Osteochondroma
- An osteochondroma is a bony bump growing off the surface of a bone, capped by a layer of cartilage — it's the most common benign bone tumor.
- The single defining feature: the lesion's marrow and cortex flow continuously into the parent bone. They're one continuous piece, not a thing stuck on top.
- It points away from the nearest joint, like a branch growing back down the tree.
- It's usually harmless. The thing to watch is the cartilage cap — a thick cap (especially one that grows after the skeleton matures) raises the worry for malignant change.
- Multiple osteochondromas? Think hereditary multiple exostoses, which carries a higher transformation risk.
Imagine a tree branch that, instead of growing outward, decided to grow as a knobby bump right out of the trunk — same wood, same bark, just pointed in a weird direction. That's an osteochondroma. It's bone growing off of bone, and it is the most common benign bone tumor you'll meet. The good news for the reader: once you know the one trick, you can spot it from across the room.
What it actually is
During childhood, your long bones grow at the growth plate. An osteochondroma is essentially a piece of that growth plate that wandered off course and set up shop on the outer surface of the bone. It keeps growing the way the rest of the bone does — laying down cartilage at its tip, then turning that cartilage into bone underneath. That tip of living cartilage is the cartilage cap, and it matters enormously, so hold onto it.
Because it grew from displaced growth-plate tissue, it stops growing when you stop growing. Skeleton matures, the osteochondroma calls it a day. That timing is your friend — it'll come back when we talk about the scary version.
The one finding that nails the diagnosis
Here's the trick, and it's beautiful in its simplicity: the cortex and the marrow of the osteochondroma are continuous with the cortex and marrow of the parent bone. The hard outer shell of the bump flows directly into the hard outer shell of the bone, and the spongy marrow inside the bump is plumbed straight into the bone's own marrow. No seam. No "stuck-on" appearance.
This is what separates a true osteochondroma from something merely sitting on the bone's surface (which is a much more worrying conversation). If you can trace the cortex running uninterrupted from bone into bump, you're basically done.
Two flavors of the same lesion: a pedunculated osteochondroma has a narrow stalk (the classic mushroom on a stem), while a sessile one is broad and flat, hugging the bone like a low hill. Sessile lesions are a bit harder to read and, as a group, get watched a little more carefully.
It classically arises at the metaphysis — the flared end of a long bone, near where the growth plate was — and famously points away from the nearby joint. Picture that branch again: it angles back down the trunk, not up toward the canopy. Most live around the knee (distal femur, proximal tibia) and proximal humerus.
When to actually worry: the cartilage cap
Most osteochondromas are happy, boring, and asymptomatic — found by accident, or because someone felt a hard lump, or because the bump rubbed a nerve, tendon, or blood vessel and made a nuisance of itself.
The real question is whether one has crossed over into a chondrosarcoma — malignant transformation. The headline clue is the cartilage cap. In a quiet osteochondroma the cap is thin. In a transforming one, the cap gets thick, and MRI is the way to measure it, because cartilage is mostly water and lights up brightly on fluid-sensitive sequences — like measuring frosting on a cupcake when the frosting is the part that can go bad.
The most useful red flag is growth after skeletal maturity. A bump that enlarges in a fully grown adult, develops new pain, or shows a thick cartilage cap has earned a closer look. Remember: a normal osteochondroma should have stopped growing when you did.
A single sporadic osteochondroma carries a low risk of malignant transformation. The risk is meaningfully higher in hereditary multiple exostoses (multiple osteochondromas, inherited) — so when you count more than one, the surveillance conversation changes.
How the imaging shakes out
You usually diagnose this on a plain radiograph alone; the cortical-and-marrow continuity is often obvious. CT shines when the lesion sits somewhere anatomically confusing — like the pelvis or scapula — and you need to prove that continuity in cross-section. MRI is the tool for the cartilage cap and for any soft-tissue mass that shouldn't be there.
| Modality | What it adds |
|---|---|
| Radiograph | First-line; shows the bony bump and cortical/marrow continuity. |
| CT | Confirms continuity in complex anatomy; great for sessile lesions. |
| MRI | Measures the cartilage cap and hunts for malignant change. |
If you ever want the bigger framework for sorting a bony lesion into "calm" versus "call someone now," it's worth a detour through aggressive vs non-aggressive bone lesions and the patterns of periosteal reaction.
The one thing to remember
Trace the cortex. If the bone's outer shell and marrow run continuously into the bump, you've found an osteochondroma — and the only follow-up question that really matters is how thick that cartilage cap is.