Snap, crackle, pop. If you’re a knuckle-cracker, that familiar sound when you consciously pop your joints is like comfort food. You know it might not be so healthy for your hands or ankles, but it feels oh-so-good.

Robert G. Cook knows that feeling. The 55-year-old Californian said he’s been popping his knuckles daily for decades. Typically, it’s when he first sits down to work at his computer keyboard. “It’s like a concert pianist or baseball player warming up,” he said. “It’s a ritual.”

And like many habitual knuckle crackers, he’s always been told that it’s bad for his joints, leading to arthritis or enlarged knuckles. That’s why Cook jumped at the chance to be one of 40 volunteers in a recent study by UC Davis radiology professor Dr. Robert Boutin and orthopedic surgery professor Dr. Robert Szabo, who also see patients clinically.

The pair wanted to resolve two persistent questions about knuckle-cracking: What causes that popping sound, and is it bad for your joints?

Ultrasound imaging

Ranging in age from 18 to 63, the volunteers were invited to sit and methodically crack their knuckles. Techniques varied: Some pulled their fingers, others flexed or bent them back. To determine what causes the crackle ’n’ pop, a tiny ultrasound device was hovered over their joints, capturing the sounds of knuckles being cracked. More than 400 ultrasound images were taken. The results were startling. “It looked like a tiny Fourth of July explosion inside the hand,” said Boutin, who presented his study in November at the Radiological Society of North America in Chicago. That flash, he explained, is caused by dissolved gas that sits in joint fluid. When you pull or bend a joint, it creates negative pressure, which releases the gas, forming tiny micro-bubbles. When released quickly (i.e. in knuckle-cracking), the escaping gas causes a bright flash that shows up in imaging.

advertisement | advertise on newsday

The UC Davis findings appear to contradict traditional explanations that the knuckle cracking sound is caused when the gas bubbles burst, like a balloon being popped. Last April, a study by Canadian researchers using MRI imaging came to a similar conclusion: The knuckle-cracking sound is created by the bubble formation itself. Until that joint fluid pressure builds up again, it can take up to 20 minutes before someone can re-crack his or her knuckle.

No short-term harm found

The second part of the UC Davis study was to assess the potential harm from knuckle-cracking. Each participant was tested — before and after each ultrasound — by two hand/wrist orthopedic surgeons who checked for range of motion, grip strength and laxity (overextension of ligaments). The surgeons examined the hands without knowing who was or wasn’t a knuckle cracker, and were not told which joints had successfully been cracked.

The physical examinations by the hand-injury specialists found no problems in the joints of knuckle-crackers. “We did not find any swelling or adverse results like decreased grip strength,” said Boutin. His conclusion: There’s no short-term harm in knuckle cracking. And there might even be a benefit: After a joint was cracked, it showed a “significantly increased range of motion” compared to joints that did not crack, Boutin said.

Best Bets

Get the scoop on events, nightlife, day trips, family fun and things to do on Long Island.

The UC Davis study appears to contradict a 1990 study that suggested knuckle-cracking can cause joint-swelling and weaken the grip and “should be discouraged.” Given the prevalence of knuckle-crackers — it’s estimated that 25 percent or more people do so — scholars have paid attention to the topic in recent years. Most studies have debunked the warnings that knuckle-cracking causes arthritis in joints.