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.
Depending on the severity of the ligament damage, acromioclavicular sprains usually are classified into three grades:
Grade I — The acromioclavicular ligament is partially torn but its companion ligament, the coracoclavicular, is uninjured, so the acromioclavicular joint remains tightly joined.
Grade II — The acromioclavicular ligament is completely torn and the coracoclavicular ligament is partially torn. In this case, the collarbone typically angles slightly out of place.
Grade III — Both the acromioclavicular ligament and the coracoclavicular ligament are completely torn, and the collarbone separation is obvious.
Some doctors classify the most severe acromioclavicular injuries into even higher grades or types, from IV to VI. With each higher grade, the collarbone is more displaced away from its normal position and the shoulder is more severely deformed.
Sternoclavicular Joint Sprain
The sternoclavicular joint is located where the inner end of the collarbone meets the breastbone. Because the sternoclavicular joint is even more tightly connected than the acromioclavicular joint, sternoclavicular injuries occur very rarely, only about one-fourth as often as acromioclavicular injuries. When the sternoclavicular joint is sprained, it is often when a driver's chest strikes the steering wheel during an auto accident, or when a person is crushed by an object. In athletes, sternoclavicular sprains sometimes are seen among football players and rugby players after a direct kick to the breastbone or some sideways tackles that impacts the back or side of the shoulder.
Sternoclavicular sprains are graded from I to III:
Grade I — The tears in the joint ligaments are mild and microscopic. The sternoclavicular joint remains tightly connected.
Grade II — Ligaments between the collarbone and breastbone are visibly torn, but ligaments between the collarbone and ribs remain intact, so the joint is slightly deformed, but retains some connection.
Grade III — All ligaments sustain severe damage, so the sternoclavicular joint is separated or deformed, and the collarbone is clearly displaced from its normal position.