Researchers pull fingers to solve why knuckles crack

Finding bursts bubble of popular theory


KNUCKLING DOWN  A knuckle at rest (left) pops when pulled. The sound of the knuckle cracking comes from a bubble (right, arrow) opening in the joint.

G.N. Kawchuk et al/PLOS ONE 2015 (CC BY 4.0)

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Knuckles crack when a bubble forms in a joint, new high-speed images reveal.

The finding, reported April 15 in PLOS ONE, may settle a decades-old debate about the source of the sound.

In 1947, two researchers used a series of X-rays to determine that the “Crack!” comes when joints rapidly separate to form an air bubble, a process called cavitation. A 1971 study used similar methods but concluded that the pop is the sound of the bubble bursting.

Gregory Kawchuk, a bioengineer and rehabilitation medicine specialist at the University of Alberta in Canada, and colleagues used faster, more detailed magnetic resonance imaging, or MRI, to watch what happened as they slowly pulled a man’s finger until his knuckle cracked. Coauthor Jerome Fryer, a chiropractor in Nanaimo, Canada, was the test subject. “We call him the Wayne Gretzky of knuckle cracking,” says Kawchuk. “He can do it in all 10 fingers.”

POP GOES THE KNUCKLE MRI images helped researchers determine how a knuckle cracks. As a man’s finger is pulled, a bubble forms in the joint, making the popping noise as it appears.G.N. Kawchuk et al/PLOS ONE 2015 (CC BY 4.0)

“What we’re seeing on the MRI supports the original 1947 study,” Kawchuk says. As the finger is pulled, tension mounts in the knuckle joint and fluid rapidly accumulates, showing up as a white spot on the MRI picture. Then a cavity suddenly opens, producing the pop, much like the way a suction cup being pulled off a window does, he says. The bubble remains for up to 20 minutes, making it impossible to crack the joint again until it goes away.

The researchers hope to repeat the study with more people, including some who can’t crack their knuckles and some with joint diseases. Despite old wives’ tales, the ability to crack knuckles may be a sign of healthy joints, the researchers say, and therefore useful for monitoring joint health.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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