Sunday, September 18, 2011

How Does Joint Cracking Work?

I'll give my best understanding here, but let the reader know that there does seem to be some ambiguity and contradiction in what I read on this subject.  Some of what you shall read is only my interpretation and hypothesizing about how the process most likely works.

Our joints are lubricated in a liquid called synovial fluid that

  • Is free of cells
  • Separated from our blood and internal matrix by a sturdy fibrous joint capsule 
  • Has non-Newtonian characteristics that make it more fluid with movement and use
  • Etymologically means that it has similarities to egg whites--'ovial' from 'ovum' for egg
  • When a joint is bent or torsioned that liquid has to move between the bone, the capsule and the surrounding areas.
It is this movement of the fluid that causes cracking.  For instance with our knuckles, by pushing them in one direction we pressurize the synovial fluid on one side of the joint.  This pressurized fluid is then rapidly released between the bones to the opposite side of the joint.  In the process a bubble cavitation (think cavity) is formed.  Cavitation happens when a shock wave of liquid moves so quickly that behind it it creates a vacuum.  This vacuum collapses with great force creating a loud popping sound we recognize as a knuckle cracking.  A very similar process happens much more intentionally with pistol/snapping shrimps: 
And boat propellers:
The controversy behind knuckle cracking shall some day be quelled as video MRI technology advances!

Some mo':

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