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CHICAGO -- The knuckle-cracking debate continues, with the latest evidence suggesting the resulting "pop" is due to cavity formation in the joint that subsequently allows dissolved gasses to escape, researchers reported here.

Knuckles cracked under the surveillance of ultrasound imaging showed a cavity forming that corresponded with the cracking sound, followed by a white flash of gas that eventually dissipated, , of the University of California Davis, and colleagues reported at the Radiological Society of North America.

"You have dissolved air [in fluid around the joint] and you're pulling it out of solution -- it's kind of like getting the bends in your knuckle," commented , of Stanford University in Stanford, Calif., who was not involved in the study.

There have been several theories over the years as to what happens when knuckles crack, Boutin said, with researchers arguing over whether the sound represents a cavity forming or an air pocket bursting.

Boutin and colleagues studied a total of 40 adults, 30 of whom habitually cracked their knuckles, and 10 who did not. They took ultrasound images of knuckles cracking and not cracking, totaling some 400 scans.

In addition, they had orthopedists evaluated grip strength, range of motion, and laxity for each patient. Two radiologists each read all of the scans to determine whether a knuckle cracked or not.

Overall, they found no clinical differences between knuckle-crackers and those who don't pop their knuckles, with no differences in grip strength or in range of motion between groups.

But those who cracked their knuckles did seem to have greater laxity, which is the ability to move beyond a commonly accepted range of physiologic motion.

None of the knuckle-crackers reported any pain or swelling, Boutin said.

"There's no evidence to indicate that it's bad for you in the short-term, and there's no evidence that it's irritating to the joint," he said, cautioning that "there may be irritation to the co-worker or family member next to you."

He added that it's still not clear what happens over time with knuckle cracking, as some studies have shown that swelling and decline in grip strength did eventually occur, even though patients didn't develop osteoarthritis, "so I think the jury is still out," he said.

When looking at the cracked joints on ultrasound, Boutin described them as a "brilliant bright flash that's very quick. We believe this is the bubble forming, not bursting like a balloon."

When two blinded radiologists reviewed the scans, they did so with a sensitivity ranging from 75% to 80%, a specificity of 94% to 95%, and an overall accuracy of 91% 93%, with high interobserver reliability at 0.87.

"More than nine times out of 10 they were getting it right," Boutin said.

They found that participants couldn't crack their knuckles again for about 20 to 30 minutes, which indicates the time needed for sufficient levels of dissolved gasses to build up again in joint fluid and tissues.

Although Boutin and colleagues said they're confident that the study indicates the popping sound and subsequent bright flash are associated with a cavity forming and gasses being released, they said further study is needed to assess any long-term hazard or benefit of knuckle cracking.

"It's just a snapshot in time, so you'd love to get this group back in 10 years and see if they continue to crack their knuckles, how does their range of motion change, and are there any secondary effects," Hovsepian said. "Intuitively, you'd think it has an effect over time, but maybe not. We've seen orthopedic studies of runners where the cartilage is healthy even though these patients have been running for more than 20 years. You'd think that would pound the cartilage into oblivion, but maybe it keeps the cartilage resilient."

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