I’ve heard a lot of names given to the sound that occurs during some joint manipulations. “Popping”, “cracking”, “snapping”, and “popcorn” are just a few descriptions given. So what is it, really? Similar to the sound, a number of reasons have been proposed over the past century about why this occurs in the first place: “A vapour cavity is created,” “air bubbles dissolve into the joint,” “it’s the recoil of ligaments.” Fortunately, the exceptional researcher Dr. Greg Kawchuk at the University of Alberta has recently investigated this topic, providing some insight into the most current ideas.
The most available joints for investigating the “cracking” sound are, of course, the knuckles! Although the joints in our body are of different shapes and orientations, the classification, or general build, at the knuckles is the same as between the bones of our spine (vertebrae), shoulders, and hips, among others — synovial. As a result, we can say with some confidence that what we find to be true of the knuckles is very similar to what we will find in joints of the same class.
In simple terms, Dr. Kawchuk used real-time MRI to monitor the experience of “cracking” knuckles, tractioning (pulling) the finger until the sound occurs. What they found was that there was no collapse of air bubbles, or dissolving of air bubbles when the sound occurred — rather, a cavity is formed when the sound occurs.
More specifically, the joints in our knuckles are synovial joints, as mentioned, which means they are surrounded by a fluid cavity. Where the two bones in our fingers come together to make the knuckle, this fluid coating resists our finger-bones being pulled away — something called “viscous attraction”. Eventually, when the force separating these bones becomes strong enough, this fluid attraction is overcome and a rapid separation occurs. POP! That rapid separation corresponds to both the sound created as well as the creation of an air bubble. This whole process is called tribonucleation — and given Dr. Kawchuk’s recent research, this appears to be the best available answer to what that noise actually is.
If you’re interested, the article is open access — so anybody can read it.