Knee MRI: Internal Derangement
- "Internal derangement" is just a fancy way of saying something inside the joint is torn or out of place — usually a meniscus, a cruciate ligament, or the cartilage.
- The menisci are two rubbery C-shaped cushions; on MRI they should be uniformly black, so any bright signal that touches a joint surface is the tear you're hunting for.
- The ACL normally looks like a taut, dark, straight band; when it tears it goes wavy, blurry, or simply vanishes.
- Read the knee systematically — menisci, ligaments, cartilage, bone — because a torn ACL loves to drag a meniscal tear and a bone bruise along with it.
The knee is a hinge that the body asked to do way more than hinge. It pivots, twists, absorbs your full weight on one leg, and then acts shocked when something inside snaps during a rec-league soccer game. When that happens, the X-ray usually shrugs — bones look fine — and the real story is hiding in the soft tissues. That's where MRI comes in. If you haven't met MRI properly yet, it's worth a quick detour through how MRI works and the T1 and T2 weighting basics, because the whole game here is which things light up and which things stay dark.
The menisci: two black crescents that should stay black
Picture two firm rubber wedges, each shaped like a C, wedged between the rounded end of the thigh bone and the flat top of the shin. They're shock absorbers and spacers. On MRI, healthy menisci are uniformly dark — low signal, like a clean black gummy candy. There's no water bouncing around inside to glow.
So a meniscal tear is almost embarrassingly simple to define: it's bright signal where there should be only black, and that bright signal has to reach a surface of the meniscus. That last part is the whole ballgame. Degeneration can make the inside of a meniscus a little hazy-bright without it being a true tear; what turns it into a tear you can call is that the abnormal signal touches the edge. If it doesn't reach a surface, you hedge — "intrasubstance degeneration," not a tear.
A couple of named patterns earn their keep here:
| Pattern | What it looks like | Why you care |
|---|---|---|
| Bucket-handle tear | A fragment flipped into the middle of the joint; the meniscus looks too small, with an extra dark band lying alongside the PCL ("double PCL sign"). | The flipped fragment can lock the knee — a surgical problem. |
| Horizontal / degenerative | A flat bright line splitting the meniscus like a hamburger bun. | Common with age; often managed conservatively. |
| Radial tear | A notch cut inward from the free edge. | Disrupts the meniscus's hoop strength, so it stops doing its job. |
The transverse meniscal ligament and a few normal vessels cross near the front of the menisci and can fake a tear at the meniscal junction. Trace the structure on adjacent slices: a real tear stays put and touches a surface; a normal ligament marches off to connect two menisci.
The cruciate ligaments: a taut rope vs. a wet noodle
The anterior cruciate ligament (ACL) is the one everyone's heard of, and the one that fails dramatically. Normally it's a straight, taut, dark band running diagonally through the joint — think of a guy-wire holding up a tent, snug and crisp. A torn ACL loses that crispness: the fibers go wavy, swollen, and hazy-bright with edema, or the band just isn't there anymore. A useful sanity check is that the normal ACL runs roughly parallel to the bony roof of the notch; a torn one droops below that line.
The ACL rarely tears alone. A classic pivot injury slams the shin bone forward, which bruises bone in two specific spots — the back of the outer shin plateau and the front of the outer thigh condyle. Spotting those paired bone bruises on a fluid-sensitive sequence is a big tip-off that the ACL is gone, even before you've stared at the ligament.
The posterior cruciate ligament (PCL) is the thick, strong one that arcs through the joint like a question mark. It's so robust that it tears much less often, and when it's torn it usually thickens and brightens rather than disappearing.
Bright = water = something's wrong. The recurring trick of knee MRI is comparing structures that should be black (menisci, ligaments) against the bright edema, fluid, and cartilage signal around them. Master that contrast and most of the joint reads itself.
Cartilage and bone: the slower-burning problems
The smooth articular cartilage capping the bone ends is the part that wears down over years. On the right sequences it shows up as a gray layer; tears and thinning show as fissures, divots, or full-blown bald spots where bone meets bone. Underneath, the bone itself can show a bone bruise (a fuzzy bright patch of marrow edema from impact) or, in arthritis, cysts and reactive changes.
This is also why sequence choice matters so much: fat-suppressed fluid-sensitive images make edema and cartilage damage glow, while the proton-density images give you crisp meniscal and ligament anatomy. If that menu is fuzzy for you, the common MRI sequences page lays out who's good at showing what.
Reading it without missing anything
The trap in knee MRI isn't seeing the obvious tear — it's stopping after you see it. So go around the joint every time, same order: menisci, then cruciate ligaments, then the side (collateral) ligaments, then cartilage, then bone. The torn ACL is delighted when you call it and quit, because the meniscal tear and the cartilage divot it brought to the party get to slip out unnoticed.
If you remember one thing: on knee MRI, the structures that are supposed to be black are your reference, and bright signal touching a surface is usually the diagnosis. Find the black things, ask whether they're still black and still where they belong, and the knee mostly tells you its own story.