Whether you enjoy or hate the sound of cracking joints, you may be surprised to find out that, until recently, it wasn’t really clear how that sound is made.
The typically ‘crackable’ joints are diarthrodial – two bones touching each other at the cartilage-covered tips, and surrounded by a joint capsule which is filled with a lubricating fluid to prevent friction from joint movement. This lubricant is called synovial fluid, and it contains dissolved gases – mostly carbon dioxide.
Joints in the fingers are built just like this, and when the two bones are pulled in separate directions, this creates a pressure drop inside the joint capsule keeping the whole thing together. As the pressure drops, the gasses dissolved in the synovial fluid escape and form a bubble or cavity – a process known as tribonucleation.
In 1947, J. B. Roston and R. Wheeler Haines described this process happening inside the joint after taking a series of radiographic images of a knuckle being cracked, and noted that the formation of the bubble makes the sound. However, in 1971 a team of scientists at the University of Leeds postulated that the collapse of this bubble, rather than the formation, would be the cause of the cracking sound.
So which team got it right?
Of course, none of them could actually observe a knuckle being cracked in real time, however, now a team of scientists has finally done just that. Using magnetic resonance imaging (yes, they pulled the finger of someone inside an MRI machine) they discovered that it is indeed the formation of the bubble that makes the sound. The results were published yesterday in PLOS ONE.
Even so, not everything is still clear about the sound of cracking knuckles. As the researchers note, one thing remains unclear – why is it so loud? Normally a bubble forms fairly quietly, but with our joints, the sound can be heard across a room. Now that’s the next step of the investigation.