Acromioclavicular (AC) joint

What it means

The acromioclavicular joint, usually shortened to the AC joint, is the small joint where the outer end of the collarbone (clavicle) meets the acromion, a bony shelf at the top of the shoulder blade. Unlike the large ball-and-socket shoulder joint underneath it, the AC joint is a small, relatively flat joint held together mainly by ligaments rather than deep muscle support. You can usually feel it yourself as a small bony bump on top of the shoulder, a few centimeters from the base of the neck.

Why it appears on a CT or MRI report

Radiologists comment on the AC joint routinely during shoulder imaging because it's a common source of pain and a frequent site of incidental findings. Reports often note whether the joint space is normal width or narrowed, whether there are small bone spurs (osteophytes) along its edges, whether the joint capsule is swollen or fluid-filled, and whether the supporting ligaments are intact. After an injury, the report will describe whether the joint has widened or the collarbone has shifted out of its normal position relative to the acromion, which points to a separation injury.

What it usually means

Mild wear at the AC joint, with small spurs or slight narrowing, is extremely common with age and is frequently found incidentally on scans done for unrelated reasons, often without causing any symptoms. When it is painful, AC joint arthritis typically causes a focused ache right at the top of the shoulder, worse with overhead reaching or lying on that side. A separation injury, usually from a fall onto the shoulder or a direct blow, is a different situation: it stretches or tears the ligaments holding the joint together, and its severity is graded from a mild sprain to a complete separation with visible collarbone displacement.

When to follow up

Mild degenerative changes noted incidentally, without symptoms, generally need no action. If you have shoulder pain localized to the top of the shoulder, especially with overhead movement, mention it to your doctor, since AC joint arthritis often responds well to activity changes, physical therapy, or a targeted injection. A suspected separation after a fall or sports injury should be evaluated promptly; lower-grade separations are usually treated with a sling and rehabilitation, while higher-grade ones with significant displacement are sometimes discussed with an orthopedic surgeon for possible repair.

A plain-language way to picture it

Picture two shelves meeting edge to edge, held snugly together by a set of straps rather than being bolted in place. That's the AC joint: the end of the collarbone butting up against the shoulder blade's bony tip, kept aligned by ligaments instead of a deep socket. Years of small movements can wear the edges of the shelves, like sandpaper smoothing a corner, while a hard fall can snap the straps and let the shelves shift out of line, which is what a separation injury looks like on a scan.