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It creates a sound that sets teeth on edge and, we're often told, raises the risk of arthritis.

But people who can crack their knuckles may actually have healthier joints than others.

Scientists say that the force involved in making fingers 'pop' is more than enough to cause damage.

Yet, studies show that regular knuckle-cracking doesn't seem to do any harm.

This suggests it is actually a sign of very healthy joints.

 The popping sound when someone cracks their knuckles is caused by a cavity forming in the fluid between the two joints as they are pulled apart. Images show MRI scans of hands and an arrow pointing to the cavity

The theory comes from Canadian researchers who used MRI scanners to peer deep inside the fingers and record just what happens when knuckles are cracked.

One of the team, a 'gifted knuckle-cracker', put one finger at a time into a tube connected to a cable.

This wire was then slowly pulled until the joint in large knuckle in the middle of each finger popped.

After about six seconds pulling the cable, the two bones suddenly shot apart, creating a cavity, or bubble, within the fluid that bathes the inside of the joint.

The cracking – which took less than a third of a second - occurred at the precise moment the bubble was formed, the journal PLOS ONE reports.

Lead author Professor Greg Kawchuk, from the University of Alberta, said: 'We call it the "pull my finger study" – and actually pulled someone's finger and filmed what happens in the MRI.

'When you do that, you can see very clearly what is happening inside the joints.

'It is a bit like forming a vacuum.

'As the joint surfaces suddenly separate, there is no more fluid available to fill the increasing joint volume, so a cavity is formed and that event is what is associated with the sound.'

The scans also showed a mysterious white flash occurs just before the noise is heard.

These MRI scans show the finger joints before the hand was cracked (A), and as the hand is being cracked, but just before the cavity forms (B), just after they were cracked (C), and the period after it was cracked (D)

Further research is needed into this streak of light – and why the cracking sound is so loud.

However, the researchers are happy to have shown that the noise is created when a bubble forms inside the knuckle.

Although this was first suggested more than 100 years ago, recent thinking has been that the crack is created when a bubble bursts.

Previous studies have shown that between 25 and 54 per cent of us crack our knuckles – with men doing it more than women.

And despite the common wisdom that the habit raises the risk of arthritis, previous studies have failed to prove this.

One comparison of Los Angeles nursing home residents found those who had regularly cracked their knuckles were actually less likely to have damaged joints.

The US researchers behind that study concluded: 'The chief morbid consequence of knuckle cracking would appear to be its annoying effect on the observer.'