‘Pull my finger!’ Canadian scientists solve the knuckle-cracking riddle


A University of Alberta study finally settles the decades-long debate about what happens when you crack your knuckles.

The study published on April 15 in PLOS ONE used MRI video to determine what happens inside finger joints.

The cause of the popping sound? A rapidly forming cavity inside the joint.

Scientists have debated the cause of joint cracking for decades. In 1947, U.K. researchers theorized vapor bubble formation as the cause. Then in the 1970s, another team of scientists said the cause was collapsing bubbles.

Chiropractor Jerome Fryer approached professor Greg Kawchuk about his knuckle-cracking theory. This discussion inspired the to skip the theories actually look inside the joint. They formed a team with University of Alberta colleagues Jacob Jaremko, Hongbo Zeng, Richard Thompson and Australian Lindsay Rowe.

Fryer’s fingers were inserted one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. MRI video captured each crack.

The metocarpophaangeal (MCP) joint of interest centred over the bore of the radiofrequency coil (middle). The participant’s hand within the imaging magnet (right).

The researchers found that the cracking and joint separation was associated with the rapid creation of a gas-filled cavity within the synovial fluid.

Cine MRI images displayed are those immediately prior to, and after, joint cracking with zoomed regions to demonstrate areas where signal intensities were measured for the region of interest as well as control regions.

More than just settling a scientific curiosity, the findings pave the way for new research into the therapeutic benefits or harms of joint cracking, explained Kawchuk.

Scientists have calculated that the amount of force when you crack your knuckles has enough energy to cause damage to hard surfaces, yet research also shows that habitual knuckle cracking does not appear to cause long-term harm.

Those conflicting results are something Kawchuk and his team plan to investigate next.


Our data support the view that tribonucleation is the process which governs joint cracking. This process is characterized by rapid separation of surfaces with subsequent cavity formation, not bubble collapse as has been the prevailing viewpoint for more than a half century. Observed previously in vitro, this work provides the first in-vivo demonstration of tribonucleation on a macroscopic scale and as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking. This framework will allow scientists to compare and contrast this process against tribonucleation observed between inanimate surfaces, an approach that may reveal how joint cracking affects cartilaginous joint surfaces. Presently, the literature in this area is confusing in that the energy produced during joint cracking is though to exceed the threshold for damage, but habitual knuckle cracking has not been shown to increase joint degeneration. Ultimately, by defining the process underlying joint cracking, its therapeutic benefits, or possible harms, may be better understood.

Citation: Kawchuk GN, Fryer J, Jaremko JL, Zeng H, Rowe L, et al. (2015) Real-Time Visualization of Joint Cavitation. PLoS ONE 10(4): e0119470. doi:10.1371/journal.pone.0119470

Copyright 2019 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.