Scientists Have Explained What Actually Happens When People Crack Their Knuckles

Most people around the world have an annoying habit. From chewing your nails to picking your nose, some quirks can seem impossible to break. And while most habits are relatively harmless, others aren’t. For example, arguments have raged for years over whether or not cracking knuckles is bad for you. Now questions about the custom’s potential ill-effects have finally been answered via an intriguing scientific study.

The debate regarding joint cracking has continued for decades in the science field. It all started with a study published back in 1947. The experiment was conducted by J.B. Roston and R. Wheeler Haines, as they looked to discover the cause of what’s still a common issue. And, using serial radiography, the pair appeared to find an interesting answer.

Following the cracking of a joint, Roston and Haines discovered that a bubble developed in the fluid between that particular joint. With that information in mind, the duo subsequently linked the cracking sound to the forming of the bubble. For the next 24 years, that interpretation was the standard. Then a new suggestion came to light, igniting the debate again.

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The counterclaim to Roston and Haines’ interpretation came from a trio named Unsworth, Dowson and Wright in 1971. Much like the previous pair, they used a similar procedure to confirm the inner workings of the joints. However, the three came to a very different conclusion. For them, the cracking sound was the result of the collapse of the bubble between the joints, not its formation.

Since that study over 40 years ago, both interpretations have been used in publications on the matter, proving the scientific divide. However, the knuckle-cracking debate took another turn in April 2015, as a number of scientists took a closer look at the issue. Surprisingly, the inspiration came from an unexpected source.

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Working as a professor of radiology at the University of California, Robert Boutin decided to kick-start the research on joint-cracking thanks to his 11-year-old daughter. Her curiosity on the matter inspired him to begin a new study, but the overall divisiveness of the debate also played a big role.

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“It’s extremely common for joints to crack, pop and snap,” Boutin said. “We were interested in pursuing this study because there’s a raging debate about whether the knuckle-cracking sound results from a bubble popping in the joint or from a bubble being created in the joint.” Subsequently, the professor began his ambitious experiment with the help of 40 adult patients.

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Aged between 18 and 63, the patients were comprised of 23 men and 17 women, but the differences didn’t end there. Of the 40 individuals, 30 were habitual knuckle crackers while the remaining ten were not. Boutin revealed that some of those patients had never cracked their knuckles willfully, while others had done so frequently for 40 years.

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Regardless of their usual habits, though, they were all tasked to crack their knuckles under the eye of an ultrasound machine. Boutin claims that an ultrasound can capture images 50 to 100 times faster, and ten times smaller, than an MRI of the fingers. From there, the professor and his colleagues could search for the answers they were looking for.

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While the patients tried to crack their knuckles at the base of each finger, known as the metacarpophalangeal joint (MPJ), orthopedists were assigned to evaluate the results. Unlike Boutin, however, they were unaware of the partakers’ knuckle-cracking habits. Despite that, the orthopedists looked at the patients’ grip strength, range of motion and laxity before and after the ultrasound.

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The in-depth study didn’t stop there, though. Via a sonographer, 400 MPJs were captured on video as the patients tried to crack their knuckles. In addition, the sonographer also caught stills of each joint both before and after the attempted cracking.

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Following that, two radiologists were tasked with interpreting the images, as they looked for evidence that lined up with the audible cracks. Of the 400 MPJ images, this happened in 62 of them, which led Boutin to describe the ultrasounds in vivid detail. “What we saw was a bright flash on ultrasound, like a firework exploding in the joint,” he said. “It was quite an unexpected finding.”

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“There have been several theories over the years and a fair amount of controversy about what’s happening in the joint when it cracks,” Boutin continued. “We’re confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint.”

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Indeed, the flashes came about so consistently that radiologists could predict which image revealed a cracking joint with 94 percent accuracy. Boutin was subsequently asked where his study sat in the big debate. Surprisingly, he was reluctant to firmly commit its position. However, he reiterated what the results likely meant in the grand scheme of things.

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“That’s a surprisingly tough question to answer,” Boutin told The Washington Post. “I will tell you that we consistently saw the bright ‘flash’ in the joint only after we heard the audible crack. Never the other way around. Perhaps that supports the bubble formation theory, not the bubble popping theory.”

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However, the tests also revealed some other interesting findings. Firstly, the patients suffered no immediate discomfort, swelling or disability after cracking their knuckles. Secondly, there was no noticeable difference in grip strength or laxity between those who cracked their knuckles regularly and those who didn’t.

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“We found that there was no immediate disability in the knuckle crackers in our study, although further research will need to be done to assess any long-term hazard – or benefit – of knuckle cracking,” Boutin said in a release. Yet when speaking to The Washington Post, the professor revealed an intriguing piece of information.

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“After a joint cracks, the range of motion for that joint increases significantly,” he told the newspaper. With that in mind, it could be that knuckle crackers enjoy some short-term benefits after practicing their habits. As for Boutin’s research, it was due to be presented at the Radiological Society of North America’s annual meeting.

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In 1947, then, Roston and Haines unwittingly kick-started a decades-spanning debate that divided the scientific community. Unsworth, Dowson and Wright’s work stoked that particular fire in 1971, but thanks to Robert Boutin the answer is somewhat clearer now.

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Many non-knuckle-crackers see the practice as a maddening habit that produces a stomach-churning sound. Even enthusiasts of the routine often view knuckle cracking as an annoying habit that’s hard to shake. For those aforementioned scientists, though, it means a whole lot more.

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