Why do knuckles crack?
I have to admit it but one of the most irritating things someone can do in my presence is crack their knuckles. I long for the moment – it will be delicious when it arrives – when they have dislocated one or more of their digits in the process. Until then, I will just have to grin and bear it.
A recent exhibition of knuckle cracking in my proximity led me to wonder why this happens and why, alas, the perpetrator is able to put them back without any apparent harm. I was astonished to find, reading a report in the ever popular journal, Plos One, that whilst the phenomenon was acknowledged, the cause had not been the subject of detailed scientific investigation until a team of Canadian scientists from the University of Alberta under the leadership of Professor Greg Kawchuk cracked their grey cells on the matter.
It was back in 1938 that Nordheim, a German physician, demonstrated that most joints in healthy people can be made to pop when pulled – research, I’m sure, the Gestapo made good use of. The why, he never addressed.
In 1947 a couple of doctors at St Thomas’ Hospital in London conducted their own research into the knotty problem. They found volunteers – there is always one – tied a cord round their fingers and told them to tug until the joint popped. The docs captured what happened on a series of X-ray images and concluded that a tension of around 7 kilogrammes was required to make the bones in the knuckle separate by about half a centimetre.
As to why, they thought that the crack sounded when the joint surfaces were wrenched apart, causing a sudden drop in pressure in the synovial fluid around the joint and the formation of a bubble. Gratifyingly, once cracked, the same knuckle wouldn’t crack again for around 20 minutes.
For anyone who cared about such things this was the commonly accepted explanation until, in 1971, scientists from Leeds University repeated the St Thomas experiments and concluded that the crack came from the swift collapse of the bubble, not its formation.
And there matters stood until the Canadian scientists came along and videoed, using a MRI scanner, what exactly happened internally when knuckles were cracked. They discovered that every crack occurred precisely when the joints suddenly separated and a gas-filled pocket appeared in the synovial fluid which lubricates the joint. As the pressure in the fluid drops, the bubble comes from the gas in the fluid. The natural reaction of the joint is to resist the pulling but at some point, probably when the tension is some 7 kilogrammes, it will suddenly give way with a crack.
So, it seems that the London doctors were right and one of life’s many little mysteries has been solved. Rather desperately in trying to find an application for this discovery, rather than just glorying in putting a thorny problem to rest, Kawchuk suggests that the regular monitoring of the ability of those who can crack their knuckles will give a clue as to the health of the joints. Get out of here!
So now we know!