disruption tear of the AC joint and a partial disruption of coracoclavicular ligament. The AC gap is >5 mm. Grades I and II never require surgery and heal by themselves, though physical therapy may be required. Grade III is complete disruption of AC and CC ligaments. On plain film the inferior aspect of the clavicle will be above the superior aspect of the acromion. This can also be assessed with an MRI scan, which will also demonstrate disruption of the coracoclavicular ligaments (the degree depending on the severity of AC joint disruption) as well as tearing of the joint capsule. The joint will be very tender and swollen on examination. Grade III separations most often do not require surgery and shoulder function should return to normal after 16–20 weeks. However, there will be some physical deformity of the shoulder with a noticeable bump resulting from the dislocation of the clavicle. Grades IV-VI are complications on a 'standard' dislocation involving a displacement of the clavicle, and will almost always require surgery. Dynamic US is critical to detect mild (grade I) dislocations, especially because they may present as a normal joint in static images. Doppler US may also be used to increase diagnostic confidence to detect low-grade injuries because it depicts reparative process to injured ligament. Osteoarthritis Osteoarthritis of the acromioclavicular joint is not uncommon. It may be caused by a prior trauma (secondary osteoarthritis) or occur as a chronic degenerative disorder often co-existing with subacromial impingement. Additional images See also Shoulder girdle (Pectoral girdle) Glenohumeral joint (Shoulder joint) Shoulder Sternoclavicular joint References External links Images from an MRI scan demonstrating AC Joint Injury - Grade II sprain What is Acromioclavicular Joint Osteoarthritis? - www.SimonMoyes.co.uk Acromioclavicular joint disease Joints Upper limb anatomy