AC Joint Arthritis

Overview

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 (the joint lining)) 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 (shoulder impingement) coming from the cuff getting irritated by those downward-projecting spurs.

This is the everyday wear-and-tear kind of arthritis. The inflammatory and systemic forms (such as rheumatoid arthritis) are a different, body-wide problem diagnosed and managed by rheumatology, and we refer out when a systemic cause is suspected.

Symptoms

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 is 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.

Diagnosis & Evaluation

exam first, imaging and a numbing test

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 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 is not clean (for example, when AC arthritis is sitting alongside rotator cuff irritation and it is 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 does not, the pain is coming from somewhere else and we look further.

How We Treat It

what we try first, in order

NSAIDs
Anti-inflammatories like ibuprofen or naproxen calm the inflamed joint lining enough to get through the day. Useful for flares; not a long-term answer on their own, and the standard cautions apply (stomach, kidneys, blood pressure if you take them daily for months).
AC Joint Corticosteroid Injection
A small injection of cortisone placed directly into the AC joint quiets the inflammation for weeks to months. For patients who are not surgical candidates, or who want to put surgery off as long as possible, repeating this injection every few months can help calm a flare, but steroid injections are generally limited to a few per joint per year, and repeating them is weighed carefully because of concern they can affect the joint over time. The same shot doubles as the diagnostic test described above.
Activity Modification
Backing off the movements that load the joint hardest (heavy bench-pressing, repetitive overhead lifting, anything that pulls your arm across your body under load) gives the inflammation time to settle and slows the rate at which the cushion keeps wearing down.

When Surgery Is Considered

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 is done arthroscopically through small incisions and is one of the more reliable arthritis surgeries; the pain relief tends to last.

If non-operative care is not enough, this procedure can address this condition:

Providers Who Treat AC Joint Arthritis

Michael S. Vrana, M.D.

David B. Templin, M.D.

Trent Twitero, M.D.

Frequently Asked

questions we hear in clinic

How is AC joint arthritis different from other shoulder problems?

The focality of the pain. AC joint arthritis is a small-joint problem with a small, focal pain footprint: a sharply localized point of pain right on top of the shoulder that most patients can put one fingertip directly on. That focality distinguishes it from the bigger, more diffuse aches of subacromial impingement or rotator cuff tendinopathy. Pain that radiates down the arm past the elbow, weakness, or numbness and tingling point toward the rotator cuff, the cervical spine, or a nerve problem rather than the AC joint itself.

Why does my shoulder also pinch when I reach overhead?

The bone spurs that form at the rim of the worn joint often project downward into the narrow space the rotator cuff tendons run through. When the cuff gets irritated by those downward-projecting spurs, AC arthritis shows up not just as point pain on top of the shoulder but also as a pinching pain with overhead motion.

How is the diagnosis confirmed?

Your provider will press directly on the AC joint; true AC arthritis is exquisitely tender right there, and that single finding is highly suggestive on its own. X-rays show the cushion-space narrowing, the bone spurs at the rim, and erosion of the outer end of the clavicle. When the picture is not clean, a small amount of numbing medicine is injected directly into the AC joint: if the pain dissolves within minutes, the joint is the source; if it does not, the pain is coming from somewhere else and we look further.

Is this the same as rheumatoid arthritis?

No. This is the everyday wear-and-tear kind of arthritis. The inflammatory and systemic forms, such as rheumatoid arthritis, are a different, body-wide problem diagnosed and managed by rheumatology, and we refer out when a systemic cause is suspected.

How long does a cortisone injection last, and can it be repeated?

A cortisone injection placed directly into the AC joint quiets the inflammation for weeks to months. For patients who are not surgical candidates, or who want to put surgery off as long as possible, repeating the injection every few months is a routine, reliable part of long-term AC arthritis care, and many patients manage well on a steady injection rhythm for years.

Which activities should I back off from?

The movements that load the joint hardest: heavy bench-pressing, repetitive overhead lifting, and anything that pulls your arm across your body under load. Backing off gives the inflammation time to settle and slows the rate at which the cushion keeps wearing down.

When is surgery considered?

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 procedure called distal clavicle excision.

Will my shoulder still move normally after distal clavicle excision?

The shoulder still moves the same way; the muscles and ligaments that anchor the AC joint stay intact. The 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. It is done arthroscopically through small incisions, and the pain relief tends to last.