AC Joint Separation:
When the Shoulder "Separates"

What the AC joint is

The acromioclavicular (AC) joint is the small joint at the top of the shoulder where the outer end of the collarbone (clavicle) meets the acromion, a bony projection of the shoulder blade. You can usually feel it as a small ridge on top of the shoulder. Two sets of ligaments hold it together: the AC ligaments, which wrap around the joint itself, and the stronger coracoclavicular (CC) ligaments, which run from the underside of the collarbone down to the coracoid process of the shoulder blade and provide most of the joint's vertical stability.

When someone lands on the point of the shoulder with the arm at the side, the shoulder blade is driven downward while the collarbone is held in place. The AC ligaments tear first; with more force, the CC ligaments tear as well, and the shoulder drops away from the collarbone. The end of the collarbone then appears to stick up — which is why people often describe a "step" or "bump". In Japanese this injury is called kensa kansetsu dakkyū (肩鎖関節脱臼).

It is worth separating this from a shoulder dislocation, where the ball of the upper arm comes out of the socket. The two are often confused because both are called "shoulder" injuries, but they involve different joints and are managed very differently.

Who gets it

AC joint injuries are among the most common shoulder injuries in active young adults, particularly men who play contact sports.

In everyday life in Tokyo, falls from bicycles, judo and rugby, skiing and snowboarding trips, and falls on stairs or icy pavements are typical causes.

Symptoms and how the diagnosis is made

Typical symptoms are pain and tenderness directly on top of the shoulder, swelling, and pain when lifting the arm overhead or reaching across the body. In higher-grade injuries the outer end of the collarbone is visibly prominent. A doctor will usually press over the AC joint, check whether the collarbone can be pushed back down, and perform a cross-body adduction test (bringing the arm across the chest), which typically provokes pain at the joint. It is also important to check for other injuries — a fracture of the collarbone, a rotator cuff tear, or nerve symptoms.

X-rays are the main test. Images of both shoulders are often taken so the injured side can be compared with the normal side, measuring the coracoclavicular distance (the gap between the collarbone and the coracoid). An additional view from above (axillary view) helps show whether the collarbone has been pushed backwards. MRI or ultrasound is not needed in most cases, but may be used when the grade is uncertain or another injury is suspected.

The grades of AC joint injury

The most widely used grading system is the Rockwood classification, which describes which ligaments are torn and in which direction and how far the collarbone has moved.

Grade	What is injured	Usual approach
Type I	AC ligaments sprained; joint not displaced	Non-operative
Type II	AC ligaments torn, CC ligaments sprained; collarbone slightly raised	Non-operative
Type III	AC and CC ligaments torn; collarbone clearly elevated	Usually non-operative first; surgery debated
Type IV	Collarbone pushed backwards into the trapezius muscle	Surgery commonly considered
Type V	Severe upward displacement with tearing of the muscle attachments over the collarbone	Surgery commonly considered; some trials included non-operative care
Type VI	Collarbone displaced downward beneath the acromion or coracoid (rare)	Surgery

Because grade III is the "grey zone", an international expert group has proposed splitting it into two subtypes.

Non-operative treatment

For grades I and II, and for most grade III injuries at first, treatment is non-operative. It typically involves a sling for comfort for a short period (often days to a few weeks, depending on pain), ice and pain relief, and then a gradual rehabilitation programme focusing on shoulder range of motion, shoulder-blade (scapular) control, and later strengthening. Heavy lifting and contact sport are usually avoided until pain has settled and strength has returned.

A natural question is whether a special brace that pushes the collarbone down helps the joint heal in a better position.

After non-operative treatment, the bump at the top of the shoulder usually remains. For most people this does not interfere with function, but it is worth knowing in advance.

Surgery versus non-operative care: what the trials show

Surgery for AC joint separation aims to put the collarbone back in position and hold it there while the ligaments heal, or to reconstruct the coracoclavicular ligaments. Methods have included hook plates, suspension devices passed through bone tunnels ("tightrope"-type devices), screws, wires, and ligament grafts, done through open or arthroscopic approaches. Several randomised trials have compared surgery with non-operative care, mostly for grade III (and some grade IV–V) injuries.

Longer-term follow-up and pooled analyses point in the same direction.

These trials have limitations: they are small, cannot be blinded, often lose patients to follow-up, and mostly enrolled young men. Several used fixation methods that have since been refined. So the evidence does not show that surgery is never useful — rather that, for a typical grade III injury, starting with non-operative care is a reasonable, well-supported option, and that a minority of people who stay painful or unstable after rehabilitation may later be considered for reconstruction.

When surgery is more commonly discussed

Surgery tends to be considered for grade IV and VI injuries, many grade V injuries, open injuries or those threatening the skin, and grade III injuries that remain painful or unstable (type IIIB) after a genuine period of rehabilitation. Some overhead athletes and manual workers also discuss earlier surgery with their surgeon. The decision is individual and depends on the injury, the person's work and sport, and their own priorities — including how they feel about the cosmetic bump.

Recovery and returning to activity

Recovery time varies with the grade. Low-grade sprains often allow a return to sport within a couple of weeks, while higher-grade injuries take considerably longer, whether or not surgery is done. Some people develop late pain from wear in the AC joint itself years after the injury. Persistent pain on top of the shoulder, clicking, or weakness with overhead activity several months after injury is a reason for review by a shoulder specialist.

When to see a doctor

Anyone with significant pain on top of the shoulder after a fall, a visible step at the end of the collarbone, or difficulty lifting the arm should be assessed, because a collarbone fracture or other shoulder injury can look similar. Urgent assessment is needed if there is numbness or weakness in the arm, the skin over the collarbone is tented or broken, or there is difficulty breathing.