Shoulder Instability

Your shoulder is a shallow ball-and-socket joint. The round head of your upper arm bone (the humeral head) sits against a small dish on your shoulder blade (the glenoid). That dish is barely bigger than a quarter. A stack of soft tissues keeps the ball centered on it: a cartilage rim around the edge of the socket that deepens it (the labrum), a tough capsule that surrounds the joint, several thickened bands inside the capsule (the glenohumeral ligaments), and the rotator cuff muscles, which actively hold the ball in the socket through every motion.

Anterior (front-direction) instability happens when those stabilizers tear or stretch out, mainly the front of the labrum and one key ligament, and the ball starts slipping forward out of the socket. It runs on a spectrum. A subluxation is when the ball partially slips out and snaps back on its own. A dislocation is when it goes all the way out and will not go back without help. The first dislocation almost always happens in one position: arm raised away from the body and rotated outward, the position you'd use to throw a ball or block a tackle. It almost always tears the front of the labrum off the rim of the socket (a Bankart lesion). It often also dents the back of the humeral head where it hit the rim on the way out (a Hill-Sachs lesion).

Once the labrum is torn and the ligaments stretched, the shoulder no longer has its front stop. The odds of it happening again depend on age. In athletes under 25, the chance of a second dislocation after a first runs 70-90%. In patients over 40, a repeat is much less common, but the first dislocation often comes packaged with a rotator cuff tear instead. Each new dislocation does a little more damage on the way out and back in. It wears down the front rim of the socket (glenoid bone loss) and enlarges the Hill-Sachs dent on the ball. Eventually there may not be enough socket left to keep the ball in, even with repair of the soft tissues alone.

Anterior is the dominant pattern, roughly 95% of shoulder instability cases, and most of this page describes it. Two other patterns deserve mention. They show up differently and can be missed when only anterior instability is on the table:

• Posterior instability: the ball slips backward rather than forward. Classic causes include a seizure, an electric shock, a heavy bench-press push, or a fall onto an outstretched hand with the arm reaching forward. The labral tear is on the back rim of the socket (reverse Bankart) rather than the front. Patients more often describe a looseness when pressing or pushing, a feeling that the shoulder is sliding out the back. The dramatic dislocation event of anterior instability is less common here. Treatment follows the same path: PT first, repair of the back of the labrum when surgery is needed.
• Multidirectional instability (MDI): the shoulder is loose in more than one direction at once, often without a single clear injury. It tends to show up in younger patients whose joints are naturally loose all over. They describe the shoulder slipping out in multiple positions. PT is the first-line treatment for the vast majority and works for most. In the rare cases where strengthening alone does not restore stability, a capsular shift operation tightens the capsule from inside.

The sections below mainly describe anterior instability. The same overall framework adapts to posterior and MDI, with the surgery flipped or modified to match. Bankart repair has a posterior version for tears at the back of the labrum, and Latarjet has counterparts for bone loss at the back in the rare patient who needs them.

The first dislocation is unmistakable. There is sharp pain, and the arm locks in a raised position that cannot be brought down. The front of the shoulder looks visibly out of shape where the ball has shifted forward. The shoulder usually has to be popped back in by a provider, using sedation and a controlled technique. Most people remember exactly where they were when it happened, years later.

What follows is usually more subtle, and that is the diagnostic puzzle. The shoulder feels loose. You may sense the joint might give out during the throwing motion, when reaching back to catch yourself in a fall, or when rolling onto that arm in bed. Many people learn to avoid one position, arm raised and rotated outward, because the shoulder feels least secure there. Repeat subluxations may not be dramatic enough to warrant an ER visit. But each brief episode of pain and slipping erodes confidence in the arm a little more. What is not typical: pain in one arc of overhead motion without instability (think rotator cuff irritation), pain at the very top of the shoulder (think AC joint), or pain that runs down the arm with numbness (think neck). Anterior instability is at its core a feeling that the joint is going to come apart, not just an ache.

The diagnosis is mostly made on history and a focused physical exam of the shoulder, in which your provider gently tests whether the joint feels like it is about to slip out of place. The pattern is often clear enough that the exam alone makes the diagnosis.

Imaging is then about measuring bone loss, because that is what sets the surgical plan. Standard X-rays show any piece of bone broken off the front of the socket and large Hill-Sachs dents on the ball. An MRI with contrast injected into the joint (MR arthrography) shows the labral tear in detail. It is the standard study before surgery. A CT scan is added when bone loss looks significant, because it measures how much bone has been lost from the socket, and that answer changes the operation. When little bone has been lost, a soft-tissue Bankart repair is usually enough. When a larger amount is gone, a bone-block procedure (Latarjet) is generally needed to restore the socket itself.

Treatment for shoulder instability is matched to the person, not just the injury. Two things shape the plan: how much the shoulder is interfering with daily life or sport, and how much bone is left around the socket. For many patients, strengthening the muscles around the joint is the right first step. For others, the recurrence math points toward surgery sooner. The path below starts with the non-operative option we reach for first.

Surgery is the answer in three settings. The first is repeat instability that interferes with daily life or sport. The second is a young contact athlete after a first dislocation; one repair costs less than the added-up damage of repeated dislocations. The third is any patient with significant bone loss at the socket. The choice of operation is driven by how much bone is left.

For shoulders with intact bone, an arthroscopic Bankart repair is used. Working through a camera and small tools, the surgeon stitches the torn labrum and ligament back to the front rim of the socket with small anchors drilled into the bone. That restores the front stop. For shoulders with significant bone loss, or when a previous Bankart repair has failed, a Latarjet procedure is used. It moves a small block of bone with its attached muscle onto the front rim of the socket. That both replaces the lost bone and creates a sling that keeps the ball from sliding forward.

External patient-education references and related OSI pages for additional background:

• AAOS OrthoInfo, patient education library
• OSI glossary, orthopedic terminology in plain language