What Is It?
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. These bones include:
The triangular shoulder blade, called the scapula
The bony knob at the top of the scapula, called the acromion
The collarbone, called the clavicle
The breastbone, called the sternum
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.
Acromioclavicular Joint Sprain
The acromioclavicular joint is supported by the acromioclavicular ligament and the coracoclavicular ligament at the outside end of the collarbone near the shoulder. They bind the shoulder blade and collarbone tightly together. It takes a lot of force to tear these ligaments. The most common causes of this type of shoulder sprain are either a strong, direct blow to the front or top part of the shoulder or trauma from a fall, especially during athletic training or competition. A shoulder sprain also can be caused when a person collides with an object, such as a goal post or a tree (when skiing). Shoulder sprains are common among athletes who participate in high-velocity or contact sports such as alpine skiing, jet skiing, football, rugby and wrestling.