Your shoulder has three bones that come together to create two separate joints. The acromioclavicular (AC) joint is formed where your collarbone (clavicle) meets the bony tip of your shoulder blade (acromion). The glenohumeral joint forms where the head of your upper arm bone (humerus) fits into your shoulder blade (scapula).
In shoulder osteoarthritis (OA) – also called degenerative joint disease – your cartilage and other joint tissues gradually break down. Friction in the joint increases, pain increases, and you slowly lose mobility and function. Shoulder OA is not as common as OA of the hip or knee, but it is estimated that nearly 1 in 3 people over the age of 60 have shoulder OA to some degree. OA is more common in the AC joint than the glenohumeral joint.