In his paper, which has become known as the “Pull my finger” study, he placed the fingers of a chiropractor into a device that literally pulled the finger to the point where a knuckle crack occurred. Even better, the reason he used that chiropractor’s finger was because the chiropractor possessed the unusual ability of being able to have his knuckle crack on demand- no refractory period, etc. Greg was able to use this on all 10 fingers of the participant. With his team, he then took cine-MRI images of the knuckle as it cracked, and was able to visualize the changes taking place. He found not that there was a bubble collapse (the prevailing theory) but that a cavity was formed. You can now see why this garneed such worldwide attention (in a Facebook post of a few moments ago, Greg noted that his aunt said that “it went virus.”).The actual paper, which is titled “Real-time visualization of joint cavitation,” is available free for download on the Public Library of Science, or PLoS. It can be found at http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0119470. From Greg’s work, we need to know a new term: tribonucleation. Here is the abstract of this most interesting paper.
Cracking sounds emitted from human synovial joints have been attributed historically to the sudden collapse of a cavitation bubble formed as articular surfaces are separated. Unfortunately, bubble collapse as the source of joint cracking is inconsistent with many physical phenomena that define the joint cracking phenomenon. Here we present direct evidence from real-time magnetic resonance imaging that the mechanism of joint cracking is related to cavity formation rather than bubble collapse. In this study, ten metacarpophalangeal joints were studied by inserting the finger of interest into a flexible tube tightened around a length of cable used to provide long-axis traction. Before and after traction, static 3D T1-weighted magnetic resonance images were acquired. During traction, rapid cine magnetic resonance images were obtained from the joint midline at a rate of 3.2 frames per second until the cracking event occurred. As traction forces increased, real-time cine magnetic resonance imaging demonstrated rapid cavity inception at the time of joint separation and sound production after which the resulting cavity remained visible. 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. Observed previously in vitro, this is the first in-vivo macroscopic demonstration of tribonucleation and as such, provides a new theoretical framework to investigate health outcomes associated with joint cracking.