AC Joint Arthritis

The AC joint is the small joint at the very top of your shoulder, where the outer end of your collarbone (the clavicle) meets a bony shelf on top of your shoulder blade (the acromion). A thin disc of cartilage cushions the two bones, and like the cartilage in any other joint, it can wear out — usually from years of overhead work, repeated heavy lifting, or as a late consequence of an old AC joint separation. As the cushion thins, the bones start to contact each other directly, the joint lining (the synovium) gets inflamed and thickens, and the body lays down small bony outcroppings at the rim of the joint (called osteophytes). Those spurs often project downward into the narrow space the rotator cuff tendons run through, which is why AC arthritis sometimes shows up not just as point pain on top of the shoulder, but also as a pinching pain with overhead motion (subacromial impingement) coming from the cuff getting irritated by those downward-projecting spurs.

The signature is a small, sharply localized point of pain right on top of your shoulder — most patients can put one fingertip directly on it. It hurts most with cross-body reaching (putting on a seatbelt, washing the opposite armpit, throwing an arm across to swat at something), with bench press or push-ups, and with the very last bit of overhead reach. Lying on that shoulder at night is often uncomfortable. The pain is usually a deep ache that flares with use and settles with rest, and many patients can grit through it for years before it becomes the thing that finally pushes them in.

What's not typical: pain that radiates down the arm past the elbow, weakness, or numbness and tingling — those point toward the rotator cuff, the cervical spine, or a nerve problem rather than the AC joint itself. AC arthritis is a small-joint problem with a small, focal pain footprint; that focality is the thing that distinguishes it from the bigger, more diffuse aches of subacromial impingement or rotator cuff tendinopathy.

Your provider will press directly on the AC joint at the top of your shoulder — true AC arthritis is exquisitely tender right there, and that single finding is highly suggestive on its own. Pulling your arm across your chest (the cross-body adduction test) reproduces the pain by compressing the worn joint. X-rays show the cushion-space narrowing, the osteophytes at the rim, and erosion of the outer end of the clavicle. When the picture isn't clean — for example, when AC arthritis is sitting alongside rotator cuff irritation and it's not obvious which one is doing what — the diagnosis is confirmed by injecting a small amount of numbing medicine directly into the AC joint. If the pain dissolves within minutes, the joint is the source; if it doesn't, the pain is coming from somewhere else and we look further.

When the injection rhythm no longer holds and the joint hurts through your daily activities, the surgical answer is to remove the worn end of the collarbone — a distal clavicle excision. Your surgeon shaves off the last few millimeters of the clavicle where it meets the acromion, leaving a small gap so the two bones can no longer grind on each other. The shoulder still moves the same way; the muscles and ligaments that anchor the AC joint stay intact. It's done arthroscopically through small incisions and is one of the more reliable arthritis surgeries — the pain relief tends to last.

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