Dynamic MRI of a Knuckle Cracking

From the medical oddities file, researchers from the University of Alberta have *finally* captured a knuckle popping on MRI in order to explain what goes on during when one "pops a knuckle" and where the sound comes from. Per the Wired article:

First, researchers at the University of Alberta found someone who could crack his knuckles over and over again, without the long refractory period most people have. Yup, he was multiply crackasmic. 

Then the scientists put this crack-addict’s fingers into a magnetic resonance imager, watching cracking events as they took place. That’s what’s in the GIF we made you from the researchers’ video. As the bones in the joint separate, negative pressure means gas (likely nitrogen) in the synovial fluid gathers together, resulting in the sudden formation of bubbles—the scientific term for that is tribonucleation. And with that comes the pop.

The GIF of the knuckle pop looks like this:

Source: Gregory Kawchuk, University of Alberta/PLOS Media via Wired

The actual research article has the more staid title of "Real-Time Visualization of Joint Cavitation" but honestly, the authors must have known that is not the source of interest in their work. If you want to know why exactly the sound occurs, the authors state:

Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble. These observations are consistent with tribonucleation, a known process where opposing surfaces resist separation until a critical point where they then separate rapidly creating sustained gas cavities.

The work also goes a long way towards explaining this video from the 1980s:

The research leads one to wonder: what other phenomenon have we been missing on MRI? If a patient has vacuum disc phenomenon in their lower lumbar spine, are they simply in need of a good back massage? Speaking of which, why do massages feel good? Clearly, we need to get a metal-free masseuse into a scanner and see what happens!

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