Anna Gragert
March 23, 2016 9:48 am

Warning: If you habitually crack your knuckles and want to remain blissfully unaware of what happens to your bones when you do so, then you should probably look away right now.

If you’re intrigued and prefer to avoid all knuckle-related warnings, then… Cool. Let’s proceed.

This is what it looks like (on the inside) when you crack your knuckles:

Surprisingly, there is a lot of controversy surrounding knuckle-cracking. Since the 1940s, two different schools of thought have emerged, leading to many different studies on the topic. According to rheumatologist Dr. Mahsa Tehran, one side believes that knuckle-cracking occurs when two bones are stretched to the point where they crack. This causes a cavity to form within the joint, possibly creating a bubble. Then again, another group believes that the cracking occurs as a result of bubbles collapsing within the joint.

So…which is it?

In April 2015, an Alberta, Canada-based research group published a PLOS One article stating that “bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon.” They used an MRI to capture a man cracking all 10 of his knuckles to see exactly what was happening, internally. They found that the cracking sound materialized when negative pressure inside the joint ended up creating a cavity. Dissolved gases in the joint fluid were also released when the joints quickly separated – creating a bubble, not popping one. This is known as tribonucleation.

However, a new study conducted by Robert Boutin – Professor of radiology at the University of California – aimed to further investigate this claim. Using ultrasound imaging (which, according to Science Alert, is 100 times faster than MRIs) and 40 adult volunteers, they analyzed what happened when the knuckle was cracked at the finger’s base. Some participants regularly cracked their knuckles 20 times a day for 40 years, while others claimed that they’d never intentionally done so before.

Boutin told Mirror“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.”

What they ended up observing was surprising: “What we saw was a bright flash on ultrasound, like a firework exploding in the joint,” revealed Boutin, adding that they aren’t sure which comes first: the flash of light or the cracking sound. Also, does the cracking sound come from a bubble that’s popping in the joint or a bubble that’s being created there?

Answer: More research is needed to figure that one out, but they are sure of one thing, according to Professor Boutin, “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.”

Despite differences in their studies, the Alberta group and Boutin’s team both found that occasional knuckle-cracking is harmless. “This group found that right after cracking, the joint space is not altered in any way, whereas previously it was thought that by cracking your joints, you increased the joint space. Occasional knuckle-cracking shouldn’t have any major ramifications,” writes Dr. Tehrani. “The finding that the joint rebounds back to its normal position after knuckle-cracking supports this.”

Professor Boutin confirms this, stating, “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.”

Yet, Dr. Tehrani warns that one shouldn’t partake in consistent knuckle-cracking if they cope with underlying inflammatory arthritis. She also adds, “There is no evidence that knuckle-cracking will lead to arthritis. However, you should be aware that part of getting knuckles to crack entails the gliding of tendons past one another. Whenever there is excessive repetitive activity in the tendons, the risk of inflammation also increases. Therefore, if you are looking to pick up a new hobby, knuckle-cracking should not be at the top of your list.”

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