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.
UC Davis radiology professor Dr. Robert Boutin and orthopedic surgery professor Dr. Robert Szabo wanted to resolve two persistent questions about knuckle cracking: What causes that popping sound, and is it bad for your joints?
“Patients do come in all the time and want to know if knuckle cracking is bad (for their joints),” Boutin said. “It’s a real-world question that a lot of patients ask.”
Although popping knuckles is arguably the most common kind of joint cracking, it can also occur in the ankles, knees, back or neck. Some say it’s a way to release tension or limber up. For others, it’s simply a habit.
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The 40 participants included 30 with a history of habitual knuckle cracking and 10 without. Some said they had never intentionally cracked their knuckles; others were habitual, cracking them up to 20 times a day for the past 40 years.
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 sound effects of knuckles being cracked. More than 400 ultrasound images were taken. The results were startling.
Feedback showed something akin to a fireworks display, according to Boutin, who presented his study last month 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. “It has a distinctive appearance on an ultrasound. Every single case, we heard the crack before we saw the physical flash.”
Last April, a study by Canadian researchers using MRI imaging came to a similar conclusion: The knuckle cracking sound is created by the bubble’s formation itself.
Until that joint fluid pressure builds up again, it can take up to 20 minutes before someone can re-crack their knuckle.
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.
The next step for the UC Davis researchers is to look at long-term effects of joint cracking on other areas besides hands. Boutin is in the midst of analyzing results from a global questionnaire of 1,800 individuals to determine if there are age, cultural or geographic differences among knuckle crackers.
Despite all the research, Boutin knows that some things don’t change about knuckle cracking: “Many people are really quite fond of knuckle cracking and find it hard to image life without its existence,” he said. “For other people, it’s like fingernails on a blackboard.”