Posterior Shoulder Instability (Batter's Shoulder)

Description

Posterior instability means the ball of the shoulder slips backward, out the back of the socket. It is far less common than the forward kind, about one in twenty cases of instability, and it behaves differently. A full dislocation out the back is rare. What I usually see instead is a teenager with deep shoulder pain, a click, and a loss of power in certain positions, who often has no idea the shoulder is slipping at all. Because of that, posterior instability is missed more often than any other shoulder problem in young athletes.

The nickname "batter's shoulder" comes from baseball. It affects the lead shoulder, the one closest to the pitcher: the left shoulder in a right-handed hitter. At the end of a swing, especially a hard swing at an outside pitch that is missed, that arm is stretched across the body and the ball is driven backward against the back rim of the socket.

How it happens (the mechanism)

The back rim of the socket is held by the same ring of cartilage (the labrum) and capsule that hold the front, but the back side is thinner and the socket is naturally a little tilted that way in many people. Two kinds of force push the ball backward:

• One big load with the arm out in front and across the body, as in the batter's swing, a fall on an outstretched arm, or a blow to the front of the shoulder.
• Many smaller loads, which is more common: offensive linemen blocking with the arms straight out, lifters doing bench press and push-ups, rowers, swimmers, and overhead athletes whose follow-through drives the arm across the body hundreds of times a week.

Over time, the back labrum tears or frays, the back capsule stretches, and the ball slips a few millimeters with each push. Each slip pinches the labrum and causes the deep ache and the click.

Symptoms

• A deep ache at the back of the shoulder, often vague and hard to point to.
• Pain and weakness with pushing: bench press, push-ups, blocking, or pushing a door open.
• A click or clunk with certain motions, sometimes with a feeling of the shoulder "shifting."
• Loss of power in the swing, the push, or the follow-through.
• Rarely, a true dislocation out the back, which often happens during a seizure or an electric shock rather than in sport.

Many athletes play through this for a season or more, because there is usually no single dramatic injury.

Doctor examination and imaging

The telling tests load the ball backward: pressing the arm across the body and pushing the elbow toward the back of the socket while feeling for a clunk or for the familiar pain (the jerk test and the posterior load-and-shift). I also check for generalized looseness in the other joints, since some teenagers are loose in every direction and need a different plan. X-rays are usually normal. An MRI with dye injected into the joint shows the back labrum tear, any stretching of the back capsule, and the shape of the socket. A CT scan is added if the socket looks tilted or worn.

Treatment

Therapy first. For most athletes, three to six months of a focused program works: strengthening the rotator cuff muscles at the back of the shoulder, retraining the shoulder blade so it supports the socket, and avoiding the loaded pushing positions that drive the ball backward while the tissue settles. My home program for shoulder instability and scapular dyskinesis is the starting point. Bench press, dips, and push-ups come out of the training plan for a time, and the athlete learns to keep the elbows a little closer to the body when pushing.

Surgery. When pain and weakness persist after a proper trial of therapy, or when the MRI shows a clear tear in an athlete who needs to push and swing, an arthroscopic repair of the back labrum works well. The torn ring is reattached to the rim with small anchors and the stretched capsule is tightened. A sling for about four weeks, therapy for four to six months, and return to sport at about six months. A tilted or worn socket occasionally needs a bone procedure, but that is rare in teenagers.

Recovery and long-term outlook

Most athletes with posterior instability return to their sport, with or without surgery, once the diagnosis is made and treated for what it is. The main cost of this condition is the time lost before it is recognized. After a repair, about nine in ten return to the same level of play, including hitters and linemen.

When to call the doctor

Call if the shoulder locks or will not move after an injury, if there is numbness or weakness in the arm, or if deep shoulder pain and loss of power with pushing are not improving after six to eight weeks of therapy.

Learn more

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

My guides to anterior shoulder instability, multidirectional and global shoulder instability, and scapular dyskinesis.

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.