An AC separation is an injury to the joint where your collarbone meets the bony tip of your shoulder blade. Unlike a dislocated shoulder, which involves the ball-and-socket joint, a separated shoulder damages the ligaments holding these two bones together at the top of your shoulder. The injury ranges from a mild sprain to a complete tear that visibly displaces the collarbone upward.
The Joint Involved
Your acromioclavicular (AC) joint sits right at the top of your shoulder, where the outer end of your collarbone (clavicle) connects to a bony projection on your shoulder blade called the acromion. Two sets of ligaments hold this joint in place. The AC ligaments wrap directly around the joint and prevent the collarbone from sliding forward or backward. A second set, the coracoclavicular (CC) ligaments, anchor the collarbone downward to another part of the shoulder blade called the coracoid process. These CC ligaments are the main reason your collarbone doesn’t ride up when you push or lift. When one or both sets of ligaments tear, the joint separates.
How It Happens
The most common cause is falling directly onto the outside of your shoulder with enough force to tear the ligaments. This happens frequently in contact sports, car accidents, and cycling crashes. A classic example is a football receiver making a diving catch and landing hard on the point of the shoulder, forcing the arm downward while the collarbone stays in place. The mismatch in force tears the ligaments apart.
In professional baseball, AC joint injuries account for a relatively small share of all shoulder injuries, but they follow an interesting pattern. Infielders and outfielders tend to suffer acute AC injuries from contact while fielding (about 67% to 80% of their AC injuries are acute). Pitchers and catchers, on the other hand, develop AC problems more evenly from repetitive overhead motion rather than a single traumatic hit.
Grades of Severity
AC separations are classified into six types, though three of them account for the vast majority of cases.
- Type I: The AC ligaments are sprained but not torn. The CC ligaments are intact. The joint looks normal on X-ray, and there’s minimal deformity. This is the mildest form.
- Type II: The AC ligaments are completely torn, but the CC ligaments are only sprained or partially torn. The collarbone may sit slightly higher than normal, but vertical stability is mostly preserved.
- Type III: Both the AC and CC ligaments are completely torn. The collarbone is visibly elevated, and the shoulder sags downward, creating an obvious bump at the top of the shoulder. This grade sits in a gray zone between conservative and surgical treatment.
- Types IV, V, and VI: These are severe injuries involving complete tears of both ligament sets plus damage to surrounding muscle and tissue. In a Type IV, the collarbone displaces backward. In Type V, it displaces dramatically upward. Type VI is rare and involves the collarbone being driven downward beneath the shoulder blade.
What It Feels and Looks Like
Pain is concentrated right at the top of the shoulder, directly over the AC joint. Swelling, bruising, and tenderness develop quickly after the injury. In mild separations (Types I and II), the deformity is usually minimal, and you might mistake it for a simple bruise. In a Type III or higher, the outer end of the collarbone becomes abnormally prominent, creating a visible bump. The normal rounded contour of the shoulder is lost because the acromion sags downward while the collarbone stays elevated.
People with acute AC separations typically have poor shoulder range of motion and moderate pain when trying to raise the arm. You’ll likely find yourself holding the injured arm close to your body or using your other hand to support it. These symptoms become more obvious when holding a weight of 10 to 15 pounds in the affected hand, which pulls the shoulder down and increases the visible separation.
How It’s Diagnosed
A physical exam is often enough to suspect the diagnosis. The most reliable test involves raising the affected arm to shoulder height and then pulling it across the body. Pain at the AC joint during this maneuver strongly suggests an AC injury. Your doctor will also press on the outer collarbone and check how stable the joint feels when pushed in different directions: front-to-back motion tests the AC ligaments, while up-and-down motion tests the CC ligaments.
X-rays confirm the diagnosis and determine the grade. Both shoulders are typically X-rayed for comparison, since the amount of separation is measured against your uninjured side. A specific angled view is sometimes needed to clearly see the joint, and a side view taken from the armpit is required to identify a Type IV injury, where the collarbone has shifted backward in a way that standard front-facing X-rays can miss.
Treatment Without Surgery
Types I and II are almost always treated without surgery, and Type III injuries are frequently managed conservatively as well. The initial phase focuses on rest, activity modification, and ice to control pain and swelling, typically lasting one to two weeks. A sling is used during this period, though there’s no standardized duration that research has firmly established.
Rehabilitation starts with gentle exercises that keep the arm close to the body. The goal is restoring fluid motion in the shoulder blade and arm without stressing the healing ligaments. Exercises that require lifting the arm overhead or shrugging are avoided for the first three to six weeks because they place excessive force on the AC joint. Early workouts typically start with one to two sets of 5 to 10 repetitions using no added weight.
As healing progresses, exercises gradually move from short-lever movements (arms close to the torso) to long-lever movements (arms extending away from the body, then eventually at or above shoulder height). Core and leg strengthening are incorporated early because trunk stability plays a direct role in how well the shoulder functions during recovery. In professional baseball, acute AC injuries typically require about three weeks before a player returns to competition.
When Surgery Is Needed
Types IV, V, and VI require surgical repair because of the severe tissue disruption involved. Type III injuries remain debated, with some surgeons recommending surgery for young athletes or heavy laborers and others preferring to try conservative treatment first.
Several surgical approaches exist. One common method uses a hook plate, a metal plate screwed onto the top of the collarbone with a curved hook that slides under the acromion to hold the collarbone down. Another technique uses a suture button system that threads a strong cord between the collarbone and the coracoid process to restore vertical stability. For more severe or chronic cases, surgeons may reconstruct the torn ligaments using a tendon graft, sometimes combined with one of the fixation methods for added support during healing. An older technique that transferred muscle attachments from the coracoid to the clavicle has largely been abandoned due to high complication rates, including fractures and nerve damage.
Long-Term Outlook
Most people recover well from lower-grade AC separations, but the injury can leave lasting effects. Damage to the cartilage inside the joint at the time of injury can lead to arthritis in that joint over time. Studies have shown that roughly 30% of heavy laborers report mild ongoing symptoms after an AC injury, though significant problems requiring further treatment are much less common. A small percentage of people with Type II injuries eventually develop enough joint degeneration to need surgery down the line.
Other potential long-term issues include decreased shoulder range of motion and reduced upper-body strength on the affected side. The visible bump from a Type III or higher separation often persists permanently, even after successful treatment, because the stretched ligaments don’t always restore the collarbone to its exact original position. This cosmetic change is usually painless and doesn’t affect function once rehabilitation is complete.