The AC joint, short for acromioclavicular joint, is where your collarbone meets the highest point of your shoulder blade. It’s one of two joints that form your shoulder, and it plays a key role in letting you raise your arm overhead, reach across your body, and transmit force between your arm and torso. It’s also one of the most commonly injured joints in the body, particularly from falls and contact sports.
Bones and Ligaments of the AC Joint
The joint sits at the very top of your shoulder, where the outer end of your clavicle (collarbone) meets a bony projection on your shoulder blade called the acromion. If you reach up and feel the bony point at the top of your shoulder, you’re touching the AC joint. Unlike the ball-and-socket joint lower in the shoulder that gives you a wide range of motion, the AC joint is a small, relatively flat connection that allows subtle gliding and rotational movements.
Three ligaments hold the AC joint together. The acromioclavicular ligament connects the two bones directly at the joint and prevents them from sliding apart horizontally. Below the joint, a pair of ligaments called the coracoclavicular ligaments anchor the collarbone down to a hook-shaped piece of the shoulder blade called the coracoid process. These deeper ligaments are the main reason your collarbone doesn’t ride upward when you push, pull, or carry heavy loads. A disc of cartilage inside the joint cushions the two bones, though this cartilage begins wearing down as early as your 20s.
How AC Joint Injuries Are Graded
AC joint injuries, often called “shoulder separations,” happen when the ligaments holding the joint together get stretched or torn. They’re graded on a scale from Type I to Type VI based on how much ligament damage has occurred and how far the bones have shifted apart.
- Type I: The AC ligament is sprained but not torn. The joint is tender but stable, and X-rays look normal.
- Type II: The AC ligament is fully torn and the coracoclavicular ligaments are sprained. The collarbone shifts slightly upward, with the gap between the collarbone and shoulder blade increasing by less than 25% compared to the uninjured side.
- Type III: Both the AC and coracoclavicular ligaments are completely torn. The collarbone displaces upward noticeably, with a 25 to 100% increase in the gap. You can often see a visible bump on top of the shoulder.
- Type IV: All ligaments are torn and the collarbone is pushed backward through the trapezius muscle. The displacement can’t be pushed back into place by hand.
- Type V: All ligaments are torn with severe displacement, more than double the normal gap. The shoulder droops visibly and doesn’t improve when you try to shrug.
- Type VI: Extremely rare. The collarbone is driven downward beneath the shoulder blade, usually from a high-energy injury with associated nerve damage.
What AC Joint Pain Feels Like
Pain from an acute injury is sharp and localized right at the top of the shoulder. Reaching across your body to the opposite side typically makes it worse because that motion compresses the joint surfaces together. Lifting your arm above shoulder height, carrying heavy bags, or sleeping on the affected side can all aggravate it. In a Type III or higher separation, you’ll see a visible bump where the collarbone has shifted upward, and the shoulder may look like it’s sagging.
Degenerative AC joint pain develops more gradually. Osteoarthritis is the most common cause of chronic AC joint pain, though many people have arthritic changes on imaging without any symptoms at all. When it does cause problems, the pain tends to be a dull ache at the top of the shoulder that worsens with overhead activity or pressing movements like push-ups and bench presses. It’s often confused with rotator cuff problems since both conditions hurt in a similar area.
How AC Joint Problems Are Diagnosed
A physical exam is usually enough to identify AC joint issues. Direct pressing on the joint reproduces the pain in most cases. A commonly used test involves bringing your arm across your chest while a clinician resists your attempt to lift it. This combination of compression and resistance is highly sensitive for detecting AC joint problems. Standard X-rays are very specific (about 90% accurate when they show something abnormal) but miss a lot of cases, catching only about 40% of AC joint problems. MRI can reveal additional details like bone swelling, fluid in the joint, and thickening of the joint capsule that X-rays miss.
For chronic pain where the diagnosis is uncertain, a small injection of numbing medication directly into the joint can be both diagnostic and therapeutic. If the pain disappears within minutes, it confirms the AC joint as the source.
Treatment for Mild Separations
Type I and Type II injuries are treated without surgery. The approach involves rest, ice, a sling for comfort, and gradual return to movement as pain allows. A Type I sprain typically heals within a few weeks. Most people with mild to moderate injuries return to all normal activities within three months. Physical therapy focuses on restoring range of motion first, then rebuilding strength in the muscles that support the shoulder blade and rotator cuff.
For chronic AC joint arthritis that doesn’t respond to rest and anti-inflammatory medications, a steroid injection into the joint can provide relief. The joint itself is small, so only a fraction of a milliliter of medication is injected along with a small volume of numbing agent. These injections can reduce inflammation for weeks to months, though they aren’t a permanent fix.
When Surgery Is Needed
Type IV, V, and VI injuries almost always require surgery because the collarbone is so far out of position that it can’t heal properly on its own, and the displacement can’t be corrected by hand. Surgery typically involves reconstructing the torn ligaments and securing the collarbone back into its normal alignment using various fixation methods.
Type III injuries fall into a gray area. Both surgical and nonsurgical treatment are widely used, and the best approach is still debated. The decision often depends on the person’s activity level and physical demands. Someone who needs full overhead strength for work or sport may benefit from surgical repair, while someone with lower physical demands may do well with rehabilitation alone.
For arthritis that doesn’t respond to conservative care, a procedure called distal clavicle excision removes a small piece of the outer collarbone to eliminate the bone-on-bone contact. This can be done through a small incision or arthroscopically, and it reliably reduces pain while preserving shoulder function since the coracoclavicular ligaments still hold everything in place.