Acromioclavicular osteoarthritis (AC joint OA) is a common degenerative joint disease affecting the shoulder. It targets the acromioclavicular (AC) joint, where the collarbone (clavicle) meets the highest part of the shoulder blade (acromion). Like other forms of OA, this condition results from the progressive wear and tear of the joint’s protective tissues. It is a frequent cause of pain in the upper shoulder, particularly among middle-aged and older adults.
The Affected Joint and Its Function
The AC joint is a small, plane synovial joint formed by the lateral end of the clavicle and the acromion (a bony projection of the scapula). This articulation is one of two joints making up the shoulder, providing stability and allowing the gliding motion essential for full arm movement. Although classified as a passive joint, its movement is controlled by other muscles.
The joint allows the arm to be raised completely overhead and moved across the body. Normally, the articulating surfaces are covered by fibrocartilage, which cushions movement. When AC joint OA develops, this protective cartilage breaks down, causing the bones to rub together. This friction leads to inflammation and often prompts the formation of bone spurs (osteophytes) around the joint margins, which narrow the joint space, causing stiffness and pain.
Identifying the Causes and Symptoms
AC joint OA is primarily driven by mechanical stress and cumulative use over time. While common in older individuals due to age-related wear, it is accelerated by specific risk factors. A frequent cause is a history of trauma, such as an AC joint separation or sprain resulting from a fall or a blow during contact sports.
Repetitive, high-stress use also contributes to degeneration. Athletes, particularly weightlifters, and individuals whose occupations require constant overhead lifting or throwing motions place excessive strain on the joint, increasing the incidence of AC joint OA. This chronic mechanical stress initiates the degenerative process.
Symptoms typically involve pain and tenderness localized directly over the AC joint. Pain is often aggravated by specific movements, such as reaching the arm across the chest (cross-body adduction) or lifting the arm above shoulder height. Patients may also experience aching discomfort at night, especially when sleeping on the affected side. A grinding, clicking, or popping sensation, known as crepitus, may be felt as the shoulder moves due to the irregular joint surfaces.
Diagnostic Procedures
Diagnosis begins with a thorough medical history and physical examination. The provider palpates the area, looking for tenderness and swelling directly over the joint. The cross-body adduction test is frequently used, where the patient’s arm is moved across the chest to compress the AC joint, typically reproducing the characteristic pain.
Imaging confirms degenerative changes. Standard X-rays are the primary diagnostic method, visualizing joint space narrowing and the presence of osteophytes, which are hallmarks of OA. However, X-ray evidence of degeneration does not always correlate perfectly with a patient’s symptoms.
If the diagnosis is uncertain, or to differentiate AC joint OA from other shoulder issues, advanced imaging may be used. Magnetic resonance imaging (MRI) or computed tomography (CT) scans provide detailed soft tissue and bone visualization, although they are not routinely required for an initial diagnosis. A diagnostic injection of a local anesthetic directly into the AC joint is highly specific; temporary but complete pain relief strongly confirms the AC joint as the source of pain.
Comprehensive Management Strategies
Management follows a progressive approach, starting with non-surgical methods to reduce pain and restore function. Initial conservative treatment involves activity modification, avoiding painful movements (like overhead lifting). This is paired with nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, to reduce pain and inflammation.
Physical therapy focuses on improving shoulder mechanics and strengthening surrounding muscles. While exercises do not reverse joint damage, improved stability decreases strain on the AC joint during daily activities. Local treatments are incorporated when oral medications and therapy are insufficient to control symptoms.
An intra-articular corticosteroid injection is a common and effective second-line treatment, delivering powerful anti-inflammatory medication directly into the AC joint space. This injection provides substantial pain relief, often lasting several months, and helps confirm the joint as the source of pain. Conservative management is typically pursued for six to twelve months before surgical options are considered.
When non-operative strategies fail to provide lasting relief, surgical intervention is considered. The primary procedure is the distal clavicle excision (Mumford procedure), which involves removing a small segment (0.5 to 2 centimeters) from the end of the collarbone. The goal is to create a gap between the clavicle and the acromion, preventing arthritic bone surfaces from rubbing. This can be performed arthroscopically or through an open incision. Following surgery, rehabilitation is necessary to restore range of motion and strength, leading to predictable pain relief and improved function.