Shoulder Instability

The shoulder is comprised of 5 joints or articulations to the body. The primary joint is the glenohumeral joint. This is the ball and socket that allows the arm to be elevated overhead, rotated out to the side or placed behind the back. To allow such large ranges of motion, the joint has a shallow socket which is less constraining. I like to think of the joint as a golf ball sitting on a tee. The shoulder socket (glenoid) is wider than a golf tee but just a shallow. Since the glenoid is part of the scapula (shoulder blade), it is also moving as the ball (humeral head) moves. This makes it more difficult for the humeral head to remain located within the glenoid. Instability commonly results when the ligaments that hold the joint together become damaged or are loose. The glenoid socket is deepened by a cartilage rim that is attached around the perimeter of the socket called the labrum. Damage to the labrum or fractures of the rim of the socket can also make the humeral head more likely to displace out of the glenoid.

Classification of shoulder instability

Shoulder instability is classified by the amount of displacement, the direction of displacement, the frequency of the instability and the overall laxity or looseness of the joint ligaments. A displacement of the humeral head to the edge of the socket without completely coming out of joint is called a subluxation. Complete displacement out of the socket is a dislocation. There are 3 primary directions that the shoulder can sublux or dislocate. The most common direction is to the front or anterior. The humeral head can also displace out the bottom of the socket or inferior. There can also be displacement to the back or posterior. Patients with ligament laxity (especially if "double-jointed") will have instability in all directions called multidirectional instability. A common test for ligament laxity is the ability to pull the thumb back to touch the forearm just above the wrist. Hyperextension or back-bending of the elbow and knee are also signs of joint laxity. A shoulder that has subluxed or dislocated more than once has recurrent instability.