Not too rare. The most common problem affecting this joint (called the sternoclavicular joint or SCJ) is osteoarthritis. Degenerative changes from aging make this a disease of the older adult.
Studies show more than half of all adults age 60 or older have moderate to severe arthritic changes in the SCJ. Part of this may be due to the fact that the SCJ is the only place where the bones attach the arms to the main skeleton. Anyone with a history of manual labor or overuse of the arms is at risk for this condition.
Some have painful symptoms, while others do not. Treatment is only needed when pain and loss of motion occur. Surgery is rarely needed. Most patients do well with rest, antiinflammatory drugs, or local steroid injection.