Science and Tech


Why do bones break?

Dr. Claire Shannon, a pediatric orthopedic surgeon, says popping your knuckles – or cracking your knee or your wrist – is considered a 'phenomenon.'
Posted at 1:49 PM, Feb 01, 2023

Bones are the framework of our bodies and make up our skeletal system. But they aren’t indestructible and they can sometimes break. Scripps News went to an expert to find out why.  

"Bones break because of either two main reasons. One, the force on the broken is stronger than the bone can take. Or two, the bone itself is weakened for some reason," said Dr. Claire Shannon, a pediatric orthopedic surgeon. "You hear a crack because much like breaking a piece of wood, the physical structure is actually cracking in half, be it all the way or only partway." 

And no matter what you call it — it means the bone is in trouble.  

"So, there are lots of different types of breaks, but ultimately cracked, broken, fractured, snapped. You pick the term. They all mean the same thing. They mean the bones' integrity has been damaged."

But that "crack" from when a bone fractures or breaks is different from the noise you might hear when you intentionally crack your knuckles. 

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"When you pop your knuckles – or crack your knee or your wrist – that is actually a phenomenon where you get little bubbles inside the liquid pouches of the joints, where you get little nitrogen bubbles, and then when you squeeze it, it pops those little bubbles. And that's why popping or cracking your knuckles sounds more like a popping noise than a real, like, loud breaking crack," Shannon said. 

That bubble phenomenon is also why some bones in your body are more prone to pop than others, according to doctor Shannon.  

"The rounder smoother that a whole joint is, so for instance, a ball and socket joint like the hip or the shoulder, generally doesn't crack as much because there's not as many spaces for the bubbles to get trapped and get popped," Shannon said.

And while cracking or popping your knuckles occasionally isn’t harmful, a broken bone can be serious.  

So why haven’t our bones evolved to withstand breaks? 

"Bones are a balance of strength and weight. If we had bones that never broke, you'd never be able to walk or run or jump because they'd be so thick and heavy," Shannon said. 

Our bones are evolving, but at a super slow rate so that we can continue to stay on the move.