Dislocations
- A dislocation is a joint that has come fully apart: the two bone surfaces that should be kissing are no longer in contact at all.
- A subluxation is the half-hearted version — the surfaces still touch a little, just misaligned.
- You almost always need two views at right angles; the bone can look perfectly seated on one view and be flung out the back on the other.
- Name the dislocation by where the distal bone went (anterior, posterior, etc.) — that direction drives the diagnosis and the reduction.
- After you spot it, hunt for the company it keeps: fractures, trapped nerves, and squashed vessels.
A joint is basically two bones holding hands. A dislocation is those hands letting go completely — the surfaces that are supposed to glide against each other are now pointing at totally different things, like two dance partners who wandered to opposite corners of the room. It is one of the more satisfying things to diagnose, because when you see it, you really see it. The hard part is not the obvious ones. The hard part is the sneaky one that hides on a single view and waits for you to look away.
Dislocation vs. subluxation
These two get muddled constantly, so let me anchor them. Think of a drawer in its slot.
- Dislocation: you yanked the drawer all the way out and it is sitting on the floor. The bone surfaces have no contact.
- Subluxation: the drawer is half-open and crooked, jammed at an angle. The surfaces still touch, just not where they should.
Same spectrum, different severity. Both matter, but a full dislocation is the one that makes the on-call orthopedist's phone ring at 2 a.m.
Two views, always
Here is the single rule that will save you the most embarrassment: one view is a lie waiting to happen. A bone can sit dead-center on the frontal film and be shoved completely behind the joint on the side film. If you only look straight-on, you will confidently call it normal and be completely wrong.
So you get two views at roughly ninety degrees to each other — usually a frontal (AP) and a lateral. The classic teaching example is the shoulder: a posterior shoulder dislocation can look almost normal head-on, and only the axillary or scapular-Y view gives it away.
If you ever feel certain about a joint from a single projection, that certainty is the trap. Find the orthogonal view before you commit.
Naming the direction
When you describe a dislocation, you name it by where the distal bone ended up relative to the proximal one. For the shoulder, that means where the humeral head went relative to the socket (the glenoid). For the hip, where the femoral head went relative to the cup (the acetabulum).
This is not just radiology pedantry — the direction tells you the likely mechanism and how someone will put it back. A few high-yield patterns:
| Joint | Common direction | Quick mental note |
|---|---|---|
| Shoulder | Anterior (most common by far) | Head slips down-and-forward, under the socket. |
| Shoulder | Posterior (much rarer, easily missed) | Classically after seizures or electric shock; looks deceptively normal on the AP. |
| Hip | Posterior (the usual one) | Femoral head rides up and back; leg often turned inward. |
| Elbow | Posterior | The forearm bones jump backward behind the humerus. |
This table is a starting map, not the whole atlas — every joint has its own quirks, and the directions above are the patterns you will meet most.
The company it keeps
Dislocations rarely travel alone, and the associated injuries are often what actually hurt the patient long-term. When bones get rammed out of a socket, the rim of that socket can chip, and the ball can get dented where it slammed against bone. Those bony bruises and chips are part of why someone's shoulder keeps popping out for years afterward.
So after you call the dislocation, do a deliberate second lap looking for fractures along the joint margins — and describe any you find using the usual fracture vocabulary. Some fracture-dislocation patterns are detailed enough that they get their own dedicated MRI workup down the line.
Don't stop reading the moment you spot the obvious dislocation — that is exactly when associated fractures get missed. The big finding pulls your eye and your attention right off the subtle chip next to it. Force yourself to finish the search.
Why it is an actual emergency
A dislocation is not just a misplaced bone — it is a clinical clock ticking, for two reasons.
First, vessels and nerves run right past joints, and a displaced bone can stretch or pinch them. Picture a garden hose pinned under a chair leg: the longer it stays kinked, the worse things get downstream. A knee dislocation in particular can threaten the artery behind the knee and the blood supply to the whole lower leg; a hip dislocation can choke off the supply to the femoral head itself, starving the bone. Either way, that is why these get reduced urgently rather than scheduled politely.
Second, the joint cartilage and surrounding tissue do not love being stretched out of position. The sooner the bone goes home, the happier everything around it stays.
A dislocation is a "reduce it soon" diagnosis. Spot it, name its direction, check the orthogonal view, then sweep for fractures and for signs that nerves or vessels are getting squeezed.
The takeaway
A dislocation is two joint surfaces that have completely lost contact; a subluxation is the partial version. Confirm it on two views at right angles, name it by where the distal bone went, and never let the dramatic main finding distract you from the small fracture — or the threatened nerve or vessel — hiding right beside it.