|MadSci Network: Anatomy|
The process by which joints produce a 'cracking' sound when manipulated is called cavitation.
Gasses, principally Nitrogen, are a normal component of the synovial fluid which lubricates all the movable joints of our body (namely fingers, wrists, elbows, shoulders, hips, knees, ankles, toes, and the spine). As we manipulate a joint by physically stretching the connective tissue capsule surrounding it we allow the dissolved gasses to rapidly escape from solution, forming a small bubble that bursts at the right moment, producing the 'pop' sound we are all familiar with. The phenomenon is quite similar to opening a can of soda and releasing the carbon dioxide dissolved in the liquid.
After a short period of time, the nitrogen is resorbed into the synovial fluid and we can again 'pop' the joint (this explains the time lag). The reason it feels different in other joints is because the joint surfaces and supporting soft tissues are different. Thus, the movement and activity required to produce the desired result is also different.
When using joint manipulation as a form of therapy to alleviate musculoskeletal pain (as Doctors of Chiropractic do), the 'crack', or 'pop' sound is used as a reliable means of knowing that a joint has been properly manipulated. In fact, studies have shown that patients experience greater relief from manipulations associated with complete cavitation than those who do not reach this threshold.
I hope this answered your most excellent question to your satisfaction. If not, please feel free to write again.
Dr. David A. Schneider
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