Anterior Shoulder Instability

Description

The shoulder is a ball sitting on a shallow saucer. The ball is the top of the upper arm bone (the humerus) and the saucer is a small socket on the shoulder blade (the glenoid). That design gives the shoulder more motion than any other joint in the body, and it also makes the shoulder the joint that dislocates most often. Anterior instability means the ball slips forward and down, out the front of the socket. It is by far the most common direction, accounting for more than nine in ten shoulder dislocations, and it is most common in teenagers and young adults who play contact or overhead sports.

A full dislocation means the ball comes all the way out and usually has to be put back. A subluxation means it slips partway and slides back on its own, often with a sharp pain and a "dead arm" feeling for a few minutes. Both count as instability.

How it happens (the mechanism)

The classic position is the arm raised out to the side and turned back, like cocking the arm to throw or reaching to block a shot, and then a force pushes it further back: a tackle, a fall on an outstretched arm, a collision with the boards. The ball levers out over the front rim of the socket.

What holds the ball in the socket is a ring of cartilage around the rim (the labrum), which deepens the saucer, and the ligaments of the joint capsule, which attach to that ring. When the ball comes out the front, it almost always peels the labrum and ligaments off the front rim. That tear is called a Bankart lesion. The back of the ball can also be dented by the hard rim as it slips out (a Hill-Sachs lesion). Those injuries do not heal tightly on their own, which is why a shoulder that has dislocated once is loose, and why it can dislocate again with less and less force.

Age matters a great deal. In a teenager, the labrum tear is the usual injury and the chance of another dislocation is high, somewhere between half and nine in ten in the first two years depending on the sport. In a child whose growth plates are still open, the growth plate at the top of the humerus is weaker than the ligaments, so a fall more often causes a fracture there than a dislocation.

Symptoms

• At the time of injury: severe pain, the arm held slightly away from the body, a visible change in the shape of the shoulder, and an inability to move it. Sometimes numbness over the outer shoulder from a stretched nerve.
• After the shoulder is back in: soreness for days to weeks, then a feeling of looseness or apprehension when the arm goes up and back, as if it might slip again.
• With repeated episodes: the shoulder may slip with everyday reaching, rolling over in bed, or putting on a shirt.

Doctor examination and imaging

I check which position makes the shoulder feel unsafe (the apprehension test) and whether pressing the ball back takes that feeling away. I also look at the opposite shoulder, since some people are loose in both, and I check the nerves and the blood supply to the arm. X-rays confirm the shoulder is back in place and show any chip off the front rim of the socket. An MRI, usually with dye injected into the joint, shows the labrum tear and the dent in the ball. If I suspect bone has been lost from the rim, a CT scan measures it, because bone loss changes the operation.

Treatment

The first time. The shoulder is put back in, usually in an emergency room with medicine to relax the muscles. A sling for one to three weeks for comfort, then physical therapy to restore motion and to strengthen the rotator cuff and the muscles around the shoulder blade. Most athletes are back in four to eight weeks with a brace that limits the risky position, depending on the sport.

Deciding on a repair. This is where age and sport matter. A teenager in a contact or overhead sport who keeps playing after one dislocation has a very high chance of another, and each episode can wear more bone off the rim and make the final repair harder. For that reason I discuss an arthroscopic repair of the labrum (a Bankart repair) after a first dislocation in a young athlete who wants to keep playing a high-risk sport, and I recommend it after a second episode in nearly everyone. My home program for shoulder instability and scapular dyskinesis is the starting point for therapy either way.

The operation. Through three or four small incisions, the torn labrum and ligaments are reattached to the rim with small anchors. The arm is in a sling for about four weeks, therapy runs for four to six months, and return to contact sports is usually at five to six months. If a large piece of the rim has been worn away, the labrum repair alone is not enough, and a procedure that moves a small piece of bone to the front of the socket (a Latarjet) is used instead.

Recovery and long-term outlook

After an arthroscopic repair, about nine in ten young athletes return to their sport, and the chance of another dislocation drops to roughly one in ten. Without a repair, the odds of another episode in a teenager are high, and repeated dislocations are the main cause of early arthritis in the shoulder. Either way, keeping up the strengthening program is the best protection the shoulder has.

When to call the doctor

Call if the shoulder slips or dislocates again, if there is numbness or weakness in the arm or hand after an injury, if the shoulder cannot be put back and the arm looks deformed (go to the emergency room), or if pain and looseness are not improving after six weeks of therapy.

Learn more

OrthoInfo (AAOS), Chronic Shoulder Instability: orthoinfo.aaos.org (opens in a new tab)

OrthoInfo (AAOS), Dislocated Shoulder: orthoinfo.aaos.org (opens in a new tab)

My guides to posterior shoulder instability and multidirectional and global shoulder instability.

This information is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. For specific concerns about your child, consult a board-certified, fellowship-trained pediatric orthopaedic surgeon.