Why Do Knuckles Crack, and Do Scientists Agree on the Reason?

The Crack Comes From Inside the Joint, Where Two Bones Meet in a Slippery Fluid

A knuckle is a joint, a place where two bones meet so that your finger can bend. The studies in this note looked at the knuckle where the finger joins the hand. Inside it, the bone ends are surrounded by synovial fluid, a slippery liquid that helps the joint move smoothly [1].

When a finger is pulled slowly, the bones are drawn apart, and at some point the joint separates suddenly and makes the crack. So the real puzzle is what happens in that fluid at that instant. Scientists use the word cavitation for a gap or bubble that forms in a liquid when the pressure drops, and the argument is about what that bubble does [2].

In 1971 an X-ray Study Led to the Idea That a Bubble Bursts, and It Lasted Over Forty Years

In 1971 a team led by Unsworth pulled on finger joints and took X-ray pictures. They saw a gap appear in the joint, and they suggested that the sound came from the bubble suddenly collapsing. According to the 2015 and 2018 papers, that explanation was widely accepted for more than four decades [1] [2].

There was a catch. The X-ray pictures showed the gap forming, but the 2015 authors say the 1971 study did not show the cavity collapsing [1]. The “burst” was a reasonable guess that had never been seen.

In 2015 an MRI Movie Showed a Gap That Opened With the Crack and Stayed Open

In 2015 Gregory Kawchuk and colleagues in Canada tried a new camera. They pulled on a volunteer’s fingers inside an MRI scanner, a machine that takes pictures of the inside of the body, and recorded the joint at about 3.2 frames per second [1].

At the moment of the crack, a dark gap opened in the joint in less than a third of a second, and the authors read it as a gas-filled cavity. It did not vanish afterward. It stayed visible until the pulling stopped and the bones moved back together. The team concluded that the sound goes with the cavity forming, not with a bubble collapsing [1].

The study was small: one man, ten knuckles, over two sessions. It could not image the whole joint, and the authors note that it does not explain why the sound is as loud as it is [1]. A surprise, yes, but one measured on very few joints.

In 2018 a Computer Model Showed a Bubble Could Make the Sound Without Fully Collapsing

If the gap stays open, is the old “collapse” idea simply wrong? In 2018 two engineers, Chandran Suja and Barakat, built a mathematical model of a bubble in the joint fluid. Their calculated sound matched measured sounds in loudness and in its main pitch. The model needed only a partial collapse, so a bubble could still be there after the sound [2].

That fits the 2015 pictures, but it does not settle the matter. The authors say that cameras are far too slow to watch the event directly: the model works at about 1,200 frames per second, while safe X-ray video and MRI manage roughly 100 or fewer. Their model also treats the event as a single bubble, which is a simplification [2]. So the exact mechanism is still open.

After One Crack, the Same Knuckle Usually Cannot Crack Again for About 20 Minutes

You may have noticed that a knuckle you just cracked will not crack again right away. The 2015 paper describes this waiting time and gives about 20 minutes as the figure [1]. It is an approximate number and not a fixed rule.

A simple way to picture it is that the joint needs time to get back to the state it was in before the gap formed. That is a picture to help you think, not a finding from the sources.

Studies Have Not Linked Cracking Knuckles to Arthritis, but Each Study Has Limits

The next question is whether the habit does harm. One doctor, Donald Unger, cracked the knuckles of his left hand at least twice a day for about 50 years, at least 36,500 times, and left his right hand alone for comparison. He reported no arthritis in either hand and no difference between them. The report earned an Ig Nobel Prize in 2009, a humorous award for research that makes people laugh and then think. It was one person, and he knew which hand was which [3].

A larger check came in 2011. Researchers asked 215 adults aged 50 to 89 about their cracking habits and compared that with X-rays of their hands. About 20 percent were habitual crackers. Arthritis in any joint was found in 18.1 percent of crackers and 21.5 percent of non-crackers, and the amount of past cracking was not tied to arthritis in those joints [4].

That study asked people to remember their past habits, only about two thirds of those contacted replied, and the people came from those who had had hand X-rays. So it does not prove the habit is harmless for everyone [4]. If a joint hurts or swells, tell a parent or another adult you trust.

Look at the Real MRI Pictures, and Think of a Camera That Could Settle the Argument

You do not need to crack anything for this. The 2015 paper is free to read online, and its MRI pictures are part of it [1]. Ask a grown-up to help you find it, then compare the joint before the crack, at the crack and after it.

1. Where is the dark gap in the picture taken at the crack?
2. Is it still there in the picture after the crack?
3. The model needs a camera of about 1,200 frames per second. What would a camera like that have to be able to do inside a living finger?

Please do not pull hard on your fingers to get a crack. Knuckles do not need to be forced, and the studies above were done with careful equipment.

Sources

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