Knuckle cracking may harm your hands

“Harmless” is overstating it, however, argue experts who have studied the fine print of the research. Even as there’s no strong link to arthritis — specifically osteoarthritis, the degeneration of the cartilage cushioning the ends of bones — cracking knuckles, they conclude, may still harm your hands.

Seattle neurosurgeon Rod Oskouian is the most recent researcher to jump into this small but lively tributary of mainstream science, as co-author of a 2018 review of knuckle-cracking studies in the journal Clinical Anatomy.

Oskouian and his three colleagues pored over 26 sometimes-contradictory papers regarding the mechanisms and effects of knuckle cracking, beginning with a 1911 German treatise titled “On the Dispute About Joint Pressure.” He did so, he said, after becoming fascinated by the universal inability of his students through the years to explain what makes that cracking noise.

Modern scholars now agree that bones themselves aren’t cracking, but rather that the movement creates a bubble of gas in the synovial fluid lubricating the joints. Researchers still don’t know if it is the bubble’s formation or subsequent pop that makes the noise, but Oskouian said the mechanics are similar to a chiropractor’s “adjustment” of the spine, which also elicits a cracking sound.

Joining with several of their predecessors, Oskouian and his colleagues concluded that researchers have yet to show any reliable association between knuckle cracking and arthritis. A 2017 study of 30 knuckle crackers offered evidence that the habit even increased range of motion.

But that still doesn’t give knuckle-crackers a pass — especially not if they do it a lot and for a long time, or have a preexisting problem.

“Knuckle cracking over the years will cause repetitive trauma to the joints and cartilage,” Oskouian said in a telephone interview.

Studies he cited in his review suggest that long-term knuckle cracking can cause significant damage short of arthritis, stressing and ultimately degenerating cartilage. In 2017, a team of Turkish scientists who examined 35 people who cracked their knuckles more than five times a day found that while it didn’t appear to affect grip strength, it was associated with a thickening of the metacarpal cartilage, a potential early sign of damage that can lead to osteoarthritis.

A more ambitious 1990 study of 300 participants over 45, including 74 habitual knuckle crackers, found that while, again, the crackers had no greater rates of arthritis, they were more likely to have swollen hands and, in this case at least, weaker grips.

“Habitual knuckle cracking results in functional hand impairment,” concluded the two authors, based at the former Mount Carmel Mercy Hospital in Detroit. For good measure, they also noted that habitual knuckle crackers were also more likely to do manual labor, bite their nails, smoke and drink alcohol.

Orthopedists vary in how seriously they regard knuckle cracking as a health threat. Oskouian ventured that the habit is probably harmless for most people, adding that most of his patients seem to abandon the practice after a few years or so.

Yet for perhaps as much as 10 percent of the population, who suffer from a preexisting problem such as rheumatoid arthritis, an inflammatory disorder, knuckle cracking is particularly ill-advised, warned Charles Kallina, an assistant professor of surgery at the Texas A&M College of Medicine who acknowledged that he cracks his own knuckles on occasion. “I cannot in good faith recommend it as a rule,” Kallina said.

Over the years, dogged researchers have exposed knuckle crackers’ knuckles to grip-tests, X-rays and MRIs. In one of the more offbeat endeavors, in 2018, a Stanford chemical engineer collaborated with a researcher from the Hydrodynamics Laboratory in Palaiseau, France, to produce “A Mathematical Model for the Sounds Produced by Knuckle Cracking.”

Even so, the most persistent member of this cottage industry of knuckle studiers is a Southern California retired allergist named Donald L. Unger. In 1998, Unger wrote a wry letter to the journal Arthritis & Rheumatism titled “Does knuckle cracking lead to arthritis of the fingers?”

“During the author’s childhood,” he began, “various renowned authorities (his mother, several aunts, and later his mother-in-law [personal communication]) informed him that cracking his knuckles would lead to arthritis of the fingers.”

Rebelliously determining to test that hypothesis, Unger, for the next 50 years, cracked the knuckles of his left hand at least twice a day — ultimately, as he calculated, “at least 36,500 times,” while cracking his right-hand knuckles “rarely and spontaneously.”

At the end of that half-century, he said, he compared his two hands, and on finding arthritis in neither of them, concluded “there is no apparent relationship between knuckle-cracking and the subsequent development of arthritis of the fingers.”

Eleven years later, Unger’s one-man research was honored with an “Ig Nobel” prize, a parody award featured at an event at Harvard University. Unger, then 83, traveled across the country to receive it, joining other winners including a team of Swiss scientists who experimented on human cadavers to find out if it would be less injurious to be hit over the head with a full beer bottle or an empty one.

In his earlier letter, Unger warned that his experiment called into question the wisdom behind other parental dictums, including the benefits of eating spinach.

Kallina, the Texas hand-surgeon, agreed — up to a point. “This may be somewhat similar to how parents tell you not to cross your eyes, or they’ll stay that way,” he said. In other words, as he suggested, sometimes elders intentionally hand down medical myths to try to scare their offspring into dropping an irritating habit.

Still, Kallina maintained his warning for the general public: Unless you get a doctor to confirm you have no preexisting conditions, it would be a lot safer to twiddle your thumbs.

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