There is loss of the articular cortical margin 1. In the early stages, x-rays may show osteopenia of the clavicle, soft-tissue swelling and loss of the distal subarticular cortex 2.
It can be due to trauma and traumatic joint separation. These are typically unilateral. Other causes may include gout or multiple myeloma. It can also be due to repetitive trauma such as in weight lifters and can be unilateral or bilateral.
Bilateral causes include rheumatoid arthritis, scleroderma and hyperparathyroidism.