What causes AC (acromioclavicular) joint swelling?

Causes of Acromioclavicular (AC) Joint Swelling

AC joint swelling results from either traumatic ligamentous injury, inflammatory arthropathy (particularly rheumatoid arthritis), rotator cuff pathology with cyst formation, infectious processes, or overuse-related osteolysis. 1

Traumatic Causes

• AC joint sprains and dislocations are the most common athletic injuries causing AC joint swelling, with variable degrees of ligamentous damage and displacement 1
• Direct trauma to the shoulder can lead to immediate swelling and joint disruption 2
• Distal clavicle fractures frequently disrupt the stabilizing ligaments of the AC joint, producing swelling even when the joint itself appears intact 1

Inflammatory and Degenerative Causes

• Rheumatoid arthritis affects the AC joint in approximately 85% of patients with shoulder involvement, causing tenderness, pain, and visible swelling 3
• AC joint destruction in rheumatoid disease shows time-dependent progression with subchondral bone erosion, tapering, and osteolysis of the acromial end of the clavicle 3
• The AC joint destruction is frequently associated with glenohumeral joint disease, suggesting systemic inflammatory involvement 3
• Distal clavicle osteolysis presents as an atraumatic, overuse condition with insidious onset, increasingly prevalent in weight-training athletes 1

Rotator Cuff-Related Causes

• AC joint cysts are rare sequelae of complete supraspinatus tendon rupture combined with AC joint arthritis, appearing as swelling over the AC joint 2
• These cysts must be distinguished from tumors of the shoulder and neck region through imaging 2
• Intramuscular ganglia arising from the AC joint can extend into the trapezius muscle and are usually associated with massive rotator cuff tears 4
• Ganglia with intramuscular extension can mimic tumors and require MRI for definitive characterization 4

Infectious Causes

• Septic arthritis of the AC joint, though rare, presents with joint swelling accompanied by systemic signs of infection 5
• Tuberculous arthritis can cause AC joint destruction and swelling, sometimes presenting as an undiagnosed extrapulmonary site in patients with miliary tuberculosis 6
• Imaging reveals joint destruction, and surgical drainage with synovectomy may be required for diagnosis and treatment 6

Diagnostic Approach Algorithm

Step 1: Determine if trauma occurred

• Recent trauma → Consider AC sprain/dislocation or distal clavicle fracture 1
• Remote trauma (years prior) → Consider late cyst formation from rotator cuff tear 2

Step 2: Assess for systemic inflammatory disease

• Known rheumatoid arthritis or polyarticular symptoms → AC involvement is present in 85% of cases 3
• Isolated AC symptoms without trauma → Consider overuse osteolysis in athletes 1

Step 3: Evaluate for infection

• Fever, systemic symptoms, or immunocompromise → Urgent arthrocentesis required to exclude septic arthritis 5
• Resistance to tuberculosis treatment → Consider undiagnosed AC tuberculosis 6

Step 4: Imaging sequence

• Plain radiographs first to assess bone integrity, joint space, and osteolysis 2
• MRI if cyst or rotator cuff pathology suspected, as this is the most useful tool for correct diagnosis 2
• Ultrasound can identify ganglia and guide aspiration if needed 4

Critical Pitfalls to Avoid

• Do not assume painless swelling is benign—AC joint cysts and ganglia must be distinguished from tumors through imaging 2, 4
• Do not delay arthrocentesis if infection is suspected, as imaging cannot distinguish infected from non-infected effusions 5
• Do not overlook rotator cuff pathology when evaluating AC joint swelling, as cyst formation is a sequela of complete supraspinatus rupture 2
• Do not miss rheumatoid involvement—AC joint tenderness occurs in one-third of rheumatoid patients with shoulder pain, but radiographic changes appear in 85% 3
• Recognize that intramuscular extension of ganglia can mimic tumors and requires MRI confirmation 4