Knee Fractures (Tibial Plateau)
- The tibial plateau is the flat tabletop on top of the shin that the thigh bone rests on — bust it and the knee loses its level floor.
- Most plateau fractures hit the lateral (outer) side, classically when a force drives the femur down into the tibia from the side.
- The big radiology jobs are: spot the fracture, measure how far the bone fragment has dropped or spread, and look for a fat-fluid level that proves you're dealing with a fracture even when you can't see the crack.
- X-rays start the conversation, but CT is what the surgeon actually plans on, because it shows the depth and the joint surface in 3D.
- A subtle plateau fracture that gets called a "sprain" is a classic miss — when a knee won't bear weight after trauma, hunt harder.
Picture the top of your shin bone as a small dinner table. Flat, sturdy, two surfaces side by side, and the bottom of your thigh bone (the femur) sits right on top like a heavy serving dish. The whole arrangement only works because that table is level. A tibial plateau fracture is what happens when someone drops the serving dish too hard and a corner of the table caves in. Now the dish tilts, slides, and the whole place setting is a mess. That, in one tablecloth-related image, is the disease.
How the table gets broken
The plateau usually breaks when the femur gets driven down into the tibia — think landing from a height, a car bumper to the side of the knee, or a fall in an older person with softer bone. Because most knees take side-impact and most people have a natural slight knock-knee angle, the lateral plateau (the outer half) takes the hit far more often than the medial side.
Two things tend to happen to the broken piece, and they're the two things you'll be measured on:
- Depression — a chunk of the joint surface gets punched downward, like a thumbprint pressed into soft clay. The table now has a dent.
- Splitting — the bone cleaves apart and the fragment slides sideways, widening the tabletop.
Plenty of fractures do both at once. Naming the pattern precisely matters, so it's worth being fluent in the vocabulary from how to describe a fracture before you start dictating.
What you see on the X-ray
On a frontal and lateral knee radiograph, you're scanning the plateau's smooth top edge for any step, dip, or break in that clean white line. A depressed fragment looks like a denser, sunken sliver where the surface should be flat. A split shows up as a lucent line running into the joint, often with the outer bone pushed apart.
Here's the catch that humbles everyone: some plateau fractures are maddeningly subtle on plain film. The crack hides, the depression is shallow, and the X-ray looks almost normal. That's where your secret weapon comes in.
The puddle of fat that gives it away
When the bone breaks into the joint, the marrow inside — which contains fat — leaks out along with blood into the joint capsule. Fat floats on blood the way oil floats on vinegar. On a horizontal-beam (cross-table) lateral X-ray, that floating fat settles into a crisp horizontal line above the denser blood below. Radiologists call this a lipohemarthrosis (lipo = fat, hem = blood, arthrosis = joint), and the visible boundary is a fat-fluid level.
A fat-fluid level in the knee means marrow fat has escaped into the joint — which means the bone is broken into the joint, even if you can't see the fracture line itself. It's the smoke that proves there's a fire.
CT: the surgeon's blueprint
X-rays start the conversation; CT finishes it. The surgeon doesn't really care that there's a fracture — they care exactly how deep the depression is and how far the fragments have spread, because those millimeters decide whether the knee gets fixed with screws and a plate or left to heal as is. CT reformats the plateau in every plane and reconstructs it in 3D, so the dent in the table becomes obvious and measurable.
A normal-looking X-ray does not clear a knee that can't bear weight. If the plain films are unconvincing but the story is worrying — especially with a lipohemarthrosis — CT is the next stop. It routinely reveals depression that the radiograph quietly hid.
Don't break the table and ignore the wiring
The plateau doesn't break in isolation. The same force that caves in the bone can shred the soft tissues — the cruciate ligaments, the menisci, and occasionally the nerves and the artery running behind the knee. A high-energy plateau fracture with a lot of displacement should make you specifically worried about the popliteal artery, because a dead-straight or dislocated knee can kink it.
The "knee sprain" trap: a patient twists the knee, the X-ray looks okay-ish, and the fracture gets missed. Two tells should stop you — refusal to bear weight, and a fat-fluid level on the cross-table lateral. Either one earns a closer look or a CT.
Once the bony emergency is handled, the ligament and meniscus damage is sorted out on MRI — that's the territory of knee MRI detail. And if the knee hurts after repetitive loading with no single big injury, you're probably thinking about stress fractures instead.
The one thing to remember
The tibial plateau is the knee's level floor, and your job is to notice when it isn't level anymore. See the step, measure the depression and the spread, and let a fat-fluid level rat out the fractures you can't directly see. When in doubt on a non-weight-bearing knee, get the CT — the table is too important to leave dented.