Acromioclavicular joint (ACJ) cysts are a benign manifestation, ganglionic or synovial.
Classification
Depending on their aetiology they are classified as type 1 and type 2 cysts.
Type 1 cysts: present in advanced ACJ arthritis, not associated with rotator cuff tear, secondary to degenerative changes of the joint surfaces that cause irritation of the synovium, with hyperproduction of fluid with subsequent formation of joint superficial cysts.
Type 2 cysts: associated with chronic rotator cuff tear with superior humeral head migration, irritation and deterioration of the ACJ capsule. Increased synovial fluid production causes fluid to pass from the glenohumeral joint to the communicating ACJ to produce cysts (geyser sign).
Treatment
Type 1 cyst: surgical excision associate to resection of the lateral clavicle.
Type 2 cyst: reverse shoulder arthroplasty (RSA).