AC Joint Arthritis

The acromioclavicular joint, or AC joint, is the small joint at the top of the shoulder where the outer end of the collarbone meets the acromion, the bony projection of the shoulder blade. It is roughly the size of a fingertip, and it is stabilized by a capsule and by strong ligaments running from the collarbone down to the coracoid process below. Unlike the ball-and-socket joint, the AC joint moves much less, but it transmits load between the arm and the chest with nearly every push, press, and overhead reach.

A thin disc of cartilage sits between the two bone surfaces. With age and use, that disc degenerates and the cartilage covering each surface thins, allowing the bone ends to contact each other. This produces pain, tenderness directly over the joint, and bone spurs that can press downward onto the rotator cuff.

AC joint arthritis is common on imaging and frequently causes no symptoms at all, which is precisely why it is over-diagnosed. Degenerative changes appear on shoulder MRI in the majority of adults past middle age. The relevant question is not whether the joint looks arthritic, but whether it is the structure generating the patient’s pain.

• Age-related wear: The cartilage disc within the joint degenerates predictably with time, beginning as early as the second and third decades of life.
• Prior AC joint separation: A previous shoulder separation, typically from a fall onto the point of the shoulder or a contact injury in sport, frequently leads to arthritis of the joint years later.
• Distal clavicle osteolysis: A distinct pattern seen in weightlifters, in which repetitive heavy loading, particularly bench press, dips, and overhead pressing, causes the end of the collarbone to resorb. It is often called “weightlifter’s shoulder” and affects a considerably younger group of patients.
• Repetitive overhead or heavy work: Occupations and sports involving sustained pressing or carrying accelerate wear.
• Inflammatory arthritis: Rheumatoid and other inflammatory conditions can involve the AC joint, often alongside other joints.
• Prior distal clavicle fracture: A fracture near the outer end of the collarbone can leave the joint incongruent and prone to arthritis.

• Pain at the top of the shoulder: Patients can usually place one finger directly on the sore spot, which is a useful distinguishing feature from rotator cuff pain that is felt more diffusely over the outer arm.
• Pain with reaching across the body: Discomfort when bringing the arm across the chest, such as reaching for a seatbelt or washing the opposite shoulder.
• Pain with pressing and overhead lifting: Bench press, push-ups, dips, and overhead pressing commonly reproduce the symptoms, particularly at the bottom of the movement.
• Night pain: Difficulty sleeping on the affected side, as the weight of the body compresses the joint.
• A visible or palpable bump: Prominence over the joint from bone spur formation or from a previous separation.
• Pain with reaching behind the back: Discomfort when the arm is brought backward and across, as in putting on a jacket.

Diagnosis begins with a thorough evaluation by Dr. Rynecki Baker. The history covers prior shoulder injuries or separations, training and lifting habits, and precisely where the pain is felt. On examination she localizes tenderness directly over the joint, performs the cross-body adduction test, and assesses the rotator cuff, biceps tendon, and neck, because AC joint arthritis very commonly coexists with rotator cuff disease and impingement. Establishing which structure is driving the symptoms is the central task, as treating an incidental finding leaves the patient no better.

With regards to imaging. X-rays are the first step. Dr. Rynecki Baker will obtain a dedicated view of the AC joint, angled to avoid overlap with the rest of the shoulder, which may demonstrate joint space narrowing, spur formation, and resorption of the end of the collarbone in weightlifters. An MRI assesses the joint itself along with the rotator cuff and biceps tendon. Swelling within the bone on either side of the joint is a more meaningful finding than degenerative change alone. Injections can be a very helpful diagnostic aid. A small injection of local anesthetic directly into the joint is often the single most valuable test. Relief of pain confirms the joint as the source; a lack of relief redirects the workup, and Dr. Rynecki Baker frequently uses this step before recommending any surgical treatment.

Arthritis of the AC joint sits at the center of one of the most common diagnostic errors in shoulder care. It appears on nearly every MRI in patients past middle age, which means it is often blamed for pain that is coming from the rotator cuff, the biceps tendon, or the neck, and patients undergo procedures that address a finding rather than their symptoms. The reverse also happens: genuine AC joint pain in a young lifter is dismissed as a strain for years.

Dr. Rynecki Baker’s training at NYU Langone Health and Hospital for Special Surgery, her dedicated focus on shoulder surgery and sports medicine, and her published research in shoulder surgery mean patients are evaluated by a specialist who works out the source of the pain before treating it, and who uses a diagnostic injection rather than an assumption when the picture is unclear.