This Is What Happens When You Crack Your Knuckles

Everybody knows somebody who cracks their knuckles. I’m one of them and you probably are too. There’s been a long argument regarding whether or not it’s actually bad for you and the general consensus is that it’s just bubble popping. However, researchers have been unsure if this popping resulted from a bubble popping within the joint or one being created in the joint.

At a meeting in the Radiological Society of North America in Chicago they determined that it’s likely the latter. Synovial fluid, which lubricates the joints, becomes tribonucleated when we stretch our fingers. this stretching allows gas into the synovial fluid, which subsequently forms small bubbles. That’s tribonucleation for ya. But that’s not all, folks! These rapid-forming bubbles tend to burst almost as quickly as they form, either when they expand to a certain size with the joint or when the joint collapses in on ’em. This is where the debate comes from: is it the sudden emergence of the bubbles or their collapse that makes the noise?

Well, an MRI study conducted in April took a very close look at the formation of the bubbles within the metacarpophalangeal joint (MPJ) at the base of the finger and found out exactly what was going on.

The mystery was declared solved when it seemed that the noise stemmed from the initial formation of the bubbles. So, that’s it, right? Wrong. Ultrasound imaging was the next method in line to test this theory, since it can see processes nearly 100 times faster than MRIs and detect things nearly 10 times smaller.

Robert D. Boutin from the University of California decided it would be a solid idea to conduct the old study using this newer method. Researchers observed images of over 400 knuckle cracks from 30 regular crackers and 10 who didn’t really do it. If you’ve already seen the video then you know what they saw.

“What we saw was a bright flash on ultrasound, like a firework exploding in the joint,” Boutin told The Washington Post. “It was quite an unexpected finding.” The frequency of the flashes was so great that radiologists were able to look at mutes videos and discern which showed a joint cracking with 94% accuracy. So, while it is the bubbles that causes the cracking it seems that researchers are still unsure of whether or not it’s the formation or the collapse that causes the noise.

“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,” Boutin continued. “Perhaps that supports the bubble formation theory, not the bubble popping theory.”

[IFL Science]

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