AC Joint Pain: Common Causes, Degenerative Conditions, and Overuse

What are common causes of AC joint pain?

Acromioclavicular (AC) joint pain commonly arises from acute traumatic injuries, such as falls or direct impacts to the shoulder, or from chronic degenerative processes and overuse, particularly in individuals engaged in repetitive overhead activities or heavy lifting.

Understanding the Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is a critical articulation in the shoulder complex, connecting the acromion process of the scapula (shoulder blade) to the distal end of the clavicle (collarbone). While seemingly small, this joint plays a vital role in shoulder stability, range of motion, and force transmission from the arm to the axial skeleton. It is stabilized by the AC joint capsule, the superior and inferior AC ligaments, and, more significantly, by the coracoclavicular (CC) ligaments (conoid and trapezoid), which provide robust vertical stability. Dysfunction or damage to these structures can lead to debilitating pain and impaired shoulder function.

Traumatic Injuries

Acute trauma is arguably the most frequent cause of significant AC joint pain, often resulting in an AC joint sprain or separation. These injuries are classified into grades (I-VI) based on the severity of ligamentous damage and displacement of the clavicle relative to the acromion.

• Direct Impact to the Shoulder: A common mechanism involves a direct blow to the top or side of the shoulder, often with the arm adducted (close to the body). This type of impact forces the acromion downwards, while the clavicle remains relatively fixed, stressing or tearing the AC and/or CC ligaments.
• Falls onto an Outstretched Hand (FOOSH): While more commonly associated with wrist or elbow fractures, a FOOSH injury can transmit force up the arm to the shoulder, potentially injuring the AC joint, especially if the fall results in the shoulder being driven into the ground.
• Sports-Related Injuries: Contact sports (e.g., football, rugby, hockey) and activities with a high risk of falls (e.g., cycling, skiing, wrestling, martial arts) frequently lead to AC joint injuries due to direct collisions or impacts.

Degenerative Conditions

Chronic pain in the AC joint often stems from degenerative changes that occur over time, exacerbated by repetitive stress and aging.

• AC Joint Osteoarthritis (Degenerative Arthritis): Similar to other joints in the body, the cartilage within the AC joint can wear down over time due to aging, repetitive microtrauma, or previous injury. This leads to bone-on-bone friction, inflammation, and the formation of osteophytes (bone spurs). Symptoms typically include gradual onset of pain, stiffness, and tenderness, particularly with overhead movements or reaching across the body.
• Distal Clavicle Osteolysis (DCO) / Weightlifter's Shoulder: This condition involves the breakdown and erosion of the bone at the end of the clavicle, specifically where it articulates with the acromion. It is most commonly seen in athletes, particularly weightlifters, who engage in repetitive heavy bench presses, dips, and overhead presses. The repetitive compressive and shear forces across the joint are thought to cause microtrauma, leading to bone resorption and pain.

Overuse and Repetitive Stress

Even in the absence of acute trauma or advanced degeneration, repetitive activities can induce inflammation and pain in the AC joint.

• Repetitive Overhead Activities: Occupations or sports that involve frequent overhead arm movements (e.g., painting, carpentry, throwing sports like baseball or volleyball, swimming) can place chronic stress on the AC joint, leading to inflammation (AC joint synovitis) and pain.
• Repetitive Lifting and Carrying: Activities requiring repeated lifting of heavy objects or carrying loads on the shoulder can contribute to microtrauma and inflammation in the AC joint, particularly if proper biomechanics are not maintained.
• Poor Biomechanics and Posture: Suboptimal shoulder mechanics, such as rounded shoulders or scapular dyskinesis (impaired movement of the shoulder blade), can alter the alignment and loading of the AC joint, predisposing it to irritation and pain during movement.

Less Common Causes and Contributing Factors

While less prevalent, other conditions can also cause or contribute to AC joint pain.

• Inflammatory Arthritis: Systemic inflammatory conditions like rheumatoid arthritis, psoriatic arthritis, or ankylosing spondylitis can affect various joints, including the AC joint, leading to pain, swelling, and stiffness.
• Infection: Although rare, an infection (septic arthritis) in the AC joint can cause severe pain, redness, swelling, and fever.
• Referred Pain: Pain perceived in the AC joint area might sometimes be referred from other structures, such as the cervical spine (neck) or surrounding shoulder musculature.
• Post-Surgical Complications: Following shoulder surgery, complications like hardware irritation (if plates or screws were used) or persistent inflammation can contribute to AC joint discomfort.

When to Seek Professional Help

If you experience persistent or severe AC joint pain, especially following an injury, or if pain interferes with daily activities or sleep, it is advisable to consult a healthcare professional. A thorough examination, often including imaging (X-rays, MRI), can help accurately diagnose the cause of the pain and guide appropriate treatment, which may range from conservative measures like rest, ice, and physical therapy to surgical intervention in severe cases.

Conclusion

AC joint pain is a common complaint with a diverse etiology, primarily stemming from acute traumatic injuries, chronic degenerative processes, and repetitive overuse. Understanding these common causes is crucial for accurate diagnosis and effective management, enabling individuals to return to their activities with reduced pain and improved function. Prioritizing proper training techniques, maintaining good posture, and addressing early signs of discomfort can help mitigate the risk and severity of AC joint issues.