Ultrasound images show a flash of light when knuckles are cracked, according to a study presented at the annual meeting of the Radiological Society of North America (RSNA).
Researchers from the University of California, Davis (UC Davis) Health System, sought to determine the cause of the noise that is made when people crack their knuckles.
"It's extremely common for joints to crack, pop and snap," coauthor Robert D. Boutin, MD, said in a release. "We were interested in pursuing this study because there's a raging debate about whether the knuckle-cracking sound results from a bubble popping in the joint or from a bubble being created in the joint."
Seventeen women and 23 men (mean age 33) participated in the study; 30 had a history of habitual knuckle cracking (KC) and 10 did not. The researchers recorded clinical history, including how often the subjects cracked their knuckles, and physical examination, which included grip strength and Beighton scoring. All subjects underwent ultrasonography of the metacarpophalangeal joint (MPJ) before, during, and after KC. The images were interpreted by two blinded radiologists who were not present when the subjects cracked their knuckles.
Four-hundred MPJs from the 40 subjects were available. The findings showed that there were no significant differences in sex, age, QuickDASH score, grip strength, or Beighton score among the subjects. There was an audible crack sound during manual distraction of 62 of the 400 MPJs, and range of motion significantly increased in these joints before and after KC. There was no immediate pain, swelling, or disability among the cracked knuckles. The researchers saw a bright flash of light on the ultrasound during the KC, as well. "It was quite an unexpected finding," Boutin said in the release.
"There have been several theories over the years and a fair amount of controversy about what's happening in the joint when it cracks," he noted. "We're confident that the cracking sound and bright flash on ultrasound are related to the dynamic changes in pressure associated with a gas bubble in the joint."
More research is needed to determine which happens first, the cracking sound or the flash of light, and if there is any long-term hazard or benefit to the cracking action, the researchers concluded.