Knuckle cracking – A Model

Interestingly, a paper was recently published challenging what we had seen in our PLOS One study. It would seem reasonable that there would be others looking to crack the code on the sound source of a joint pop. It is such a common phenomenon. In their study, they used a mathematical model to simulate what they believe is going on inside a joint when it makes an audible release sound. You know, that cracking sound that chiropractors generate to help those with joint pain.

To take you back a bit on what all the fuss is about, it was a researcher by the name of Roston

1 that brought forward the idea that the cracking sound from a knuckle was due to the formation of a cavity within the joint. It wasn’t until 1971 when another researcher by the name of Unsworth 2, challenged this cavity formation idea with a study explaining that the sound was from a cavity collapsing phenomenon.

The collapsing cavity theory remained the best explanation until we conducted an MRI study in 2015 3 that challenged the sound source was related, again, to the formation of cavity, resorting back to what Roston had proposed in 1947.  However, in our conclusions, tribonucleation, which is the formation of a cavity from the rubbing of structures, did not have strong supporting references contributing to the magnitude of the classic cracking sound. That is, when some cracks their knuckles, people can often hear it from across the room. Tribonucleation has not been shown to create such loud sounds.

This is what got the researchers of this recent publication curious. They used a mathematical model study using the Rayleigh-Plesset equation in the attempt to resolve the debate of whether the cracking sound is related to the formation or the collapse of a cavity. In their calculations, they believe it is to be related to a partial collapse.

Again, like with our PLOS One study, the media has gone wild with this publication. But the truth is, the plot has only thickened. The debate gets even more intense.

Quietly, I have developed a physical model that may settle the debate once and for all. Data collection is underway at UBC. I look forward to contributing to the science of sounds from our joints.


knuckle cracking

Investigating the sound source of knuckle cracking.


  1. Roston, J. & Haines, R. W. Cracking in the metacarpo-phalangeal joint. Journal of anatomy 81, 165 (1947)
  2. Unsworth, A., Dowson, D. & Wright, V. Cracking joints – A bioengineering study of cavitation in the metacarpophalangeal joint.
    Annals of the rheumatic diseases 30, 348 (1971)
  3.  Kawchuk, G. N. et al. Real-time visualization of joint cavitation. PloS one 10, e0119470 (2015)
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