Mystery solved: Why knuckles crack

Hint: It’s not due to the popping of bubbles in the joints

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

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

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

We’ve all heard it — that loud “Crack!” when someone pulls on stiff knuckles. But what happens to make that sound? It’s something that scientists have puzzled over for decades. A new study has now used a high-speed camera to watch what happens to the joint. That popping sound comes from the formation of a bubble in the fluid between two bones in the finger, it finds.

And these new data may just settle an age-old debate.

In 1947, two researchers used a series of X-rays to probe why knuckles “Crack!” Their images indicated that the sound occurs when the bones at a joint rapidly separate, forming an air bubble. It’s a process known as cavitation. Twenty-four years later, a second study looked into the issue. Using similar methods, it concluded the pop was due to some bubble in the joint bursting.

Two conflicting explanations. Both involved bubbles. Although most people came to believe the second explanation, some Canadian researchers wanted to know the real truth. So they formed a research team and conducted tests to settle the dispute.

Gregory Kawchuk is a bioengineer and rehabilitation-medicine specialist at the University of Alberta in Canada. His team turned to magnetic resonance imaging, or MRI. It can study very fast processes. The team chose one of its members, Jerome Fryer, to lay on his stomach and put his hand inside the MRI machine. Why Fryer? He has an unusual ability. “We call him the Wayne Gretzky of knuckle cracking,” explains Kawchuk. “He can do it in all 10 fingers.”

The team stuck one of Fryer’s fingers into a tube. As the MRI recorded what was happening, that tube slowly pulled on Fryer’s finger until it cracked. What this revealed “supports the original 1947 study,” says Kawchuk.

As a finger is pulled, tension mounts in the knuckle joint. Fluid rapidly accumulates there. This shows up as a white spot on the MRI picture. Suddenly, a cavity — or bubble — opens. As it does, the knuckle makes a pop. It’s much like the sound that a suction cup makes as someone pulls it off of a glass window, Kawchuk says. The joint’s bubble can last for up to 20 minutes. And until it goes away, the knuckle will not be able to crack again.

The researchers hope to repeat their study with more volunteers. It would include some people who can’t crack their knuckles and others with joint diseases. By the way, the researchers say that despite “old wives’ tales,” being able to crack your knuckles could be a sign of healthy joints.

Kawchuk’s team shared its new findings April 15 in PLOS ONE.

Power Words

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bioengineer      Someone who applies engineering to solve problems in biology or in systems that will use living organisms.

cavitation The formation and quick collapse of bubbles in a liquid, usually caused by mechanical force.

cavity     A large open region surrounded by tissues (in living organisms) or some rigid structure (in geology or physics).

knuckle   A finger joint, especially the one between the phalanges (the first set of bones spanning out from the hand), and the metacarpals (the next set of middle finger bones).

magnetic resonance imaging (MRI)   An imaging technique to visualize soft, internal organs, like the brain, muscles, heart and cancerous tumors. MRI uses strong magnetic fields to record the activity of individual atoms.

old wives’ tale    A colloquial term used to describe an urban legend, “conventional wisdom” or other explanation that has been passed down through a community or between the generations. It may sound sensible, but these explanations are not based on science or data. Indeed, some may be grounded on little more than superstition.

X-ray   A type of radiation analogous to gamma rays, but of somewhat lower energy.

Tina Hesman Saey is a senior staff writer and reports on molecular biology at Science News. 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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