Despite probably being told otherwise thousands of times, cracking your knuckles is unlikely to cause arthritis, Harvard Medical School's Healthbeat explains.

The sound has long defied simple explanation, although a 2015 study used an MRI scanner to reveal that it's caused by the rapid creation of a cavity in the fluid inside the joints. And in 2018, another study used a mathematical model to shed further light on how exactly the phenomenon works.

"The cavity in between the two knuckles is filled with a fluid that is called the synovial fluid, and when you suddenly change the pressure in that fluid as a result of increasing the spacing between the knuckles, some of the gases in that fluid can nucleate into a bubble," study co-author Abdul Barakat, of France's Ecole Polytechnique, tells the Guardian. "As you form this bubble you can cause pressure changes, and that can produce sound."

As for the reputed link between knuckle popping and arthritis, science writer Steve Mirsky took a closer look for Scientific American in 2009. Mirsky cites several studies in debunking the connection, including a 1998 paper by researcher Donald Unger, who for 50 years cracked the knuckles on his left hand at least twice a day while leaving the other hand at rest to serve as a control.

According to Mirsky, that means Unger cracked his knuckles on his left hand at least 36,500 times, “while those on the right cracked rarely and spontaneously.”

So why conduct this seemingly off-the-wall experiment?

Because, as Unger explains, during his childhood “various renowned authorities (his mother, several aunts and, later, his mother-in-law) informed him that cracking his knuckles would lead to arthritis of the fingers.”

In other words, Unger didn't want his relatives giving him advice without being able to back it up with evidence. And, during the five decades of his test, Unger had the satisfaction of telling those relatives simply that the results weren’t in yet.

Upon completing his study, Unger found the following: “There was no arthritis in either hand, and no apparent differences between the two hands,” which caused him to conclude that “there is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers.”

But though the study almost certainly gave Unger personal satisfaction when presenting his findings to his relatives, he could have saved some time by simply referring to other scholarly studies of knuckle cracking that have already been done, as the article points out.

For example, Robert Swezey’s own 1975 study — co-authored by his then 12-year-old son in an apparent attempt to get the kid’s grandma to stop the worrying about the cracking — also found that cracking one’s knuckles did not cause arthritis.

Bone development expert David Kingsley at Stanford University has also looked into the theory, and his results support other researchers' doubts that knuckle cracking leads to arthritis. His method involved visiting nursing homes, where he asked residents both whether they cracked their knuckles and whether they had arthritis. He found no increased incidence of arthritis among the knuckle crackers.

Not all joints have the ability to make the sound, according to Barakat, nor do all people. "Some people cannot crack their knuckles," he explains, "because the spacing between their knuckles is too large for this to happen."

Editor's note: This article has been updated since it was published in December 2009.

The truth about cracking your knuckles
Scientists are revealing why knuckles pop, and why it probably won't lead to arthritis — despite what your grandmother said.