Pain and tenderness at the top of the shoulder and difficulty reaching the arm across the body—to swing a golf club or put on a seat belt, for example—may be a sign of acromioclavicular arthritis.
The acromioclavicular (ah-kro-mee-o-klah-vik-u-ler) joint, also called the AC joint, is located at the top of the shoulder where the clavicle (collarbone) and highest part of the scapula (shoulder blade) meet. Most people are not familiar with this joint in the body, but arthritis in the AC joint is actually more common than arthritis in the glenohumeral joint, the shoulder’s large ball-and-socket joint.
For symptoms and treatments for glenohumeral arthritis, see What Is Shoulder Osteoarthritis?
Typically, arthritis is indicated by a gradual onset of pain, tenderness and limited range of motion. Acromioclavicular arthritis comes in many forms, most commonly osteoarthritis.
AC Joint Osteoarthritis and Cartilage Loss
The defining feature of osteoarthritis is the breakdown and loss of articular cartilage. In the AC joint, slippery, flexible articular cartilage covers and protects the ends of the acromion and clavicle bones where they meet.
During the development of osteoarthritis, cartilage may thin or disappear. The body may attempt to produce new cells, but it is not enough to replace the missing cartilage.
AC Joint Osteoarthritis and Shoulder Pain
Cartilage does not contain nerves, so the degeneration of cartilage in the AC joint does not necessarily cause pain. In fact, many people have acromioclavicular arthritis and have no symptoms. Research 1 suggests that people who do experience symptoms of acromioclavicular arthritis are more likely to have:
- Bone marrow lesions. The bone underneath the damaged or missing AC joint cartilage may develop areas of abnormal swelling called bone marrow lesions. These lesions seem to be associated with pain.
- Minor shoulder dislocation. If the acromion is positioned just a few millimeters out of place, termed AC subluxation, it may indicate the shoulder pain is coming from AC joint arthritis. A minor dislocation will not necessarily be obvious to the patient or even the doctor without an x-ray or other medical imaging. AC joint arthritis can also occur after a complete dislocation.
- Pain from AC joint osteoarthritis is less common than pain from osteoarthritis of weight-bearing joints such as knees and hips. Those who do have painful AC joint arthritis are advised to seek treatment. Chronic discomfort or minor pain in the shoulder should not be ignored.Understanding the symptoms and causes of acromioclavicular osteoarthritis, getting an accurate diagnosis, and following an appropriate treatment program can encourage healthy joint function and may minimize or halt the progression of symptoms.