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The science behind knuckle cracking



There are no prizes to be found for solving the little secrets of science – why yawning is contagious, why puppies make us melt – but that does not mean that we do not want the answers anyway. Add the question of cracking to these everyday puzzles. Why exactly should some of the smallest joints of the body produce such an oversized bat?

Now a study in the scientific reports finally gives an explanation. It is one that was first introduced in 1971 but has been heavily debated ever since. However, thanks to some creative thinking and a new mathematical model, things could be resolved.

The authors of the paper ̵

1; a research team from Stanford University in California and the Ecole Polytechnique at Palaiseau – begin with a quick review of ankle-crack literature (yes, such a thing exists). In 1939, the leading theory was that the click sound is the result of a tightening of the fibrous capsule that surrounds the joints after so-called joint release, when the joint is adjusted or moved at a certain angle or to some degree. In 1947, there was the theory that the signature click sound was caused by vibration in tissues when joints were moved beyond their usual range.

But in 1971, the Knuckle Tear Field had its Eureka moment, when a team of researchers from Leeds University announced that ankle cracking is caused by the collapse of blisters in the synovial fluid, which surrounds and protects joints. The theory made sense, especially as a newly cracked knuckle can not be cracked for an average of 20 minutes, indicating that the bubbles are bursting and need to reform.

However, an international research team from Canada and Australia published a study using magnetic resonance imaging to show that even after the tear, the blisters remain in the joint. If they do not pop, how are they making noise?

To clarify the question, the current researchers came in their work differently. There are limits to how well an MRI or, in previous studies, X-rays could represent vesicles that have an average radius of only 200 microns – or 200 one-thousandth of a millimeter. Instead, they developed a mathematical model that would enable them to simulate and manipulate the joints, fluids, and bladders, and the interplay of all of them in various combinations.

The first step was to create a computer model of the third metacarpal phalangeal joint (MCP) – the one at the base of the finger that does the true job of producing the pop. With this virtual knuckle, the researchers then simulated the so-called tribo-cation process – making and breaking the contact between solid surfaces immersed in liquid. The bones in the MCP joint touch and separate all the time, resulting in decreased pressure in the synovial fluid, which in turn forms bubbles.

Once this acoustic ammo has been loaded into the knuckle, you still have to fire it. To investigate how this happens, researchers have relied in part on the so-called Rayleigh-Presset equation, a formula that "controls the dynamics of a spherical bubble in an infinite body of incompressible fluid." When they added Rayleigh-Presset to their model They moved their virtual knuckles so that the real knuckles are moved as they crack, the mathematical bubbles collapsing in a way that would produce the right sound.

So, problem solved, right? Not quite. All researchers had at that time proved what the researchers of 1971 already theorized. What about the dis proof – the later findings that the bubbles dwell in the ankle after the tear? The new model also answers that: Yes, the bubbles collapse, but only partially. The sudden, albeit incomplete, contraction of the bladder is sufficient to produce an easily audible tone while leaving enough bladder to be recognized by MRIs. The approximately twenty minutes the next crack takes is the time it takes for tribon nucleation to form new bubbles and expand the survivors.

The last puzzle – how a microscopic process can produce such an incredibly loud sound is actually the simplest. Existing acoustic pressure equations show that given the speed of the collapse of the bladder and the environment in which it takes place – the surrounding bone, the surrounding meat – it is quite possible to create a crack reaching 83 decibels. This roughly corresponds to the volume of a diesel truck, which flies at 40 mph from a distance of 50 feet. When the investigators compared the virtual sound waves that generated and drew their mathematical ankles with actual waves as real ankles cracked, the tracks were almost identical.

The practical application of this work is immediately apparent. It's, uh … OK, it's not so obvious. The researchers suggest that the methods they have developed could be used in modeling other sounds, and that is certainly possible. But not every bit of science has to be groundbreaking. Sometimes it makes the everyday world a little more meaningful – and sometimes that can be more than enough.


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