How is the AC Joint usually injured?

shoulder1_1001 (Photo credit: Ce nest pas un JB.)

The AC joint is injured most often when one falls directly on the point of the shoulder. The trauma will separate the acromion away from the clavicle, causing a sprain or a true AC joint dislocation. In a mild injury, the ligaments which support the AC joint are simply stretched (Grade I), but with more severe injury, the ligaments can partially tear (Grade II) or completely tear (Grade III). In the most severe injury, the end of the clavicle protrudes beneath the skin and is visible as a prominent bump.