Although knuckle cracking hasn't been strongly linked to arthritis, the habit still might harm your hands, according to a review of knuckle-cracking research published in the journal Clinical Anatomy, Katherine Ellison writes for the Washington Post.

What happens when you crack your knuckles?

As many as 54% of Americans crack their knuckles, according a study published in the Journal of the American Board of Family Medicine in 2011. But far fewer Americans likely understand what's happening when they engage in the practice, Ellison writes.

According to modern scholars, people aren't cracking their bones when they crack their knuckles. Instead, Ellison writes, knuckle crackers are creating a gas bubble in their synovial fluid, which is responsible for lubricating joints.

Researchers haven't yet figured out whether "the bubble's formation or subsequent pop" causes the cracking sound that accompanies knuckle cracking, according to Ellison. However, Rod Oskouian, a neurosurgeon who conducts research on the habit, said the mechanics involved in knuckle cracking are comparable to those involved in spine adjustments conducted by chiropractors, which produce a similar sound.

Although knuckle cracking may sound like bones popping, experts have long said the habit likely won't hurt anyone. In fact, Robert Shmerling, an associate professor of medicine at Harvard Medical School, in a 2018 blog post wrote that knuckle cracking "is probably harmless," citing evidence showing the practice did not result in a higher incidence of arthritis.

Why knuckle-cracking may not be 'harmless'

But a recent review of research on knuckle cracking led by Oskouian and his colleagues suggests the habit may not be as "harmless" as Shmerling concluded.

For the review, Oskouian and his colleagues examined more than 26 papers focused on the mechanisms and effects of knuckle cracking. Overall, the researchers that found the papers did not show a reliable association between knuckle cracking and arthritis—and at least one study published in 2017, which involved 30 participants, offered evidence suggesting the practice may help increase a person's range of motion, Ellison writes.

However, the researchers also found evidence suggesting knuckle cracking may be harmful for at least some people, including those who engage in the practice frequently, Ellison writes.

For example, a study conducted by Turkish scientists in 2017 that involved 35 people who cracked their knuckles more than five times a day found that, although the practice didn't appear to affect grip strength, it was associated with the metacarpal cartilage's thickening—which is a potential early sign of damage that could result in osteoarthritis, according to Ellison.

In addition, a separate study published in 1990 that involved 300 participants older than 45, including 74 participants who were considered habitual knuckle crackers, found that although participants who cracked their knuckles did not have higher rates of arthritis, they were more likely to have swollen hands and weaker grips, Ellison reports. While the study's two authors, who were based at the former Mount Carmel Mercy Hospital in Detroit, noted that the participants who cracked their knuckles most frequently also were more likely to drink alcohol, do manual labor, bite their nails, and smoke, the researchers ultimately concluded that "[h]abitual knuckle cracking results in functional hand impairment."

"Knuckle cracking over the years will cause repetitive trauma to the joints and cartilage," Oskouian told Ellison. And according to Ellison, the research included in Oskouian's review suggests "that long-term knuckle cracking can cause significant damage short of arthritis, stressing and ultimately degenerating cartilage."

Separately, Charles Kallina, an assistant professor of surgery at the Texas A&M College of Medicine, told Ellison that knuckle cracking may cause harm to up to 10% of the population who has certain preexisting medical conditions, such as the inflammatory disorder rheumatoid arthritis. While Kallina admitted that he, too, cracks he knuckles on occasion, he said, "I cannot in good faith recommend it as a rule."

Still, Oskouian acknowledged that knuckle cracking likely is harmless for most people, particularly since many people eventually stop engaging in the habit.

Overall, Kallina said, "This may be somewhat similar to how parents tell you not to cross your eyes, or they'll stay that way."

Put another way, Kallina said elders may be purposely spreading a medical myth that knuckle cracking is dangerous to keep their family members from engaging in the "irritating habit." But Kallina added that he does recommend people avoid the practice—and instead twiddle their thumbs—unless they've received confirmation from a doctor that they don't have a preexisting medical condition that might make knuckle cracking risky, Ellison writes (Ellison, Washington Post, 10/17/20).