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We’ve Finally Figured Out What Makes Knuckles Crack

The real reason why your knuckles crack is called “tribonucleation,” and it overturns an idea we’ve held for decades.

Before lead author Gregory Kawchuk and colleagues published what they call the “Pull My Finger” study yesterday in the journal PLOS ONE, for the last 40 years we thought that the sound knuckles make when cracked is the result of a bubble popping inside the fluid that lines our finger and toe joints. But before that, way back in 1947, another theory prevailed — cracking your knuckles made a sound because of the formation of a bubble in joint fluid. Sixty eight years after this theory was first proposed and then “discredited,” Kawchuk and his team has confirmed it. Knuckle cracking should be called “making finger bubbles.”

KnuckleCrack_PIC1The experimental set-up. (Click to enlarge.)

To get a subject for the study, Kawchuk asked for help from a colleague, known as the “Wayne Gretzky of finger-cracking,” Kawchuk told NPR. He had this habitual cracker place his hand inside an MRI machine and attached what was basically a Chinese finger trap to his pointer finger. Then, under tension, the MRI spun and scanned as knuckles cracked.

KnuckleCrack_PIC2The subject’s hand before and directly after cracking. (Click to enlarge.)

To their surprise, the team found no bubbles or voids in the joint fluid before cracking, but some forming during and directly after. In the picture above, the image on the right shows the void created, designated with the yellow arrow. Below, in slide C, another angle shows a bubble forming in joint fluid immediately after cracking. This is tribonucleation.

KnuckleCrack_PIC3Slide C shows a dark void form in the subject’s joint fluid immediately after the knuckle cracks. (Click to enlarge.)

Tribonucleation is the formation of tiny bubbles between two submerged surfaces when those surfaces are suddenly pulled apart. Think of it like the famous Mentos and Diet Coke reaction. When there is dissolved gas in a fluid, those gases will form bubbles, and rapidly, if given the chance. The Mentos give the carbon dioxide dissolved in the Coke “nucleation sites,” or spots to gather, and this causes a cascading effect that gives us a geyser.

Although the specific mechanism of tribonucleation isn’t known, what’s clear is that moving objects submerged in a fluid (past a certain threshold depending on speed, fluid viscosity, and other factors) can also provide nucleation sites between those objects allowing bubbles to form. It is this rapid formation of a sustained bubble in between your knuckles as bones and tissue quickly separate that makes the tell-tale cracking sound. The small pops are likely from the pressure waves produced from moving joint fluid around.

We know at least one other thing about knuckle cracking too: it’s totally safe, so crack away!

IMAGES: Crackly by Jaysin Trevino; Kawchuk et al.

STUDY: Real-Time Visualization of Joint Cavitation

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