Why knuckles go crack has been revealed by Canadian researchers.
In 1947, two UK scientists speculated that joints click and crack when low pressure triggers a bubble to pop into existence inside the joint space.
Then, in the 1970s, a second group of British researchers instead claimed that the sound was produced not by a bubble forming in the joint, but instead when it collapsed in on itself afterwards.
This is a process called cavitation, and it became the benchmark explanation for why joints crack. Surprisingly, however, scientists had no direct observations to support either theory.
Now University of Alberta researcher Gregory Kawchuk and his colleagues, writing in PLoS One, have finally provided those observations, by clicking the joints of one of their team inside an MRI scanner.
By capturing 3 images per second inside the scanner as they tugged on a rope attached to the index finger of their volunteer, they were able to capture what happened inside the knuckle joint at the precise moment when it went crack.
The images clearly show the abrupt appearance of a dark void, corresponding to a bubble of gas inside the finger joint, simultaneously with the production of the sound.
"The bubble is gas that was dissolved in the joint fluid but comes out of solution when the pressure drops as the joint is stretched," says Kawchuk.
"It then hangs around for quite a long time afterwards and slowly disappears as it dissolves again. While this is happening, the joint can't be cracked again."
This shows that the sound can only be produced by the bubble appearing, rather than disappearing as the later theory had speculated.
"Rather than cavitation, when bubbles collapse noisily, our results fit more with a process called tribonucleation," explains Kawchuck.
"This is where two surfaces - in this case the bone surfaces in the joint - resist separation until a critical point when they separate rapidly. This produces a low-pressure area and that's what leads to the bubble appearing."
The results mean that scientists can now consider an equally long-standing joint-cracking-conundrum: whether the process is linked to damage to the joint or even arthritis...