→ damage to the joint next to the collarbone.
The acromioclavicular joint, also known as the AC joint, is the point where the top of the shoulder bone (the acromion) meets the collarbone (the clavicle). It allows the arm to be raised above the head, and to rotate in the shoulder.
There are two main causes of AC joint problems. The first is injury. Like the ball-and-socket joint, the AC can be dislocated by a forceful impact – typically this happens in a contact sport like football or rugby, or during a fall onto an outstretched hand. This may be accompanied by a partial or complete tear of the ligaments in the joint. The second major cause is osteoarthritis. Here the cartilage in the joint breaks down, causing the bones to rub against each other. As the cartilage deteriorates, the risk of a shoulder impingement increases.
Injuries to the AC joint tend to cause pain at the top of the shoulder, particularly during lifting or overhead movements. It may be accompanied by swelling, bruising and loss of arm mobility. There may also be a visible lump if the collarbone has been displaced. Osteoarthritis symptoms appear gradually around the top of the shoulder; you may feel tenderness, or low-level pain that intensifies during a ‘flare-up.’ Reaching across the body – when tightening a seatbelt, for example – can be a particular cause of pain, as can movements above the head, or lifting. Swelling may also occur.
Diagnosing acromioclavicular issues begins with an examination, where you’ll be asked about your recent history and given some basic mobility tests to assess your pain (such as a cross-body movement). Other useful diagnosis tools for osteoarthritis include x-rays, MRI scans and anaesthetic injections (where a local anaesthetic is injected into the joint; temporary relief of the pain indicates that arthritis is present).
Treatments vary depending on the type and severity of the problem. AC joint injuries may heal naturally, particularly if accompanied by physical therapy to strengthen and improve movement in the joint as it heals. Surgery may be advised for a severe ligament tear, or to pin the joint after dislocation. Osteoarthritis in the AC joint can be managed in a number of ways, including change of activity, physiotherapy, pain relief, steroid injections and anti-inflammatories. More severe cases of osteoarthritis are treated with surgery to relieve symptoms, from removal of damaged cartilage to removal of the tip of the clavicle to prevent friction in the joint.