What makes a knuckle-cracking sound

Partly settled. Some of it is understood, some is still argued about.

The idea that knuckle cracking involves gas bubbles in the joint fluid goes back to X-ray studies by Roston and Wheeler Haines in 1947, and a 1971 study concluded the sound came from the sudden collapse of a cavitation bubble. In 2015 Kawchuk, Fryer and colleagues used real-time MRI to image finger joints as they were cracked and found the sound coincided with the sudden appearance (formation) of a gas cavity rather than its collapse, challenging six decades of consensus. This did not fully settle the matter: in 2018 Chandran Suja and Barakat built a mathematical model showing that a partial, rather than complete, collapse of the newly formed bubble best reproduces the measured acoustic signature, effectively reconciling elements of both the older 'collapse' and newer 'formation' pictures rather than declaring either fully correct, so the precise mechanism is still described by researchers as not completely settled.

A sound almost everyone has made with their own hands took seventy years, X-rays, real-time MRI and a mathematical model to even partly pin down.

Explanations that circulated

• The pop is simply a bubble collapsing (implosion) in the joint fluid, full stop — this was the dominant view since a 1971 study, but real-time MRI in 2015 instead showed the sound coincides with rapid cavity formation (not collapse) in the synovial fluid; a 2018 mathematical model then showed the sound is actually best explained by only a partial collapse of the newly formed bubble, bridging rather than simply confirming either older camp.

References

• Real-Time Visualization of Joint Cavitation, G. N. Kawchuk, J. Fryer, J. L. Jaremko, H. Zeng, L. Rowe, R. Thompson, PLOS ONE, 2015
• A Mathematical Model for the Sounds Produced by Knuckle Cracking, V. Chandran Suja, A. I. Barakat, Scientific Reports, 2018