Why do knuckles crack?

The old story: a popping bubble

Look at your hand. The big knuckles where your fingers meet your palm are joints, and the space between the bones is filled with a liquid called synovial fluid¹. When you pull or bend a finger, those bones are drawn apart. In 1947, researchers proposed that this creates a vapour bubble. In the 1970s another team argued that the noise comes from a bubble collapsing: the 'popping bubble' explanation⁸.

Filming a crack from the inside

In 2015, a team at the University of Alberta looked instead of guessing. Their volunteer was a co-author, the chiropractor Jerome Fryer, who can crack his knuckles on demand. Each of his fingers went into a tube attached to a cable, which was pulled slowly until the knuckle cracked, while an MRI scanner filmed the joint⁸. All ten joints they imaged cracked¹. The scanner took about three pictures per second, and each crack happened within a single frame: in under about 310 milliseconds¹.

At the moment of the crack, a cavity suddenly appeared in the joint fluid¹. The gap between the bones jumped from about 0.93 millimetres just before the crack to about 1.89 millimetres just after¹. The key observation: the cavity did not disappear with the sound. It 'persisted past the point of sound production' and was never seen to collapse; it only faded from view after the pulling stopped¹. The authors call this tribonucleation: two surfaces cling together until they suddenly snap apart, leaving a cavity behind¹. Gregory Kawchuk, who led the study, said: 'It's a little bit like forming a vacuum'⁸.

A 2017 ultrasound study of 40 adults (400 knuckles) points the same way. Of 62 knuckles that cracked audibly, 50 showed a bright spot appearing in the joint during or right after the pull, which the authors read as support for bubble formation³.

So what actually makes the sound?

The argument is not over. In 2018, two researchers built a mathematical model of the knuckle and argued that a bubble collapsing only partly could explain the crack². In their model the loudest moment comes about 0.1 milliseconds after the crack begins, and a partial collapse was enough to match sound recordings they made from three people². Because part of the bubble survives, they argue, their idea also fits the MRI images that still showed a cavity after the crack². So both sides agree there is a bubble. They disagree about whether you hear it being born or partly collapsing^{1, 2}.

Why you can't crack it twice in a row

You may have noticed that a knuckle you just cracked won't crack again right away. Researchers have described this pause since 1947: about 20 minutes before the same joint can crack again^{1, 2}. The MRI team says its images cannot tell whether the old cavity re-forms or a new one appears¹.

What about arthritis?

The most famous test was done by a doctor on himself. Donald Unger of California cracked the knuckles of his left hand at least twice a day for about 50 years, leaving his right hand as a comparison (its knuckles cracked only rarely, on their own), and reported in 1998 that neither hand had arthritis⁷. In 2009 he received the Ig Nobel Prize in Medicine; by then the prize citation put his experiment at 'more than sixty (60) years'⁶. But one person is not proof, and a statistician pointed out that the experiment was not blinded⁷.

Larger studies point the same way. A 1990 study of 300 patients aged 45 and older found no more hand arthritis among the 74 habitual crackers⁵. A 2011 study of 215 adults aged 50 to 89 found that knuckle cracking did not seem to be a risk factor for hand osteoarthritis⁴. A 2017 study of 40 younger adults was not designed to look at arthritis, but it saw no swelling right after cracking and says long-term effects still need to be studied³.