The human body is host to a number of minute phenomena that seem easy to explain, but are somehow impossible to, from mysteries like why humans itch or why pain even hurts. However, one of these conundrums is a conundrum no more: scientists from the Radiological Society of North America (RSNA) have figured out in part what causes humans to crack their knuckles with the help of sonographic real-time video footage, confirming that gas bubbles are in part to blame.
The average Joe might not be aware that controversy has brewed for decades among medical professionals and other scientists in physiognomy-related fields for decades. The kerfuffle over knuckles dates back to 1947, when researchers took a series of radiographs to prove that metacarpophalangeal (MCP) joints — or the development of gas bubbles in the joints — are what cause the bodily sensation. Scientists have been flip-flopping between that and the theory that the crack is created from the gas bubble popping, and as a result, have been arguing for decades since.
"It's extremely common for joints to crack, pop and snap," said Robert D. Boutin, M.D., one of the leading researchers of the study and a professor of radiology at University of California, Davis Health System, in a press release issued by RSNA. "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."
With the help of a small transducer, a the team took a sample of 23 men between the ages of 18 and 62 and recorded a video sonograph of each cracking his knuckles at 400 MPJs. Two radiographers were able to study the images, which featured the formation of a bubble within the joint. A bubble pop only accompanied a sound 94 percent of the time — meaning that the burst bubble was not the source of the sound but that the formation itself was the cause of a knuckle crack.
To top it all off, the scientists discovered another surprise in their knuckle-cracking experiment: the bubbles weren't the product of gas, but of tribonucleation, in which a thin layer of viscous fluid adheres to two separate bones in the knuckle; when they pull apart from each other (like when a knuckle bends), a bubble-like membrane is born, as is a cracking sound.
"What we saw was a bright flash on ultrasound, like a firework exploding in the joint," Dr. Boutin said. "It was quite an unexpected finding."
Check out this video to see what happens during — and after — your knuckles crack, courtesy of the Daily Mirror, in the clip below.
Via: ARS Technica
Photo: Jaysin Trevino | Flickr
