While estimates vary greatly, between 25 and 54% of people crack their knuckles, with men more likely to do so than women. For some, joint cracking is a nervous habit; for others the sensation brings relief from stiffness.

The joints that are cracked most often include the knuckles, knees, ankles, back, and neck. However, is there a link between cracking one’s joints and arthritis?

Joint cracking occurs due to cavities in the joint fluid

Scientists still do not completely understand the phenomenon of joint cracking. Nevertheless, it is believed that this is due to the formation of bubbles in the synovial fluid of the joints when a finger or joint is extended and causes the pressure inside the finger to be lowered. This creates a vacuum that the gases present, such as carbon dioxide, will fill, in the form of a bubble.

In a 2015 study, researchers from the University of Alberta in Canada studied what happens to the knuckles when being cracked. Volunteers' fingers were inserted into a flexible tube that could be used to apply traction to the joint. They took images using MRI before and after traction and took 3.2 frames per second when traction was applied to the point of cracking.

They observed the rapid creation of a cavity in the joint at its point of separation, which produced the sound. The cavity remained visible after the noise, which supports the synovial fluid and cavitation theory, but suggests that the noise happens not during the collapse of the bubbles, but during their formation when the joints separate. It takes about 25 to 30 minutes for the released gases to dissolve back into the synovial joint fluid, which is why the knuckles will not crack then.

People tend to crack their joints in one of three ways: bending them backward or forward; turning them sideways; or pulling on the bones around the joint. There is evidence of increased mobility in joints right after popping. When joints are manipulated, the Golgi tendon organs, a set of nerve endings involved in the sense of motion, are stimulated. This then relaxes the muscles surrounding the joint.

No arthritis fears, but researchers still suggest not to

There are several studies that disprove the link between knuckle-cracking and arthritis. For example, researchers at the Uniformed Services University of the Health Sciences examined 215 people to find out if there was any such correlation, with 20% of the participants being regular knuckle-poppers. Of those knuckle crackers, 18.1% of them had arthritis in their hands, compared to 21.5% of the study participants who did not crack their knuckles.

Still, knuckle-crackers have other concerns to face. Researchers at Harvard Medical School looked at several studies that compare rates of hand arthritis among daily knuckle-crackers compared to those who did not, with the conclusion that “there's still good reason to let go of the habit,” as people with this regular habit were more likely to have swollen hands and reduced grip strength.

For example, in 1990, there was a large study performed by the Mount Carmel Mercy Hospital in Detroit, which involved 300 participants over the age of 45; 74 of whom cracked their knuckles. It was found that although both groups showed similar rates of arthritis in their hands, the 74 who cracked their joints showed a higher rate of inflammation of the hands and a weaker grip. So, while joint cracking may not cause arthritis, it can have a negative impact on the overall health and strength of hands.

Other milder injuries may occur

Additionally, there are at least a couple of reports published in medical literature of people suffering injury of the ligaments surrounding the joint or dislocation of the tendons, which were attributed to attempts made to crack the knuckles. These injuries improved with conservative treatment.

Doctors are probably not immune to knuckle-cracking tendencies as well. Dr. Donald Unger famously reported the results of him cracking the knuckles of only his left hand at least twice daily over a 50-year period – he was a chronic knuckle-popper.

He concluded, "There is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers."

Moreover, the cracking and popping of joints is thought to be normal, and is no cause of worry unless accompanied by pain and swelling, or a loss in joint function. MIMS

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Sources:
http://www.jabfm.org/content/24/2/169.long
http://www.medicaldaily.com/cracking-your-knuckles-bad-you-chronic-knuckle-crackers-beware-habit-could-397924
http://www.medicaldaily.com/snap-crackle-pop-mri-shows-why-joints-make-knuckle-cracking-pop-sound-329556
http://www.hopkinsarthritis.org/arthritis-news/knuckle-cracking-q-a-from/
http://www.care2.com/greenliving/why-do-people-crack-their-knuckles-and-is-it-a-bad-thing.html