In a recent study published in PLOS ONE, a group of researchers found that cracking a knuckle forms a temporary cavity in the body, disproving a long prevalent theory that the crack is due to the collapse of a bubble. The study focused on the fingers of Canadian chiropractor Jerome Fryer, who has the ability to crack the knuckles in every one of his fingers on demand (not everyone can). He also has a particular interest in the science behind knuckle cracking.
“Fryer is so gifted at it, it was like having the Wayne Gretzky of knuckle cracking on our team,” said Greg Kawchuk, the study’s lead author and a professor of rehabilitation medicine at the University of Alberta, in a statement released by the school.
To observe what was happening when his knuckles cracked, Fryer inserted each of his fingers into a tube connected to a cable that was slowly pulled until his knuckle joint cracked (see photo above). An MRI video captured each knuckle joint crack in real time.
The researchers observed the same phenomenon occurring in every one of Fryer’s fingers: when a joint separated and made the “cracking” sound, a gas-filled cavity formed within the synovial fluid, a slick substance that lubricates the joints.
“It’s a little bit like forming a vacuum,” said Kawchuk, who holds a doctorate in bioengineering. “As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is created and that event is what’s associated with the sound.”
Fryer’s finger demonstration may help scientists better study joint problems, including arthritis and injury. Kawuck said in the statement that the findings have implications for new research into the therapeutic benefit — or harm — of cracking joints. At the moment, there is conflicting data in research: Scientists have found that the force at which knuckles are cracked has enough energy to damage hard surfaces, but also that frequent knuckle cracking doesn’t seem to have any long-term negative effects. Kawuck and his team plan to investigate further.