AC Joint Separation

The AC joint sits at the very top of your shoulder — it's where the outer end of your collarbone (the clavicle) meets a bony shelf on top of your shoulder blade (the acromion). What holds those two bones together is a small set of ligaments: the AC ligaments wrapping the joint itself, and — most importantly — two stout straps called the coracoclavicular ligaments (the conoid and trapezoid) that tether the underside of the collarbone to a hook of bone on the shoulder blade called the coracoid. Those CC ligaments are the main thing keeping the collarbone from riding up.

An AC separation happens when you fall directly onto the point of your shoulder — a tackle, a bike crash, a fall onto an outstretched hand that loads the shoulder from the side. The blow drives the acromion downward while the collarbone, anchored to the chest by the sternoclavicular joint, can't follow. The ligaments tear in a stepwise sequence: first the AC ligaments (the joint capsule), then the coracoclavicular ligaments, then the thick fascia of the deltoid and trapezius muscles laying over the top. How far down that sequence goes is what determines the grade — and the grade is what determines whether your shoulder can heal on its own or needs surgical reconstruction.

Surgeons describe these injuries on the Rockwood scale, I through VI. Grades I and II are sprains — the AC ligaments are stretched or partially torn, the CC ligaments are intact, the bone hasn't moved on X-ray. These heal with rest. Grade III is a complete tear of both the AC and CC ligaments, with the collarbone visibly riding up — the surgical-vs-non-surgical decision here is genuinely debated and depends on what you do for a living and how active you are. Grades IV, V, and VI involve the collarbone displacing in dramatic directions (backward through the trapezius, severely upward, or downward beneath the coracoid) and almost always need surgery.

You'll know something happened the moment it happens — a sharp, well-localized pain right on top of your shoulder after the fall or hit. Higher-grade separations come with a visible bump: the outer end of the collarbone sits higher than the acromion, creating a step you can see and feel. Lifting the arm overhead is sharply painful, and reaching across your chest to the opposite shoulder reproduces the pain. Lying on that side at night is uncomfortable for weeks.

What's not typical: numbness, tingling, or weakness running down the arm. Those point to a different injury — a brachial plexus stretch, a clavicle fracture pinching a nerve, or a separate shoulder dislocation — and need to be sorted out separately. AC separations are mechanical pain at a single point on top of the shoulder; that focality is the giveaway.

The diagnosis is mostly made on exam and X-ray. Your provider will look for a step deformity (the bump on top of the shoulder), tenderness right on the joint, and pain when your arm is pulled across your chest. Standard shoulder X-rays show the joint clearly; sometimes a special view with a small weight in your hand (a weighted view) is used to unmask the displacement that closes up when the arm hangs free — that's how a borderline grade II/III gets sorted out. MRI isn't routine for low grades but is useful for high-grade or atypical injuries to confirm exactly which ligaments are torn and rule out an associated rotator cuff or labral problem.

1. AC Joint Injection

   A small cortisone injection placed directly into the AC joint quiets the inflammation when low-grade sprains leave you with lasting joint pain — usually because the cartilage took a small hit at the time of injury and the joint is now mildly arthritic. For patients in this situation, repeating the injection every several months is a routine, reliable way to stay ahead of the pain without surgery.

Grade III separations sit in a genuine gray zone — current evidence doesn't clearly favor surgery over non-operative care for the average patient, and many grade IIIs do well with rehab alone. Heavy laborers, throwing athletes, or patients whose cosmetic deformity bothers them may still elect surgical reconstruction. Grades IV, V, and VI almost always need it: the collarbone is displaced too far for the ligaments to ever heal back to anatomic position, and the surgical answer is to rebuild the coracoclavicular ligaments — usually with a graft passed around the coracoid and over the top of the clavicle to pull the bone back down where it belongs.

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

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