Anterior 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's barely bigger than a quarter. What keeps the ball centered on that small dish is a stack of soft-tissue stabilizers: a fibrocartilage 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 that actively hold the ball into the socket through every motion.

Anterior instability is what happens when those stabilizers — primarily the front part of the labrum and the inferior glenohumeral ligament — tear or stretch out 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 won't go back without help. The first dislocation almost always happens in a specific position — arm raised away from the body and rotated outward, the position you'd be in to throw a ball or block a tackle — and it almost always tears the front of the labrum off the glenoid rim (a Bankart lesion) and often dents the back of the humeral head where it impacted 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 anterior stop. The recurrence rate is age-driven: in athletes under 25, the chance of a second dislocation after a first runs 70–90%; in patients over 40, recurrence is much less common but the first dislocation often comes packaged with a rotator cuff tear instead. Each subsequent dislocation does a little more damage on the way out and back in — wearing the front rim of the glenoid down (glenoid bone loss) and enlarging the Hill-Sachs dent on the humeral head — until eventually there's not enough socket left to keep the ball in even with surgical repair of the soft tissues alone.

The first dislocation is unmistakable: a sharp pain, the arm locked in an abducted position, an obvious deformity at the front of the shoulder where the humeral head has shifted forward, and an inability to bring the arm down. It usually has to be reduced (popped back in) by a provider with appropriate sedation and technique, and the patient remembers exactly where it happened years later.

What follows is usually more subtle and is the diagnostic puzzle. The shoulder feels loose — patients describe a sense the joint might give out during the throwing motion, when reaching back to catch themselves in a fall, or when rolling onto that arm in bed. Many patients learn to avoid specific positions — arm raised and externally rotated — because that's the position the shoulder feels least secure in. Recurrent subluxations may not be dramatic enough to warrant an ER visit but produce repeated brief episodes of pain and instability that gradually erode confidence in the arm. What's not typical: pain in a specific arc of overhead motion without instability (think rotator cuff irritation), pain at the very top of the shoulder (think AC joint), or pain that radiates down the arm with numbness (think cervical spine). Anterior instability is fundamentally a feeling that the joint is going to come apart, not just an ache.

The diagnosis is mostly made on history and exam. Your provider will put your arm into the throwing position — abducted to 90 degrees and externally rotated — and watch your face. If the shoulder genuinely feels like it's about to come out, you'll grab their arm to stop them; that's the apprehension test. Then they press down on the front of the shoulder from behind (the relocation test) — if the apprehension goes away, you almost certainly have anterior instability. The pattern is so reliable that the exam alone usually makes the diagnosis.

Imaging is then about quantifying bone loss, which is what determines the surgical plan. Standard X-rays show any bony Bankart fragment broken off the front of the socket and large Hill-Sachs dents on the humeral head. MR arthrography (MRI with contrast injected into the joint) shows the labral tear in detail and is the standard pre-op study. CT arthrography is added when bone loss looks significant — CT measures glenoid bone loss in three dimensions to within a millimeter, and the answer changes the operation: less than ~15% bone loss is usually fixable with a soft-tissue Bankart repair, more than ~20% generally needs a bone-block procedure (Latarjet) to restore the socket itself.

Surgery is the answer for recurrent instability that's interfering with daily life or sport, for young contact athletes after a first dislocation (the recurrence math makes a single repair cheaper than the cumulative damage of repeated dislocations), and for any patient with significant glenoid bone loss. The choice of operation is driven by how much bone is left.

For shoulders with intact bone, an arthroscopic Bankart repair reattaches the torn labrum and ligament back to the front rim of the glenoid using small suture anchors drilled into the bone — restoring the anterior stop. For shoulders with significant glenoid bone loss (more than about 15–20%), or for revision cases where a previous Bankart repair has failed, a Latarjet procedure physically transfers a small block of bone (the coracoid process) with its attached muscle from inside the shoulder onto the front rim of the socket — both replacing the lost bone and creating an active sling effect that prevents the ball from sliding forward.

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

• Britannica — Dislocation
• Britannica — Shoulder (anatomy)
• AAOS OrthoInfo — patient education library
• OSI glossary — orthopedic terminology in plain language