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Why do our knuckles and joints make a popping sound when cracked? Does it do us any good or harm?

Nick Vale
Hartley Wintney, Hampshire, UK

I have no idea why popping knuckles makes this noise, but it was , an achievement for which Donald Unger won the Ig Nobel medicine prize in 2009.

Every day for 50 years, Unger cracked the knuckles of his left hand, but never his right, and ultimately observed no difference in the function of either hand.

David Nye
Eau Claire, Wisconsin, US

Pulling on the end of a finger produces negative pressure in the joint fluid that fills the knuckle’s joint space, causing dissolved gasses to come out of solution rapidly with a pop. This is harmless as long as you don’t pull so hard that you damage the joint.

David Muir
Edinburgh, UK

Old kettles and boilers sometimes produce bangs and pops through cavitation. This is caused by small bubbles of gas suddenly collapsing and propagating a sound.

It was thought that stretching a knuckle caused gas formation through decreased pressure in the joint and the knuckle crack was caused by cavitation as pressure returned to normal.

A at the University of Alberta, Canada, and his colleagues has proved otherwise. They used real-time magnetic resonance imaging to show that the mechanism of joint cracking is related to cavity formation rather than bubble collapse. Yes, real fingers, attached to real people, were stretched in an MRI scanner.

Cracking occurred during joint separation, with the resulting cavity remaining visible. The knuckle-cracking comes from tribonucleation, the creation of small bubbles by the sudden breaking of contact between solid surfaces (in this case, bone) immersed in liquid (synovial fluid) that contains dissolved gas.

Whether knuckle-cracking is damaging or therapeutic is still to be resolved.

@ManorMill
via Twitter

My son cracks his knuckles all the time. He told me his science teacher said it does no harm and is all to do with gas in the joints.

I always believed that knuckle-cracking would encourage arthritis, but apparently, according to his teacher, that is a myth.

@realjohnmurphy
via Twitter

Whether it is a misconception that knuckle-cracking causes arthritis is the subject of some debate.

It seems like it ought to do harm over the long term, simply because wear and tear does harm our joints and intentionally cracking your knuckles is intentionally increasing wear and tear.

Whether the cracking is more harmful than any other wear and tear is another question. I am also a science teacher, for the record.

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