Cracking Your Knuckles Doesn't Cause Arthritis. One Doctor Cracked One Hand 36,500 Times in 50 Years, and 215 X-Rays Back Him Up

The Warning Everyone Has Heard

Stop that, you will get arthritis. Nearly everyone who has ever cracked a knuckle has heard some version of it, usually from a parent wincing across the table. The warning feels like common sense: a joint makes a violent little pop, something inside must be taking damage, and damage repeated thousands of times must add up to arthritis. Nobody checks it.

The warning is also unsupported by every study that has ever tested it, and researchers have examined the hands of habitual knuckle crackers for half a century, in samples from 28 people to 300, without finding more arthritis in the crackers.

The Fifty-Year Experiment

Donald Unger was a California allergist with a grudge against a piece of folk wisdom. As a boy he had been told by his mother, several aunts, and later his mother-in-law that cracking his knuckles would give him arthritis, so he settled the question with the only laboratory he had: himself. Starting in his teens, he cracked his left hand's knuckles at least twice a day, leaving his right hand alone as a control. The right hand cracked only rarely.

Two cracks a day, 365 days a year, for 50 years comes to at least 36,500 cracks on one hand and almost none on the other. In 1998 Unger published the result as a one-page letter in Arthritis and Rheumatism: there was no arthritis in either hand, and no apparent difference between the two, work that won him the 2009 Ig Nobel Prize in Medicine. Unger called the work preliminary, and he was right, because one man, unblinded, judging his own hands, cannot settle a medical question, though the larger groups that followed said the same thing.

The X-Ray Test

The strongest test arrived in 2011, when Kevin deWeber and colleagues looked at 215 adults aged 50 to 89 who had all had a hand X-ray in the previous five years and asked each person how often and for how long they had cracked their knuckles.

Hand osteoarthritis showed up in 18.1 percent of the crackers and 21.5 percent of the non-crackers, a difference that was not statistically significant (p = 0.548), and neither the number of years someone had cracked nor their total lifetime exposure, daily frequency multiplied by years, was linked to arthritis in the joints they cracked, so the authors concluded that habitual cracking does not seem to be a risk factor for hand osteoarthritis.

The numbers carry more weight than the p-value alone suggests, because if cracking really doubled the risk of hand arthritis, from the 21.5 percent seen in non-crackers to 43 percent, then among the 43 crackers in the study you would expect about 18 or 19 cases of osteoarthritis, and the researchers found about 8. A study this size cannot rule out a small effect, but it can rule out the large one the folk warning implies.

A 1990 study of 300 outpatients, 74 of them habitual crackers with an average habit of 35 years, and a 1975 survey of 28 nursing home residents found the same.

What the Pop Actually Is

Part of the myth's staying power is the sound: a crack sounds like something breaking, and for decades the standard explanation was a bubble collapsing in the joint fluid. In 2015, Gregory Kawchuk and colleagues at the University of Alberta put a volunteer's finger inside an MRI scanner and filmed it happening, at 3.2 frames per second, and what they saw was the opposite of a collapse: as the joint surfaces pulled apart, a gas cavity formed rapidly in the synovial fluid, the joint's lubricant. The pop is the cavity opening. Not a bubble bursting, and not bone grinding on bone, but tribonucleation, the same physics behind a suction cup's pop.

A 2017 follow-up at UC Davis watched the same event with ultrasound, and in 40 adults, 30 of them daily crackers, researchers found a bright flash at the moment of the crack, like a firework going off inside the joint. Two orthopedic surgeons who did not know who cracked examined every hand and found no swelling, no difference in grip strength, and no difference in disability scores.

The Strongest Counterargument

The honest case against cracking your knuckles does not come from arthritis at all, but from the 1990 Castellanos study, the largest ever run on the question, which found that habitual crackers were more likely to have hand swelling and lower grip strength than non-crackers even though their arthritis rates were identical, and the authors concluded that habitual cracking results in functional hand impairment. Crackers there were also likelier to do manual labor, bite their nails, smoke, and drink, so the habit or its company could explain the weaker hands.

A small 2017 ultrasound study added a second wrinkle: thinner metacarpal cartilage in habitual crackers, the kind of change that could in theory precede osteoarthritis, though the study measured thickness, not arthritis. The blinded 2017 UC Davis study found no swelling or grip loss at all, which undercuts the 1990 result. No study has found that cracking causes arthritis, but one large unblinded study found weaker, puffier hands, and the evidence is not yet strong enough to call the habit entirely harmless.

What We Didn’t Prove

Every study on this question shares the same weakness: nobody has ever randomly assigned people to crack their knuckles for decades, and nobody ever will, so all of the evidence is observational. The 2011 study relied on older adults remembering years of cracking, and memory is a shaky instrument, while the samples are modest, 300 people at most, so a small increase in risk could hide in the noise. The research covers hand osteoarthritis, the wear-and-tear kind, and says little about rheumatoid arthritis, while Unger's hands were examined by their owner, with no X-ray described in his letter.

What You Can Do

If you crack your knuckles and enjoy it, the published evidence gives you no reason to stop on arthritis grounds: the habit has been tested in 215 X-rayed hands, 300 clinic patients, and one very patient doctor, with none finding more arthritis in crackers.

Pay attention to pain, not sound. A painless pop is a gas cavity forming in joint fluid, while a crack that comes with pain, swelling, locking, or loss of movement is a different matter, and that is the one worth showing a clinician.

If you want to quit anyway, treat it like any habit: notice the trigger, keep your hands busy, and give it a few weeks.