State of the art ultrasound technology has revealed what causes knuckles to “crack” out loud.
In the first study of its kind, researchers recorded simultaneous audio and visual evidence of knuckles cracking.
A total of 40 healthy adults - 17 women and 23 men whose age ranged from 18 to 63 - were examined using ultrasound imaging, as they attempted to crack the knuckle at the base of each finger, known in medical parlance as the metacarpophalangeal joint (MPJ).
The participants included 30 people with a history of habitual knuckle cracking and 10 without.
Robert Boutin, Professor of radiology at the University of California, said: “It’s extremely common for joints to crack, pop and snap.
“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.”
He said while some participants said that they had never intentionally cracked their knuckles, others did it frequently every day - up to 20 times per day for the past 40 years.
Orthopaedists blinded to the participants’ knuckle-cracking history evaluated the participants for grip strength, range of motion and laxity of each MPJ both before and after the ultrasound examination.
Using a small transducer, a sonographer recorded video images of 400 MPJs, as participants attempted to crack their knuckles.
The sonographer also captured static images of each MPJ - before and after participants tried their hands at cracking.
Two radiologists interpreted the ultrasound images, looking for sonographic evidence that would correlate with the audible cracks, which occurred in 62 of the 400 joints imaged.
Prof Boutin said: “What we saw was a bright flash on ultrasound, like a firework exploding in the joint.
“It was quite an unexpected finding.”
Using the flashes on the ultrasound images, the radiologists were able to identify the joints with audible cracks with at least 94 per cent specificity.
Prof Boutin said: “There have been several theories over the years and a fair amount of controversy about what’s happening in the joint when it cracks.
“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.”
But as for which comes first - the cracking sound or the flash of light, Prof Boutin said that more research is needed.
The physical examinations revealed no immediate pain, swelling or disability in the knuckle-cracking group and no immediate difference in laxity or grip strength between the participants who cracked their knuckles and those who didn’t.
Prof Boutin said: “We found that there was no immediate disability in the knuckle crackers in our study, although further research will need to be done to assess any long-term hazard - or benefit - of knuckle cracking.”
The research was due to be presented at the annual meeting of the Radiological Society of North America (RSNA).