A University of Alberta study is challenging theories about what happens when joints crack, with video evidence.
My whole childhood, people told me not to crack my knuckles. "It's rude," they said. "You'll get arthritis," they said. "The other kids are already intimidated and now you're just terrifying them," they said. Although the first and third reasons may be valid, the second has been disproven since 1998, when physician Donald Unger published a study documenting how he had cracked the knuckles on only one hand every day for 60 years, with no damage.
After that, the prevailing theory for the distinctive cracking noise was that air bubbles were popping inside the joint. But now, a new study by Greg Kawchuk and other researchers at the University of Alberta is challenging that. Filming a knuckle crack with an MRI machine for the first time, they have concluded that the noise actually comes from the rapid separation of the bones. However, there is still no risk of damage.