The AC joint is where the collarbone (clavicle) meets the end of the shoulder blade (scapula), known as the acromion. The AC joint contains cartilage between the bones, which are stabilized by three ligaments.
The acromioclavicular ligament and coracoclavicular ligament, made up of the trapezoid and conoid ligaments, create stability inside the AC joint. These ligaments can stretch or tear during activity. For instance, a fall or hard impact can cause your shoulder to move beyond its normal range of motion and stretch or tear one of the ligaments. A shoulder separation is when the ligaments within the AC joint fully tear.
An AC joint injury can also be caused by repetitive joint overuse.
Typically, an acute injury causes AC joint pain on top of the shoulder. You can also experience swelling, tightness, and a loss of motion.
The severity of your symptoms can vary. Less intense ligament sprains may cause only mild pain and limited swelling, while full tears could result in intense pain and the inability to move the shoulder.
A noticeable bump on the shoulder can also form due to a full AC joint tear. When the ligaments are torn, the bones move and the bump forms.
Lifting the arm overhead, away from the body, or across the body will generally increase AC joint pain. Putting pressure on the shoulder, such as during sleep, intensifies pain.
Athletes like weightlifters may also experience inflammation, a condition called distal clavicle osteolysis, or arthritis in the AC joint from overuse and strain. In these situations, there isn't one injury that leads to symptoms, but over time the cartilage in the joint that allows the bones to move smoothly wears away. AC joint arthritis normally occurs in people over the age of 40. With AC joint arthritis, pain and swelling can intensify.
AC joint injury types are classified into three grades. Each grade accounts for the severity of stretched and torn ligaments. The level of sprained AC joint symptoms is often tied to the grade of AC joint injury.
A grade 1 AC joint sprain is the mildest of the three grades. It occurs when a ligament has been slightly stretched with no tears. A grade 1 sprain often includes only mild AC joint pain, swelling, and tenderness at the shoulder. The injured person can still use the shoulder.
A grade 2 AC joint sprain usually results from a partial tear of the ligaments or even a full tear of the acromioclavicular ligament and partial tear of the coracoclavicular ligament.
Grade 2 AC joint sprain symptoms include pain, tenderness, and swelling. A bump can also form as the scapula and upper arm bone (humerus) are lowered. A grade 2 sprain can make it difficult to use your shoulder, cause pain even without movement, and compromise shoulder stability.
A complete AC joint tear or rupture of both main ligaments defines a grade 3 AC joint sprain or shoulder separation. With severe pain, tenderness, and swelling of the shoulder, moving the shoulder becomes quite difficult. A bump on your shoulder will be heavily pronounced. Range of motion of the shoulder will be hampered, and stability is often greatly affected by the lack of ligament strength.
An AC joint sprain can occur in any athletic activity where falling or hard contact with an opponent is possible. This contact can suddenly push the shoulder blade away from the collarbone, stretching or tearing the ligaments that support the AC joint.
Common causes of an AC joint injury during athletic activity:
The most common sports that see AC joint injuries include football, cycling, mountain biking, martial arts, skiing, snowboarding, and hockey.
Once an athlete sprains an AC joint, the likelihood of injuring it again increases. If not left to heal properly, the AC joint injury can cause further damage around the ligaments, such as the bones and cartilage. Returning to activity before the AC joint has fully healed risks escalating the injury. Even if the joint is allowed to recover fully, the risk of re-injury remains high.
Athletes should be in proper physical condition to decrease the risks of injury.
A doctor will diagnose an AC joint injury using a physical examination. They may decide to add imaging technology for a closer look at your shoulder.
A physical examination to diagnose an AC joint injury includes the AC joint compression test. Your doctor will place one hand on the front of the AC joint and one behind it and compress the joint. Extreme AC joint pain is typically a sign of an AC joint separation. Your doctor may also ask questions about your injury and the extent of your pain. They’ll also look for swelling, the presence of a bump, and reduced range of motion.
A doctor may order an AC joint X-ray to rule out a bone fracture. A magnetic resonance imaging (MRI) scan can also help evaluate whether other areas in the shoulder have been damaged.
Get in touch with a Mass General Brigham Sports Medicine specialist to learn more about diagnosing, treating, and recovering from shoulder and AC joint injuries.
In the first 24 hours following an AC joint injury, athletes should rest the AC joint, use ice to limit swelling and decrease pain, and potentially use a sling to help immobilize the ligaments. Further treatments include activity modification to ease the stress on the ligaments, and physical therapy can help strengthen the AC joint as it recovers.
Highly unstable AC joints—typically ones that have experienced a full tear of the ligaments (sometimes more than once)—may need surgery. In these cases, surgeons secure the bones in the AC joint into their proper location and reconstruct the ligaments.
During recovery, patients will focus on AC joint range-of-motion exercises, isometric strengthening, and stretching.
As athletes return to activity, they must carefully monitor AC joint pain and limit activities that increase the likelihood of another injury before the AC joint is fully healed. Wearing protective equipment may help give support as an athlete returns to activity.
Most AC joint injuries will start to improve within a week of the injury, but it can take up to six weeks for a full recovery. After an AC joint surgery, patients may wear a sling for multiple weeks and recovery can take multiple months, sometimes up to six, with a return to activity dependent on the strength of the joint.
A strong AC joint and proper warm-up can help limit the risk of an AC joint injury. Tips include:
Avoid placing pressure on your shoulder. You may want to sleep in a comfortable chair with an armrest or on your back with your arm laying across your body. Use pillows to support your body and create the most comfortable position.
Arthritis can develop in an AC joint with a repetitive overuse of the same motion. Over time, the cartilage in an AC joint can wear away, leading to degeneration.
The bump caused by an AC joint injury can be reduced through AC joint surgery, which reattaches the bones in the joint by reconstructing the ligaments, but will not go away entirely.
An AC joint injury can range from a mild sprain that heals with rest in a few weeks to a major tear of multiple ligaments that requires surgery to repair fully.
The AC joint compression test is a quick way to tell if you've torn your AC joint. Also, extreme AC joint pain and the loss of range of motion can be symptoms of a full tear.
An AC joint injury can heal on its own; taking proper precautions during recovery and exercising smartly can help the process.
No, while both are part of the shoulder, the AC joint is where the shoulder blade and clavicle meet, whereas the rotator cuff is where the upper arm bone (humerus) connects with the shoulder blade. Learn more about rotator cuff injuries.