Osteoarthritis: This is the most common cause of arthritis in the acromioclavicular joint, leading to pain, stiffness, and potentially subluxation due to the degeneration of the joint cartilage and surrounding structures.

Traumatic Injury: Direct trauma to the shoulder or a fall onto an outstretched hand can cause a separation of the acromioclavicular joint, leading to subluxation or dislocation.
Rheumatoid Arthritis: Although less common than osteoarthritis, rheumatoid arthritis can affect any joint, including the acromioclavicular joint, causing inflammation, pain, and potential subluxation.
Psoriatic Arthritis: This form of arthritis can cause joint inflammation and damage, potentially affecting the acromioclavicular joint and leading to symptoms similar to those described.

Septic Arthritis: An infection within the joint can cause rapid destruction of the cartilage and bone, leading to severe pain, swelling, and potential long-term damage if not promptly treated.
Gout or Pseudogout: These conditions involve the deposition of crystals within the joint, causing acute inflammation and pain, and can occasionally affect the acromioclavicular joint.
Ligamentous Injury: Significant injury to the ligaments surrounding the acromioclavicular joint can lead to instability and subluxation, which might be mistaken for other forms of arthritis.

Spondyloarthritis: A group of diseases that can cause inflammation in the joints, including the acromioclavicular joint, though this is less common.
Neoplastic Processes: Tumors, either benign or malignant, can occasionally affect the bones or soft tissues around the acromioclavicular joint, leading to pain and instability.
Charcot Joint (Neuropathic Arthropathy): Although rare, this condition can cause progressive degeneration of a joint due to sensory loss, potentially affecting the acromioclavicular joint in the context of underlying neurological disease.