Shoulder Trauma & Dislocations
- The shoulder is a golf ball (humeral head) on a golf tee (glenoid) — great range of motion, lousy stability, so it pops out a lot.
- The vast majority of dislocations are anterior — the head ends up down and in front. Posterior is rarer and the one everybody misses.
- On the AP view a posterior dislocation hides in plain sight; you need a second view (axillary or scapular Y) before you call the shoulder "located."
- Two classic dents go with anterior dislocation: a Hill-Sachs divot in the humeral head and a Bankart chip off the front-lower glenoid rim.
- Don't forget the bone the shoulder is bolted to — clavicle fractures and AC joint separations live next door.
The shoulder trades stability for freedom. Your hip socket is a deep cup that grips the femoral head like a hand around a doorknob; your shoulder socket is more like a saucer trying to balance a melon. That shallow design is exactly why you can scratch the middle of your own back — and exactly why the shoulder is the most commonly dislocated large joint in the body. Freedom has a price, and the price is that this thing comes apart if you look at it wrong.
The golf-tee problem
The humeral head (the ball) sits against the glenoid (a small, flat-ish socket on the shoulder blade). Picture a golf ball resting on a tee. As long as everything's lined up, it's fine — but it doesn't take much of a shove in the right direction to send the ball off the tee entirely. The muscles and a rim of cartilage called the labrum are what keep the ball where it belongs, and trauma is in the business of overpowering them.
When the ball leaves the tee, that's a dislocation. Which direction it leaves matters enormously, because it changes both what you see and what got damaged on the way out.
Anterior: the common one
The overwhelming majority of shoulder dislocations are anterior — the humeral head slides forward and down, ending up in front of and below the glenoid. The classic mechanism is an arm forced into the "stop, in the name of the law" position: abducted and externally rotated. On the AP radiograph the head sits inferomedially, often tucked under the coracoid, no longer stacked against the socket.
Two souvenirs frequently come along for the ride:
| Finding | What it is | Where to look |
|---|---|---|
| Hill-Sachs lesion | A dent in the back-outer humeral head, dinged against the glenoid rim on the way out | Posterolateral humeral head |
| Bankart lesion | A chip or tear off the front-lower glenoid rim (the labrum, sometimes with bone) | Anteroinferior glenoid |
If you see a Hill-Sachs dent, your brain should immediately whisper "this shoulder has been anteriorly dislocated" — the divot is a footprint left at the scene.
Posterior: the one everybody misses
Posterior dislocations are much less common, but they are famous for slipping past tired eyes — the radiology equivalent of stepping over a coin on the sidewalk. The reason is sneaky: on a straight AP view, a posteriorly dislocated head can still look roughly centered over the glenoid, so at a glance nothing screams "wrong."
The tells on the AP are subtle. The head rotates internally and takes on a rounded, symmetric "light bulb" shape instead of its normal asymmetric club look. The smooth half-moon overlap between head and glenoid gets disrupted. Classic associations: seizures and electrocution — the kind of violent, all-at-once muscle contraction that yanks the head backward.
Never clear a shoulder on the AP view alone. A posterior dislocation can masquerade as a located joint. Always get a second, orthogonal look — an axillary view or a scapular Y — before you say the head is where it belongs.
The second view is the whole game
This is the single habit that saves you: the AP tells you something is off, and the axillary or scapular Y view tells you which way. On the scapular Y, the glenoid sits at the center of a "Y" formed by the scapular body and processes, and the humeral head should be parked right over that center. Anterior = head in front of center; posterior = head behind it. One extra picture turns a guess into an answer.
Reduction is not the finish line. Always re-image after the head is put back to confirm it's truly relocated and to catch a fracture that the dislocation was hiding — or that the reduction itself created.
Don't forget the neighbors
"Shoulder trauma" is bigger than the ball and socket. The same fall that dislocates a shoulder loves to break the bones bolted to it:
- Proximal humerus fractures — common in older patients with thinner bone; describe them like any fracture using the fracture-description framework.
- Clavicle fractures — usually the middle third, often from a direct fall onto the shoulder.
- AC (acromioclavicular) joint separation — the ligaments tethering the collarbone to the acromion tear, and the clavicle rides up. A "shoulder separation" is a different animal from a "shoulder dislocation," even though patients use the words interchangeably.
And the soft-tissue story — rotator cuff, labrum, the works — is a job for MRI, covered over in shoulder MRI. For the broader bony cast of characters in this region, see upper extremity fractures.
The one thing to carry out the door
If you remember nothing else: a shoulder is only "located" once a second view says so. Most dislocations are anterior and obvious; the posterior ones are quiet and that's exactly why they get missed. Get the orthogonal picture, every time, and the saucer-balancing-a-melon joint will stop embarrassing you.