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Scientists have used ultrasound machines to figure out exactly what's going on in our joints when we crack them, a decade-old debate about where that distinctive cracking sound comes from.
Back in April, researchers from the University of Alberta published a paper based on MRI imaging of finger joints being said that popping sound is due to the fall of air bubbles that form the fluid that surrounds our joints - called synovial fluid. But ultrasound machines can record what is going on inside our bodies up to 100 times faster than MRIs, so scientists of another team decided to investigate this claim further. Furthermore, by the researcher Robert D. Boutin from the University of California, Davis, the team recruited 40 healthy participants, 30 of whom were regular joint-crackers, and 10 who were not. Of those who were habitual crackers, older people admitted to their knuckles up to 20 times a past 40 years.
The participants were asked to crack the knuckle at the base of each finger, known as the metacarpophalangeal joint (MPJ), while being observed through an ultrasound machine. They ended up imaging 400 MPJ cracks, and recorded the sounds so they knew which ones came with 'pop'.
While researchers expect to see something happening within the joints - Boutin told Rachel Feltman at the Washington Post that ultrasound machines can capture events over 10 times smaller than MRIs capable of capable of - they are not ready for the result of this ... explosive.
"What we saw was a bright flash on the ultrasound, like a firework exploding in the joint," Boutin said. "It was quite an unexpected finding."
The flashes in the ultrasound were coupled so consistently with the popping sound that the researchers could predict 94 percent accuracy which MPJ cracks 'popped' just by looking at the images.
The researchers suspect that the cracking and visual flash in the ultrasound images are related to changes in the pressure that occurs in the synovial fluid, as Boudin explained to Richard Hartley-Parkinson at Metro:
"There are many theories over the years and a fair amount of controversy about what's happening in the joint when it cracks. We are confident that the cracking sound and the bright flash on the ultrasound are associated with the pressure associated with a gas bubble in the combined. "
But a big mystery still remains. Back in 1947, a paper was published that the popping sound occurred when a bubble first formed in the synovial fluid of the joint. This hypothesis was refuted 30 years later when another group of researchers said it was more sense that the sound came from the bubble bursting.
In April, the University of Alberta team backed up the bubble-collapsing hypothesis with their MRI recordings, but they still did not come up with any conclusive evidence. So what is it, does the sound result from a bubble popping in the joint or a bubble being created in the joint?
"That's a surprisingly difficult question to answer," Boudin told Feltman at The Washington Post. "I'll tell you that we have consistently seen the bright 'flash' in the joint only after we heard the cracker. Never the other way around. Perhaps that supports the bubble formation theory, not the bubble popping theory. "
What the researchers say is that they found no immediate pain, swelling, or damage to joints as they were cracked, and found no difference between their habitual knuckle-crackers and those who never did it. This backs up of an experiment by a Californian medical doctor who spent 60 years cracking the knuckles on one hand, and not only the other, just to find no difference between the two.
While Boudin said more research needs to be done to confirm that no long-term damage is done, there is also a possibility that joint-cracking is really good for us. "After a joint cracks, the range of motion for that increase is significantly significant," he told Feltman.
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