The acromioclavicular (AC) joint is a common site for degeneration, a condition that often leads to shoulder pain and restricted movement. This degeneration occurs when the smooth cartilage within the joint begins to wear away, causing the bones to rub against each other. Understanding the factors that contribute to this breakdown is important for recognizing and addressing the condition.
Anatomy and Function of the AC Joint
The AC joint is located at the top of the shoulder, connecting the collarbone (clavicle) and a part of the shoulder blade called the acromion. It functions to provide stability to the shoulder complex and plays a role in the arm’s overall range of motion.
The AC joint allows for subtle gliding and rotational movements of the shoulder blade, which are necessary for full arm elevation and movement across the body. It also helps transmit forces from the arm to the rest of the skeleton. The joint surfaces are covered by cartilage, which acts as a cushion, and are held together by a network of ligaments that provide crucial support and maintain joint integrity.
Key Drivers of Degeneration
Degeneration of the AC joint primarily stems from age-related changes, repeated mechanical stress, and acute traumatic injuries. These distinct mechanisms can individually or collectively lead to the breakdown of the joint’s protective structures.
Age is a significant contributor to AC joint degeneration. As people age, the cartilage cushioning the joint surfaces naturally wears down, becoming thinner and less effective at preventing friction between the bones. This gradual wear and tear can also lead to changes in the underlying bone, such as bone spurs.
Repetitive stress and overuse, particularly from activities involving frequent overhead movements or heavy lifting, can accelerate the wear on the AC joint. Athletes involved in sports like weightlifting, hockey, rugby, or throwing sports often place significant strain on this joint, leading to microtrauma. This continuous stress can cause the cartilage to fray and the joint to become inflamed, sometimes leading to a condition known as “weightlifter’s shoulder.”
Acute trauma to the shoulder is another major cause of AC joint degeneration. Direct impacts, such as falls onto the shoulder or contact during sports, can injure the joint’s ligaments and surfaces. A common example is a shoulder separation, which involves tearing of the ligaments supporting the AC joint, potentially leading to misalignment and increased wear over time.
Factors Influencing Degeneration Progression
Several elements do not directly cause AC joint degeneration but can significantly influence its progression or severity. These factors contribute to the ongoing breakdown and symptomatic presentation of the condition.
Genetic predisposition can play a part in an individual’s likelihood of developing arthritis, including in the AC joint. A family history of arthritis may increase susceptibility, though environmental factors tend to have a greater impact on AC joint osteoarthritis than genetics alone.
Certain inflammatory conditions can also contribute to joint breakdown and worsen existing degeneration. Systemic diseases causing widespread inflammation can affect joint tissues, accelerating cartilage damage and bone changes in the AC joint. Chronic inflammation can lead to reduced synovial fluid production, increasing friction and pain within the joint.
Poor biomechanics, including issues with posture or movement patterns in the shoulder, can place abnormal stress on the AC joint. Imbalances in muscle strength or coordination can alter the way the joint moves, leading to uneven loading and accelerated wear. This increased mechanical stress can exacerbate the degenerative process.
Previous injuries, even minor or long-healed, can contribute to ongoing degeneration. Damage to the joint surfaces or supporting ligaments from earlier trauma may predispose the AC joint to develop arthritis years later. Such past injuries can lead to subtle instability or altered joint mechanics, fostering continued wear.
When to Suspect AC Joint Degeneration
Recognizing the signs of AC joint degeneration involves observing specific symptoms related to shoulder movement and comfort. The condition manifests as localized pain on top of the shoulder. This pain often worsens with activities like raising the arm overhead, reaching across the body, or sleeping on the affected side.
Patients may also experience clicking, grinding, or popping sensations within the shoulder joint. Tenderness to the touch directly over the AC joint is a common finding, and sometimes a visible bump or swelling may develop. Diagnosis involves a physical examination by a healthcare provider. Imaging tests, such as X-rays, are often used to confirm degenerative changes, though some individuals may show changes on imaging without experiencing symptoms.