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See Inside December 2009

Crack Research: Good news about knuckle cracking

One man's long, noisy, asymmetrical adventure gets him a high five



Matt Collins

The latest physical anthropology research indicates that the human evolutionary line never went through a knuckle-walking phase. Be that as it may, we definitely entered, and have yet to exit, a knuckle-cracking phase. I would run out of knuckles (including those on my feet) trying to count how many musicians wouldn’t dream of playing a simple scale without throwing off a xylophonelike riff on their knuckles first. But despite the popularity of this practice, most known knuckle crackers have probably been told by some expert—whose advice very likely began, “I’m not a doctor, but ...”—that the behavior would lead to arthritis.

One M.D. convincingly put that amateur argument to rest with a study published back in 1998 in the journal Arthritis & Rheumatism entitled “Does Knuckle Cracking Lead to Arthritis of the Fingers?” The work of sole author Donald Unger was back in the news in early October when he was honored as the recipient of this year’s Ig Nobel Prize in Medicine.

The Igs, for the uninitiated, are presented annually on the eve of the real Nobel Prizes by the organization Improbable Research for “achievements that first make people laugh, and then make them think.” In Unger’s case, I thought about whether his protocol might be evidence that he is obsessive-compulsive. From his publication: “For 50 years, the author cracked the knuckles of his left hand at least twice a day, leaving those on the right as a control. Thus, the knuckles on the left were cracked at least 36,500 times, while those on the right cracked rarely and spontaneously.”

Unger undertook his self and righteous research because, as he wrote, “During the author’s childhood, 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.” He thus used a half-century “to test the accuracy of this hypothesis,” during which he could cleverly tell any unsolicited advice givers that the results weren’t in yet.

Finally, after five decades, Unger analyzed his data set: “There was no arthritis in either hand, and no apparent differences between the two hands.” He concluded that “there is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers.” Evidence for whether the doctor himself was cracked may be that he traveled all the way from his California home to Harvard University to pick up his Ig Nobel Prize in person.

Actually other scholarly studies of the phenomenon had been done. Responding to the Unger paper, Robert Swezey, M.D., wrote to the journal to report that his own 1975 study—co-authored by his then 12-year-old son in an apparent attempt to get the kid’s grandma to stop the kvetching over the cracking—also found no crack case for arthritis. Swezey further consulted Rand Corporation statistician John Adams, who noted that “it appears that the [Unger] study was not blinded. Blinding would only be possible if the investigator didn’t know left from right. This is not likely since studies indicate that only 31 percent of primary care physicians don’t know left from right.”

The knuckle kerfuffle reminded me that Stanford University bone development expert David Kingsley got dragged into this field a few years back when his son’s fourth grade class asked him if cracking was bad for you. He challenged them to come up with ways to find out while he searched the medical literature. “One kid said that we could divide the room in half,” he recalled, “and some of us could really crack our knuckles a lot and the others couldn’t, and we could see whether we end up with arthritis—an intervention experiment. I said that this was a great idea. The only problem was that it might take 20 years.” Or even 50.

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