An AC sprain is an injury to the ligaments that hold together the acromioclavicular joint, the small connection point where your collarbone meets the top of your shoulder blade. It’s one of the most common shoulder injuries, especially in contact sports like football, hockey, and rugby. AC sprains range from a mild stretch of the ligaments to a complete tear that visibly shifts the bones out of alignment.
What the AC Joint Actually Does
Your acromioclavicular joint sits right at the top of your shoulder, where the outer end of the collarbone (clavicle) meets a bony projection of the shoulder blade called the acromion. It’s a small joint, but it plays a key role in transmitting force between your arm and the rest of your skeleton. Two separate ligament groups hold it together: the AC ligaments (a set of four that wrap around the joint capsule and prevent the bones from sliding forward or backward) and the coracoclavicular ligaments (a pair that run from the collarbone down to a hook-shaped part of the shoulder blade, preventing the bones from separating up and down). When people talk about AC sprain “grades,” they’re really describing which of these ligament groups got damaged and how badly.
How AC Sprains Happen
The most common cause is a direct blow to the top of the shoulder, typically from falling onto the point of the shoulder with the arm tucked against the body. This is why it’s so prevalent in collision sports: a football tackle, a check into the boards in hockey, or a hard fall off a bicycle. Less commonly, an AC sprain can result from falling onto an outstretched hand, which sends force up through the arm and into the joint indirectly.
Grades of AC Sprain
AC sprains are classified using the Rockwood system, which identifies six types based on which ligaments are torn and how far the bones have shifted apart.
- Type I: The AC ligaments are stretched or partially torn, but the joint remains stable. There’s local tenderness and mild swelling, but no visible deformity.
- Type II: The AC ligaments are completely torn, but the coracoclavicular ligaments are intact. You may notice a slight bump at the top of the shoulder where the collarbone sits a little higher than normal.
- Type III: Both the AC and coracoclavicular ligaments are torn. The collarbone is noticeably displaced upward, creating a visible bump. The shoulder blade drops downward and forward, and the shoulder often looks asymmetrical compared to the uninjured side.
- Types IV, V, and VI: These are severe injuries where the collarbone is displaced in different directions: backward (type IV), significantly upward (type V), or downward beneath the shoulder blade (type VI). All involve complete disruption of both ligament groups plus surrounding muscle attachments.
Types I and II are by far the most common. Types IV through VI are rare and usually result from high-energy trauma.
What It Feels and Looks Like
The hallmark symptom is sharp pain right at the top of the shoulder, precisely where the collarbone meets the shoulder blade. With a mild sprain, you’ll feel tenderness when pressing on the joint and pain when reaching across your body. With higher-grade injuries, the pain is more intense and you may struggle to lift your arm overhead.
The most distinctive sign of a significant AC sprain is the “bump” at the top of the shoulder. This isn’t the collarbone popping up so much as the shoulder blade and acromion dropping downward and inward, making the end of the collarbone look prominent by comparison. In high-grade injuries, you can sometimes push the collarbone down and feel it spring back up, a sign of complete ligament disruption. You may also notice that your shoulder blade sits in an abnormal, forward-tilted position at rest or moves unevenly when you raise your arm.
How It’s Diagnosed
A doctor can often identify an AC sprain through a physical exam alone. Two common tests help confirm the diagnosis. In the cross-body adduction test, your arm is raised to shoulder height and then pushed across your chest. Pain at the top of the shoulder during this movement points to the AC joint. In the Paxinos test, the examiner places a thumb under the back of the acromion and fingers on top of the collarbone, then squeezes. A sharp increase in pain confirms AC joint involvement.
X-rays are the standard imaging study. A specialized view called the Zanca view angles the X-ray beam 10 to 15 degrees upward to get a clearer picture of the joint, revealing how far the bones have separated and whether there are any bone spurs or fractures. For mild sprains, the X-ray may look normal since the ligament damage doesn’t always show up as visible bone displacement. Comparing the injured side to the uninjured side helps clarify the severity.
Treatment for Mild to Moderate Sprains
Type I and type II sprains are treated without surgery. The initial approach centers on rest, ice, pain relief, and wearing a sling to take weight off the shoulder. This acute phase typically lasts one to two weeks, during which the goal is simply to let the initial inflammation settle down.
Rehabilitation starts early, even in the first week for type I injuries. Early exercises focus on gentle range of motion: pendulum swings, passive stretching, and gradual work on external rotation and forward elevation up to shoulder level. During the first three to six weeks, you should avoid movements that stress the joint directly, including reaching across the body, rotating the arm behind the back, and raising the arm fully overhead.
As pain decreases, rehabilitation progresses to strengthening. The focus is on the muscles that stabilize the shoulder blade, including those responsible for pulling it back (retraction), pressing it down (depression), and rotating it properly. Exercises like rows, resistance band work, and closed-chain movements (where your hand stays fixed on a surface, like wall push-ups) help rebuild stability around the joint without overloading it. Core and lower body work is often introduced at the same time, since shoulder function depends on the whole chain of muscles from your legs through your trunk.
The final phase adds sports-specific or activity-specific training: push-ups, bench press (without locking out the elbows), plyometric drills like chest passes, and graduated throwing programs for overhead athletes.
When Surgery Is Needed
There is broad consensus that type IV, V, and VI injuries require surgical reconstruction because the ligaments are too severely disrupted to heal on their own in a functional position. Type III injuries fall into a gray area. Most doctors favor trying conservative treatment first, but early surgery may be recommended for people with high physical demands, such as overhead athletes or manual laborers. If a type III sprain doesn’t improve with rehabilitation and the person continues to have pain, weakness, or abnormal shoulder blade movement, surgery becomes the next step.
Surgical techniques generally involve reconstructing the coracoclavicular ligaments to restore the normal relationship between the collarbone and shoulder blade. After surgery, return to sport timelines typically range from two to twelve months, with six months being the most common benchmark in published studies.
Recovery Timeline
For type I sprains, many people return to normal activity within two to three weeks and full sport within four to six weeks, depending on the demands of the activity. Type II sprains generally take longer, often six to eight weeks before you’re back to unrestricted use. The primary marker for return to activity in nonsurgical cases is not a fixed number of weeks but rather being pain-free and having full strength and range of motion.
Type III injuries managed without surgery can take two to three months for meaningful recovery, sometimes longer. Surgical cases for high-grade injuries have the longest recovery windows, with most protocols targeting six months before clearing return to sport.
Long-Term Outlook
Most type I and II sprains heal fully without lasting problems. However, some people develop chronic issues at the AC joint over time. The two most common long-term complications are osteoarthritis of the joint, where the cartilage wears down and causes persistent stiffness and aching, and distal clavicle osteolysis, where the end of the collarbone gradually breaks down from repetitive stress. Both conditions tend to cause a deep, nagging pain at the top of the shoulder that worsens with overhead activity or reaching across the body. Chronic AC joint instability from a higher-grade injury can also alter how the shoulder blade moves during arm elevation, potentially contributing to secondary problems like impingement or rotator cuff strain.