ACJ

Aetiology

Post-traumatic (type III clavicle fractures)

Idiopathic

4 patterns 

1. OA with osteophytes 

-  contribute to impingement

2. Osteolysis with resorption & gross osteoporosis 

Anatomy

Synovial joint with hyaline cartilage

Has fibrocartilage intra-articular disc

- complete or incomplete

- usually degeneration by 4th decade

Clavicle may lie superior to acromion in normal population

Acromioclavicular Ligaments