Causes / typical injury mechanism: Acromioclavicular joint arthritis is degenerative osteoarthritis at the acromioclavicular (AC) joint. Degeneration is usually the result of chronic, repetitive, heavy axial loads across the small contact area of the joint. Alternatively, it could be posttraumatic osteoarthritis from a single significant injury to the AC joint, such as a clavicle fracture or AC separation.

Classic history and presentation: Many individuals have asymptomatic AC joint arthritis, seen incidentally on x-rays. This condition can become painful in patients who have been working overhead, pushing / pulling heavy loads, throwing, or swimming. A single impact, as from a fall, to an AC joint with pre-existing arthritis can also flare pain.

Patients present with pain and/or a clicking or catching sensation on the "top" of their shoulder that worsens with cross-body movement and overhead activity, and may wake them from sleep when rolling onto the affected side. Pain improves with rest by unloading the upper extremity and using it near the body.

Patients may also have pain around the scapula from muscle strain, or at the mid-upper arm, referred from the rotator cuff.

Prevalence: Joint degeneration increases with age. Symptoms appear predominantly in individuals in their 40s.

Risk factors:

• Heavy loading of the shoulder, especially overhead or pushing / pulling
• Prior clavicle fracture
• Prior AC separation
• AC joint instability
• Inflammatory arthropathy

Pathophysiology: Osteoarthritis is a progressive, degenerative joint disease caused by destruction of joint cartilage and subsequently underlying bone. Osteoarthritis of the AC joint is caused by deterioration the AC joint articular cartilage from heavy axial loads across the small contact area of the joint. Such trauma also damages the fibrocartilaginous disk located within the joint between the acromion and clavicle. The degree of structural change does not correlate with the severity of clinical symptoms.

Grade / classification system: No generally accepted classification system is used for AC joint arthritis. Most practicing physicians simply classify this as mild, moderate, or severe, which is somewhat subjective.