Shoulder Instability (Shoulder Dislocation)
Shoulder instability occurs when the head of the humerus (arm) is moved out of the glenoid (socket).
The shoulder is the most mobile joint in the body, but to have this amount of motion, it is also less stable and more likely to dislocate than other joints. The shoulder works like a ball and socket joint, but the bones are much more like a golf ball on a tee. The socket is called the glenoid, and the ball is the upper termination of the humerus (arm) bone. The labrum is a ring of specialized soft cartilage (similar to meniscus tissue in the knee) around the rim of the glenoid. It helps to make the glenoid socket slightly deeper and makes the shoulder much more stable. The rotator cuff muscles are also very important for shoulder stability—they actively pull and compress the humeral head against the socket.
In a younger person’s shoulder, the labrum is often torn away from the bone when the shoulder dislocates (the ball comes all the way out of the socket) or the shoulder subluxes (the ball comes partially out of the socket). The labrum can also be damaged or torn with repetitive overhead use of the arm such as in throwing, tennis, and rock climbing. Damage to the labrum can be painful by itself, but most of the pain and disability comes from abnormal motion when the shoulder is used. The shoulder can also be unstable due to ligament or capsular laxity (looseness) and not specifically from a torn labrum.
Labral injuries and shoulder instability can occur in a variety of locations and patterns. The most common of these is an anterior labral tear (in the front of the shoulder), usually the result of a subluxation or dislocation in the front of the shoulder:
The next most common pattern of injury is a superior labral tear (at the top of the glenoid) which usually happens from repetitive overhead activities like baseball, volleyball, tennis, surfing/swimming, and even rock climbing. It can also occur from an acute traction injury, such as grabbing a rock hold or the rung of a ladder while falling, causing the arm to be pulled upward. These tears are referred to as SLAP (superior labrum, anterior to posterior) tears.
The shoulder can also dislocate or be unstable in a posterior direction (toward the back of the shoulder). This is either the result of repetitive “pushing” stress as with football linemen or from an injury that pushes the head to the back of the socket.
The last pattern of instability occurs as result of general laxity (looseness) in the ligaments and the shoulder capsule. This is usually accompanied by laxity in other joints (patients are typically female and often very flexible). For these patients, their shoulder is inherently unstable. As a result, it can become irritated or injured by a change in activity or strength, without a definitive labral tear.
The goal of all treatment for instability and labral injuries is to decrease pain and restore shoulder function. The specific treatment that your doctor recommends will depend on your age, sport and activities, and the type of injury.