The acromioclavicular (AC) joint is located at the top of the shoulder where the collarbone (clavicle) meets the shoulder blade (acromion). AC joint arthritis is a degenerative condition involving the gradual breakdown of the cartilage within this small but functionally important joint, causing pain and tenderness at the top of the shoulder.
AC joint arthritis is one of the most frequently overlooked contributors to shoulder pain because its symptoms can overlap with those of subacromial impingement syndrome and rotator cuff tears. Inferior bone spurs from the AC joint can directly narrow the subacromial space and mechanically irritate the underlying rotator cuff tendons — making AC joint arthritis both a condition in its own right and an aggravating factor in neighboring pathology. It is also a common consequence of prior AC joint separation, even when the original injury was considered mild.
Causes
Age-related degeneration.
AC joint arthritis is extremely common and frequently begins in early adulthood — earlier than most patients expect. The joint's cartilage is thin (typically only 3–4 mm) and subject to significant compressive and shear loading with everyday shoulder use. Degenerative changes including cartilage loss, distal clavicle cyst formation, and bone spur development are often visible on imaging by the fourth decade of life, even in the absence of symptoms. This means that incidental AC joint degeneration found on MRI ordered for another reason must be carefully correlated with the clinical examination before being attributed as the source of pain.
Post-traumatic arthritis.
Prior AC joint injuries — including separations and direct blows to the joint — disrupt the joint's cartilage and ligamentous anatomy, accelerating degenerative breakdown and predisposing the joint to earlier and more symptomatic arthritis. Overhead athletes such as weightlifters, rugby players, and throwing athletes, as well as heavy laborers who place repeated compressive stress across the AC joint, are at elevated risk for earlier-onset symptomatic degeneration.
Symptoms
Pain localized to the very top of the shoulder is the defining feature of AC joint arthritis, and patients can often point to the exact spot with one finger — a useful clinical clue distinguishing it from the more diffuse, lateral shoulder pain of rotator cuff pathology. The pain worsens with cross-body movements (reaching across the chest), overhead pressing exercises, and sleeping on the affected side. Bench pressing and push-ups are particularly provocative in active patients. In advanced cases, a visible step-off or prominence may be apparent at the joint from osteophyte formation, and inferior bone spurs may directly contribute to subacromial impingement on the underlying rotator cuff, producing a mixed pain pattern that can be diagnostically challenging.
Diagnosis
A physical exam by a doctor typically reveals exquisite point tenderness directly over the AC joint — easily identified as the bony prominence at the very top of the shoulder. The cross-body adduction test (pulling the arm across the chest) reliably reproduces AC joint pain and helps distinguish it from glenohumeral or subacromial sources. An AC joint lidocaine injection that abolishes the pain provides strong diagnostic confirmation. X-rays demonstrate joint space narrowing, osteophyte formation, and distal clavicle cystic changes. An MRI provides critical additional detail: the degree of cartilage loss, presence of intra-articular and periarticular fluid, bone marrow edema within the distal clavicle or acromion, inferior bone spur size and subacromial encroachment, and the condition of the surrounding acromioclavicular and coracoclavicular ligaments. MRI also simultaneously evaluates the rotator cuff for associated tendon pathology that may be contributing to the overall pain picture — a particularly important assessment when surgical planning involves addressing multiple shoulder pathologies through a single arthroscopic procedure.
Arthritis severity
Doctors use a grade to indicate the severity of AC joint arthritis based on imaging findings and symptom burden.
- Mild: Joint space narrowing with early osteophyte formation. Distal clavicle edema may be present on MRI. Symptoms are typically intermittent and activity-related, responding well to conservative management.
- Moderate: More significant joint space loss with larger osteophytes, possible distal clavicle cystic changes, and periarticular soft tissue thickening on MRI. Symptoms are more persistent and may limit sport or occupational activity.
- Severe: Near-complete or complete loss of joint space with large inferior osteophytes encroaching on the subacromial space, subchondral cyst formation, and constant or near-constant pain. Conservative measures are less likely to provide durable relief at this stage.
Treatments
Treatment depends on the severity of arthritis, response to conservative care, and patient activity level.
Initial management includes activity modification — particularly avoiding cross-body reaching, overhead pressing, and end-range elevation — along with ice and anti-inflammatory medications.
Mild to moderate arthritis: Physical therapy targeting periscapular strengthening and rotator cuff conditioning helps offload the AC joint during overhead activity. A targeted corticosteroid injection directly into the AC joint — a small, precise target requiring careful technique — provides highly effective short-term relief and simultaneously confirms the AC joint as the primary pain generator. Most patients with mild to moderate disease achieve adequate symptom control with one or two injection cycles combined with activity modification.
Persistent arthritis not responding to conservative care
Surgery is advised for patients with persistent, functionally limiting symptoms despite a thorough course of conservative treatment. Arthroscopic distal clavicle resection (the Mumford procedure) removes approximately 8–10 mm of the distal end of the clavicle, eliminating the arthritic joint surface entirely and decompressing any inferior spur impingement. The procedure is reliable and well-tolerated, with most patients returning to full activity within 2–3 months. A thorough physical therapy program following surgery is important to restoring full shoulder strength and range of motion.
AC joint arthritis with concurrent rotator cuff or subacromial pathology
When AC joint arthritis coexists with subacromial impingement or rotator cuff tears — as is very commonly the case — distal clavicle resection can be performed simultaneously with arthroscopic subacromial decompression and rotator cuff repair through the same arthroscopic portals. Preoperative MRI is essential for identifying all coexisting pathologies so that a single comprehensive surgical procedure can address the complete anatomic picture.
Get an MRI to Confirm Your Diagnosis
Before surgical planning or starting treatment, a clear MRI diagnosis ensures the right path forward. First Look MRI offers self-pay Shoulder MRI scans — no doctor's order or insurance required — at our locations in Georgia, Texas, and Colorado.