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What Exactly Is Happening When Your Knuckles Crack

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What Exactly Is Happening When Your Knuckles Crack?

by Justine Alford
Photo credit: University of Alberta/ Screenshot of YouTube Video

Knuckle cracking: immensely satisfying to some, revolting and cringe-worthy to others. Regardless of whether you love it or hate it, have you ever wondered what is actually making that loud noise? Well, wonder no more, as scientists think they have finally cracked it. After filming a man’s joints using an MRI scanner, researchers discovered that the popping sound is actually due to the rapid formation of a cavity in the fluid surrounding the joint. You can read about their findings in the open access journal PLOS ONE.
While it may seem trivial, scientists have debated the source of the distinctive sound produced when joints are cracked for some time now, and several different hypotheses have been put forward. Back in 1947, for example, scientists published a study in which they had used radiography to visualize what’s going on. They concluded that the formation of a clear space, or bubble, was responsible for the cracking noise, and this idea remained unchallenged for some 25 years.
But confidence in this idea began to wane, so a team of scientists decided to repeat the experiment in the ‘70s. Although they used a similar protocol, they came to a different conclusion: the noise was coming from the collapse, rather than the formation, of a bubble. And to make matters more complex, other groups later proposed several more ideas, such as the recoil of the ligaments or the formation of a vapor cavity.
Since nobody seems to be able to make up their mind, scientists from the University of Alberta decided to design a more robust and detailed study that they hoped would finally settle the argument. For their investigation, which was nicknamed the “pull my finger study,” they captured joint cracking in real-time using cine magnetic resonance imaging (MRI). To do this, one of the study authors was placed face down inside an MRI machine and his fingers were slowly pulled by a cable until they cracked, an event that occurred in less than 310 milliseconds.
Each time, they found that the sound was associated with the rapid formation of a gas-filled cavity within the lubricating fluid surrounding the joint, which remained visible after the noise was produced.
“It’s a little bit like forming a vacuum,” explains lead author Greg Kawchuk. “As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what’s associated with the sound.”
Interestingly, they also observed a transient white flash that appears just before the joint cracks, which had never been observed before. Although they’re not sure yet what this is, they hope further investigations using more detailed scans will shed some light on this.
Alongside finally figuring out what’s going on inside those noisy joints, the researchers hope that this work will spur further research into the potential harms or benefits of joint cracking, which we are often told “gives you arthritis,” despite a lack of evidence for this. It may be the case that the ability to crack knuckles could be related to joint health, Kawchuk explains, which could have implications for other joints.

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