“What doesn’t kill you makes you stronger”—at least, that’s what Kelly Clarkson claims. And it’s often said to be true for bones. But, there’s no evidence that bones are harder to break after they’ve fully healed from a fracture—they might even be weaker.
There is a brief moment when a broken bone might be tougher while it’s healing, though, which might explain why the idea that broken bones are stronger came about. Usually, the first step to healing a broken bone is to make sure the fragments are in the right place, or set, and then to keep them there while the tissue regrows —hence, a cast or splint. And that regrowth happens in stages.
Right after a break, the space fills with blood which quickly clots. This basically hold down the fort until a special clean-up crew of cells can come in and remove any dead bone cells and broken fragments. Then, bone-forming cells start to fill in gaps caused by the fracture, making a soft callus.
Over 6-12 weeks, the addition of tougher materials converts this soft callus into a hard callus. This is probably where the myth of the stronger broken bone came from, because this area can be temporarily stronger than the surrounding bone. But that’s just because it’s bigger. It’s made of what doctors call woven bone because the bony fibers are kind of randomly tangled.
That makes it weaker per unit volume than normal lamellar bone, but it can be thicker than the original bone, so it takes more force to break than the bone nearby. And that callus doesn’t stick around for long. Within a month or so, the hard callus is remodeled into new lamellar bone. If all goes well, this new bone has the same strength as the rest of the bone which wasn’t fractured. In fact, it can be hard to tell where the break even occurred.
But things don’t always go well. And that’s probably part of why breaking a bone actually makes you more likely to break the same bone again. Though estimates vary between studies and by the type of bone, a 2012 study of women estimated that a fracture generally increases your odds of fracturing that same bone—sometimes, by six or more times. But such statistics don’t necessarily mean the new bone is weaker.
It’s not usually recorded whether a second break is on the exact same stretch of bone, for example. And even if it is, it’s not clear if that’s because the new bone there is weaker, or because the person engages in some physical activity that makes that spot the most likely one to break.
Also, a second break can happen simply because the bone as a whole is weaker from the time it spent resting while it healed. When bones aren’t pushed or pulled against by muscles, the body essentially assume that strength isn’t needed, so it starts to take the minerals back from the bone and use them elsewhere.
The flip side to that is that you can make your bones stronger in general by working them out. In fact, the science of strengthening bones is routinely used by athletes and astronauts. Weight-bearing exercises like jogging, and strength training exercises, like lifting, put just enough stress on bones to keeps bone-forming cells active and engaged, ensuring they lay down more of the hardening minerals like calcium that give bones their density and strength.
So stressing your bones a little makes them stronger—but breaking them doesn’t.