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Joint study: scientists use MRI to examine knuckle cracking

shutterstock_201633158.jpgA team of crack researchers finally may have solved the mystery of knuckle-popping.

In a study published recently, University of Alberta scientists describe how modern imaging technology has shed new light on the age-old riddle of why some joints crack when you pull them.


April 29, 2015
By Bob Weber The Canadian Press

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“It’s something that every culture, every society is interested in,”
said Dr. Greg Kawchuk, chiropractor and lead author of the paper
published in the online journal Plos One.

“We all do it. People love it or are repulsed by it.”

But nobody had actually looked closely at how the noise beloved by annoying uncles is created.

The
first paper on the subject dates back to 1947, said Kawchuk. It
theorized the crack comes from an air cavity created by the sudden
separation of the two affected joints.

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Nonsense, said a 1971
rejoinder in scientific knuckle-cracking literature, which suggested the
pop is the result of an air pocket collapsing.

Enough with theories, decided Kawchuk.

With
the help of a colleague – fellow chiropractor Dr. Jerome Fryer – whom
he describes as the Wayne Gretzky of knuckle-cracking, Kawchuk decided
to bring MRI technology to bear. No one had ever before looked inside a
knuckle as it cracked.

“When we saw that, we said, ‘Wow! There’s a real opportunity here.’ It’s been sitting there waiting for someone to do (it).”

The
team designed a carefully calibrated knuckle-puller that resembles the
kind of woven finger-trap on offer in novelty stores. The subject digit
was placed under a magnetic resonance imager and the requisite tug
applied.

Video of the event – which lasted about 310 milliseconds – was carefully analyzed.

At first, the surface tension of the fluid in the joints kept the bones together.

“As
we increased the pull, suddenly you reach the point where you overcome
that surface tension and the two joint surfaces suddenly fly apart,”
said Kawchuk. “In that moment, we saw the creation of an air cavity that
happens at the same time the sound is produced.”

A similar effect can be created by pressing two hands together at the palm, then quickly separating them.

The
imager wasn’t able to determine what’s in the cavity, air or a gas
released by surrounding tissues. Nor can Kawchuk be completely sure the
cavity is what creates the sound.

“We have to be a little bit careful,” he said.

“We’re
only imaging a very small slice of what’s happening inside the joint.
There’s things that are happening to the left and right and in between
our images that may be causing the sound, but we don’t see them.

“But what we did observe is consistent with what was proposed in 1947.”

The research does have real importance. Fingers aren’t the only joints in the body that crack.

“By using this technique to look into people’s joints, we have a little bit of a window to better understand joint health.”

As well, not all people are able to crack their knuckles. The paper could help scientists understand why.

Kawchuk’s
study is silent on the question of why some individuals enjoy cracking
their knuckles or whether it’s as bad for them as their mothers say.

“I’ve had people tell me, ‘It drives me crazy,’” he laughed. “It’s the kind of thing people lose marriages over, I think.”


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