The reasons have long been debated by scientists, so Canadian researchers used MRI video to observe what happens inside a finger joint when it cracks.
The MRI was used to observe the 10 finger joints of one person. The volunteer inserted the fingers one at a time into a tube connected to a cable that was slowly pulled back until the knuckle joint cracked. MRI video recorded each knuckle crack in the less than 310 milliseconds it took to occur.
A cavity forming rapidly inside the joints appeared to be the cause of the familiar popping sound, according to the study published online April 15 in the journal PLOS ONE.
It turns out the culprit behind a cracking knuckle is a quickly-forming, gas-filled cavity between the bones of the knuckle joint.
And while a very forceful pop or crack can do some damage, said Donald Condit, MD, habitual knuckle cracking hasn’t been shown to increase joint degeneration or cause arthritis.
“As a parent, I find the sound completely annoying,” said Dr. Condit, a hand and wrist specialist and chief of orthopaedic surgery for Spectrum Health Medical Group. “But as a doctor, I can’t give my patients a medical reason not to do it.”
Dr. Condit explained that cartilage, which lines the insides of your joints, is like a sponge. It relies on synovial fluid (also found inside your joints) to deliver nutrients, provide moisture and keep the joint well-lubricated.
“Healthy joints are flexible joints,” Dr. Condit said. “By putting your joints through a full range of motion—stretching and bending—you keep them flexible, lubricated, nourished and healthy. If they crack or pop in the process, well, that’s OK. I don’t know that there’s any benefit to doing it intentionally, but it’s not harmful.”
Specifically, the cracking was associated with the rapid creation of a gas-filled cavity within the synovial fluid that lubricates the joints.
“It’s a little bit like forming a vacuum. 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,” study author Gregory Kawchuk, from the University of Alberta, said in a journal news release.
There was also a white flash before the knuckle cracked, something that had never been observed before. It’s likely caused by water suddenly being drawn together just before the joint cracks, according to Kawchuk.
The findings may open the way for new research into the potential benefit or damage of cracking your knuckles, the researchers said.