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What's Really Happening Inside Your Knuckles When You Crack Them? Therapeutic or Damaging?

Scientists have calculated that the amount of force at work when you crack your knuckles has enough energy to cause damage to hard surfaces, yet studies show that the habit does not appear to cause long-term harm. By defining the process underlying joint cracking, scientists seek to discover its therapeutic benefits, or possible harms, their study published last week in PLOS ONE found.

Scientists in Canada have taken a step closer to an answer by discovering what exactly happens when a knuckle is cracked that causes the distinctive popping sounds.

Through magnetic resonance imaging (MRI) video, the University of Alberta researchers observed that the cause is a cavity forming rapidly inside the joint.

Gregory Kawchuk at the University of Alberta, Canada, and colleagues used a cable to slowly pull a man's fingers in an MRI scanner until the joints cracked. The sound was thought to come from the collapse of an air bubble, but in the scans the air cavity that formed in the fluid around the separating joints persisted after the noise.

A mysterious flash also appeared just before the crack. Kawchuk thinks it may be caused by cartilage releasing fluid as the tension on the joints rises.

"The scans don't allow us to explore the mechanism of sound production but like all sound, it has to come from a production of energy that causes vibrations to travel through the air," says Kawchuk. Higher resolution MRI scans might help solve the mystery, he says.

Does It Cause Arthritis?

From fingers and toes to necks and knees, everyone knows a "cracker." And most habitual joint poppers have heard rumors their habit may cause arthritis. But they may be one of many rumors that are not true.

First, a quick anatomy lesson: Many of your joints--including those that allow your fingers to beckon or point--feature small pockets or gaps that are filled with synovial fluid. Like axle grease, this fluid allows the bones that commingle in your joints to glide close to one another without grating, explains Dr. Pedro Beredjiklian, chief of hand and wrist surgery at Philadelphia's Rothman Institute.

When you pull, twist or otherwise "crack" a joint, you're expanding the volume of space between your bones, Beredjiklian says. That volume expansion creates negative pressure, which sucks the synovial fluid into the newly created space. This sudden inflow of fluid is the popping you feel and hear when you crack a knuckle, he adds.

So is this bad for your joints? Almost certainly not, he assures. Multiple studies have looked into the prevalence of "crackers" among large groups of osteoarthritis patients. They found no evidence that finger pullers and poppers are more likely to suffer from arthritis than those who don't crack their knuckles. One devoted researcher--a man who habitually cracked the joints on his left hand--actually studied himself. After roughly six decades of lopsided joint popping, this case study of one showed no increased presence of arthritis in his left hand as opposed to his right.

"Finger cracking is so common you would expect to see a lot of causal reports if it was harmful," Beredjiklian says. "But you don't. So I think it's unlikely cracking joints in hands leads to arthritis."

Scientists have debated the cause of joint cracking for decades, dating back to 1947 when British researchers first theorised vapour bubble formation as the cause. In the 1970s, another team of scientists instead pointed to collapsing bubbles as the cause.

In 1998, a Californian doctor, Donald Unger, reported the results of a 50-year self-controlled study during which he cracked the knuckles of his left hand at least twice daily, and never cracked those on his right.

Writing in a letter to the editors of the journal Arthritis & Rheumatism, Unger said he had no arthritis in either hand, and no apparent differences between the hands.

"The ability to crack your knuckles could be related to joint health," says Kawchuk, who believes this work could have implications for other joints in the body, including the spine, and help explain why joints become arthritic or injured.

He plans to use even more advanced MRI technology to understand what happens in the joint after the pop, and its health implications.

"It may be that we can use this new discovery to see when joint problems begin long before symptoms start, which would give patients and clinicians the possibility of addressing joint problems before they begin," says Kawchuk.

Habitual Knuckle Cracking Might Impair Your Hand Function

While cracking your knuckles might not lead to arthritis, it does appear to have other consequences. In a study of 300 people aged 45 and older, habitual knuckle crackers were again not found to have an increased risk of arthritis in their hands. They were, however, more likely to have hand swelling and lower grip strength.

They also found that knuckle cracking appears to be associated with manual labor, nail biting, smoking, and drinking alcohol… they concluded that habitual knuckle cracking results in functional hand impairment. The damage was likely the result of the repeated stretching and loosening of the ligaments during repeated knuckle cracking.

Interestingly, those researchers noted that cracking your knuckles has been shown to produce "rapid release of energy in the form of sudden vibratory energy, much like the forces responsible for the destruction of hydraulic blades and ship propellers." This hardly sounds like a completely innocuous habit.

In fact, there are reports in the literature of various injuries that have occurred from knuckle cracking, including overstretching of ligaments in the fingers, dislocated fingers, and a partially torn ligament in the thumb.


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