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Why Do Knuckles Crack?

Scientists may have discovered what causes the crackling noise people make with their knuckles.

By Dominic Umile

Do you ever pop your knuckles? It might seem like a simple, familiar habit, but scientists have disagreed for years about what actually causes the sound. However, a recent study done by scientists in Canada may have cracked the case.

TO CRACK OR NOT TO CRACK?

Why do scientists care about cracking? People have long suspected that knuckle-cracking might lead to arthritis, a disease that causes a person’s joints to become swollen and painful. However, studies have shown that popping your knuckles doesn’t appear to cause arthritis. 

Richard Thompson is a biomedical engineering professor at the University of Alberta who worked on the recent study. “I personally do not like the cracking sound at all,” he told the Reuters news service. “I think primarily because I always imagined there was damage being done.”

BUBBLING BETWEEN THE BONES

For the study, researchers had a machine pull on a man’s finger to crack his knuckles. Scientists used a combination of video and magnetic resonance imaging (MRI) techniques. MRI uses radio waves to produce pictures of organs and structures inside a person’s body.

The scientists discovered that the knuckle-cracking sound comes from the creation of a bubble of gas within fluid that exists between our finger bones. The bubble forms when the bones stretch apart. When this happens, there is often not enough fluid available to fill the space between the bones. It takes a bit longer than 300 milliseconds for the sound to be produced. (That’s one third of a second.)

The new study confirms results from an older study done in 1947. That study also found that the knuckle-cracking sound was caused by the formation of a bubble. But a study in 1971 had different results. It concluded that the cause was the collapse of the bubble. For decades, scientists couldn’t agree whether the sound came from the collapse of a bubble or the formation of one.

This new knuckle knowledge may not seem very important. But it could help scientists study joint problems that can cause people chronic (long-lasting) pain.

Greg Kawchuk, the study’s lead author, is hopeful about the results. “It may be that we can use this new discovery to see when joint problems begin long before symptoms start,” he says. “[This] would give patients and clinicians the possibility of addressing joint problems before they begin.” 

Image courtesy iStockphoto.com

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