What causes the sound when you crack your knuckles?
General information only. This article explains the basics and is not medical advice. For anything about your own health, talk to a doctor or other qualified professional.
Quick answer
The pop of a cracked knuckle comes from the fluid inside the joint. Pulling or bending the finger stretches the joint capsule, the pressure drops, and a gas-filled cavity suddenly forms in the synovial fluid. A 2015 MRI study filmed this happening in real time. You usually can’t crack the same knuckle again for about 20 minutes.
Inside a finger joint
The joints in your fingers are synovial joints. The ends of the bones are covered in smooth cartilage and wrapped in a capsule filled with synovial fluid, which lubricates the joint and contains dissolved gases such as carbon dioxide and nitrogen.
Bubble forming, or bubble bursting?
For decades, scientists argued about the source of the sound:
| Year | Idea |
|---|---|
| 1947 | Researchers suggested a bubble forms in the fluid |
| 1971 | A study proposed the sound came from a bubble collapsing |
| 2015 | MRI videos at the University of Alberta showed a cavity forming at the moment of the crack |
Later computer models suggest the sound may come from a partial collapse of the bubble. The full picture is still debated, but everyone agrees it involves gas in the joint fluid.
Why you have to wait
After a crack, the gas takes time to dissolve back into the fluid. Until it does, the joint can’t crack again, which usually takes about 20 minutes.
Does it cause arthritis?
Current research has not found a link between habitual knuckle cracking and arthritis. In a famous self-experiment, US doctor Donald Unger cracked the knuckles of only his left hand for about 60 years and found no arthritis in either hand. He won a humorous Ig Nobel Prize in 2009.
Other joint noises
Knees, necks and backs can also click or pop. A crack that comes with pain, swelling or reduced movement is different from a harmless pop and is worth checking with a health professional.
For more about joints and bones, see the smallest bone in the human body.
Sources
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