AC Joint Arthritis

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The AC joint is a small synovial joint subject to significant mechanical stress — every overhead movement loads it. Osteoarthritis of the AC joint is common, especially in older adults and those with a history of AC joint separation or heavy overhead work. It causes pain directly over the joint, particularly with cross-body reaching and overhead activity. Bone spurs from AC arthritis can also project downward into the subacromial space, contributing to rotator cuff impingement.

Point tenderness directly over the AC joint, pain with cross-body adduction (cross-chest test). X-rays show joint-space narrowing, osteophytes, and distal clavicle erosion. The diagnosis is confirmed by marked pain relief after a diagnostic injection of local anesthetic directly into the AC joint.

Refractory AC arthritis that does not respond to injections and activity modification is treated with distal clavicle excision.

External patient-education references and related OSI pages for additional background:

• Britannica — Arthritis
• Britannica — Shoulder (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language