Human beings—well, the rude ones—crack their joints. It’s a thing. But scientists have never really understood the physics behind that chilling noise. (And yes, they care.) In the 1970s, most experts thought it had to do with the collapse of air bubbles in the synovial fluid that lubricates joints. But new evidence suggests the sound is actually caused by precisely the opposite: the formation of a gas-filled cavity when the bones in joints stretch apart.
But how would one study such a thing? First, researchers at the University of Alberta found someone who could crack his knuckles over and over again, without the long refractory period most people have. Yup, he was multiply crackasmic.
Then the scientists put this crack-addict’s fingers into a magnetic resonance imager, watching cracking events as they took place. That’s what’s in the GIF we made you from the researchers’ video. As the bones in the joint separate, negative pressure means gas (likely nitrogen) in the synovial fluid gathers together, resulting in the sudden formation of bubbles—the scientific term for that is tribonucleation. And with that comes the pop.
Look closely at the GIF. You can see a flash in the space between the joint surfaces as they separate. That’s an MRI signal that indicates cavitation via tribonucleation. As the joint surfaces come together, the bubble rapidly collapses—but that happens after the sound has already been emitted, contrary to earlier understandings of the popping phenomenon.
This isn’t just weird; the researchers think there’s some real medical insight that can be pulled from this study. Jacob Jaremko, a radiologist at the University of Alberta and a co-author, says bubble collapse can result in shock and damage to surrounding structures. That’s one of the reasons cavitation is something to avoid around, let’s say, boat propellers. This latest study provides some new information on the physics behind joint cavitation, and researchers hope to follow up with more research that looks at the effects of bubble collapse in any places of the human body that exhibit flow dynamics, like the heart and bloodstream. “Any environment where a fluid is under pressure, there might be tribonucleation,” says Jaremko, creating effects at small level that might have large consequences.
It’s a tough problem, but maybe they’ll be able to crack it.