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What actually makes our fingers crack
Cracking fingers are a real horror for many people. In some people, the sound even gives goosebumps. It is popularly said that this also causes osteoarthritis. However, there is no proof of this. But what makes our fingers crack at all? There is now a new theory.
Experts have been arguing about the causes for decades
For a long time, there was a controversy among experts about the causes of finger cracking. In 2015, an international team of researchers around lead author Professor Greg Kawchuk from the Faculty of Rehabilitation Medicine at the University of Alberta (Canada) published the results of a study in the specialist magazine "PLOS ONE", which, according to the scientists, determined what triggered the cracking . Now other experts reject the thesis of that time.
Crack the joints
For decades, researchers have been asking what makes our fingers crack. There is agreement that it is the joints that crack - and that you have to pull them apart for this.
In 1947, two British scientists from St Thomas’s Hospital Medical School in London found on X-rays that a gas bubble had formed in the gap between the joints.
In an article published at the time, they reported that, in their opinion, this made the noise.
In the early 1970s, however, researchers from the University of Leeds (Great Britain) claimed in a technical paper that it was not the formation of the bubble, but its bursting that caused the sound.
Cavity in the joint
In 2015, however, there was a setback for the Zerplatz theory when scientists led by Professor Kawchuk from the University of Alberta used MRI scans to observe for the first time how a cavity that causes finger cracking quickly forms in the joint when tensile forces are applied.
The joint surfaces surrounding the joint capsule (ends of the bones) are therefore separated from one another by a gap which is filled with viscous joint fluid.
If the finger is pulled, this gap widens significantly and a cavity is suddenly created in the synovial fluid, which is associated with the cracking sound.
Finger clicks observed in the MRI
According to their own statements, the researchers first needed someone who could have their ten fingers cracked at will.
Fortunately, there was a colleague in their ranks who had this ability and so they pushed him, lying on his stomach and with his finger ahead, into the MRI tube.
They attached a pulling device to his fingertip, which allowed a clear view of the joint in the following pictures. Then they pulled the explorer's finger until the crackling sound was heard.
The MRI captured the action at 3.2 frames per second, so that a film was created on which the process could be followed exactly.
At low tensile forces, according to the researchers, no changes in the joint were initially noticeable, but the finger was pulled sufficiently hard, the joint gap widened and a gas-filled cavity in the synovial fluid (synovial fluid) developed very suddenly.
Formation of a vacuum in the joint
"It's a bit like a vacuum is forming," said Professor Kawchuk in the University of Alberta press release.
Since the connection surfaces separate at once, there is not enough liquid available to fill the increasing volume, so that a cavity is created.
The occurrence of the cracking can be clearly assigned to this event on the basis of the MRI images, while the regression of the bladder was noiseless after the finger was released.
With this, the researchers confirmed the original thesis of the British scientists from 1947 and refuted subsequent, different study results.
But now doubts arise again. The two theoreticians V. Chandran Suja, who does research at Stanford University (USA), and his French colleague A.I Barakat (CNRS Palaiseau) tested the hypothesis of the Canadian researchers using mathematical model calculations.
They recently published their results in the scientific reports journal.
According to the scientists, the complete or even the partial imploding and not the formation of the bubble can cause the cracking. So it may well be that in the future further theses about the real causes of finger clicking will be made. (fp, ad)