A shoulder sprain is a tear of shoulder ligaments, the tough bands of fibrous tissue that connect bones to one another inside or around the shoulder joint. Although most people think of the shoulder as a single joint between the upper arm bone (humerus) and the torso, the shoulder actually has several smaller joints outside the arm bone's socket. Ligaments connect the four bones that are important to the shoulder's function.
A sprain that tears ligaments in the shoulder most often occurs at the joint between the acromion and collarbone, called the acromioclavicular joint. This injury sometimes is called a shoulder separation. Less often, a shoulder sprain involves the joint between the breastbone and collarbone, called the sternoclavicular joint. This joint is within an inch of the midline of the chest. Many people would not guess that it's part of the shoulder.
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