# Scientists Still Don't Know Exactly Why Knuckles Crack

It has something to do with bubbles?

The study, Vineeth Chandran Suja confesses, was born in a moment of frustration.

As a graduate student at École Polytechnique, he had to design a research project for a class on biomechanics. He was drawing a blank—until he instinctively cracked his knuckles. “I thought, ‘This is something interesting and maybe fun,’” Suja says. Of all the studies published on knuckle cracking, no one had mathematically described exactly how the knuckle produces the crack. Suja thought it had to do with a drop in pressure inside the knuckle when it is stretched, so he decided to do the math.

While he chipped away at this idea, a paper came out in 2015 that seemed to overthrow 40 years of knuckle-cracking orthodoxy. “Mystery solved,” the headlines proclaimed. Scientists at the University of Alberta had put a man in an MRI machine while he cracked his knuckles. Stretching the joint caused the pressure inside to drop and a bubble to form. From the MRI, it looked like the sound came at the moment bubbles formed in the knuckles—not when the bubbles collapsed, as researchers previously thought.

But this did not make sense with Suja’s math. Suja—who is now a PhD candidate at Stanford—had not stuck anybody in an MRI machine, but he and two friends spent some time recording their knuckle cracks in an anechoic chamber, a special room that is built to be completely echo-free. The auditory signature of those recordings matched a mathematical model he had devised to describe how bubbles behave in the egg-white-like synovial fluid that lubricates joints. This model said that an audible crack should happen when the bubble is partially collapsing, but microbubbles may still remain.

Now that we are several paragraphs deep in the nuances of bubble collapse, are you thinking what I’m thinking? I have seen many bubbles form and pop in life, and I have never heard anything as loud or sharp as the sound of a knuckle cracking. Really? Bubbles?

It turns out that the bubbles forming in knuckles are not quite like the bubbles we ordinarily encounter in water or air. After all, there is no air rushing your knuckle to form a bubble. Rather, these bubble form in a process called cavitation, when negative pressure “pulls” the joint fluid apart.

“It’s like the rupture of water,” says Philippe Marmottant, a physicist at the Laboratoire Interdisciplinaire de Physique, in France. Marmottant has studied how cavitation in trees will produce ultrasonic cracks. Cavitation can also happen when a boat’s propellers tear through the water, and its collapse can generate shock waves strong enough to tear a hole through metal propellers over time.
When cavitation occurs in the knuckle, the negative pressure causes gases dissolved in the synovial fluid to come out. It takes time for the gases to dissolve back in, which is why you cannot crack your knuckles again right away.

Suja did not mathematically model how bubbles first form in synovial fluid. He acknowledges this is the next step for ruling out bubble formation as the source of audible cracks.

Robert Boutin, a radiologist at the UC Davis, who has studied knuckle cracking with ultrasound imaging, says he still thinks bubble formation causes cracking based on his research, which came on the tail of the 2015 MRI study. “It’s very interesting, but incremental,” he says of Suja’s mathematical model, given the small number of participants involved. “Stay tuned in this space.”

The scientist who led the MRI study, Greg Kawchuk, called the new model “exciting.” He acknowledges that MRI is limited by how infrequently it can take an image—only once every 310 milliseconds in his case. In his study, the knuckle crack itself was captured in just one frame of the MRI video. It’s possible the bubble may be forming and partially collapsing all in that time.

“I don’t think we completely understand it yet,” Kawchuk says. “Anything for the topic area is a win and adds to what we need to know about this really important phenomenon.”