Acromioclavicular arthritis is a degenerative disease characterized by painful inflammation of the acromioclavicular (AC) joint, the joint where the clavicle, or collarbone, meets the acromion process on the top of the shoulder blade. A type of synovial, or movable, joint known as a gliding joint, the AC joint contains fibrous cartilage intended to pad the bones against one another. With time, repetitive movements, and injury, this cartilage can wear down, resulting in pain and joint stiffness during shoulder movement. Acromioclavicular arthritis pain may be managed with therapy and anti-inflammatory medications, but moderate to severe cases of arthritis may require surgery.
The human shoulder is in fact two separate joints: the glenohumeral and acromioclavicular joints. The glenohumeral joint is the ball-and-socket joint formed by the humerus or arm bone and a cavity on the scapula or shoulder blade called the glenoid fossa. Just above the glenohumeral joint is the AC joint, which is made up of the lateral or outside end of the clavicle and the acromion process of the scapula. The latter is a projection of the shoulder blade that juts upward and outward from the scapular spine on the posterior side of the bone and curves forward over the top of the glenohumeral joint. On the acromion process’s inside surface, the end of the clavicle articulates with the shoulder blade, the two bones separated only — and sometimes not at all — by a thin layer of cartilage much like the menisci of the knee joint.
Together, these two joints work to facilitate movement of the arm during such motions as raising and lowering the arms to either side of the body and pressing overhead. Since the acromioclavicular joint is a gliding joint, it allows the clavicle to slide and rotate against the scapula so that the shoulder blade and therefore the arm can move about freely. Over time, however, repetitive arm-lifting motions can wear down the cartilage between the bones. Like the osteoarthritis that afflicts the lower back and knees, excessive loading of the joint coupled with age are major factors leading to acromioclavicular arthritis.
Acromioclavicular arthritis is also more likely to occur in people who have suffered a blunt force trauma to the joint, such as a fall with the arm outstretched. Whether the damage to the tissue in the joint is caused by gradual wear and tear or an acute injury, the symptoms are the same. Pain may be experienced during particular movements, such as drawing the arm horizontally inward so that it crosses the chest, or on movement following long periods of inactivity. The front aspect of the shoulder may feel stiff or inflamed, and the range of motion may feel limited. To treat acromioclavicular arthritis, doctors may recommend anti-inflammatory treatments like ice, ibuprofen, and stretching for mild cases and surgery in more severe cases to remove the end of the clavicle bone and therefore eliminate bone-on-bone contact.