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Kraaken · Posted: Thu Jan 24, 2008 3:38 am Post subject:

Cracking joints
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This article has been tagged since January 2007.
See also: Joint manipulation
Cracking joints is the practice of manipulating one's joints such that it produces a sharp sound, likened to cracking (also likened to popping, etc.). To produce the clicking sounds, many people bend their fingers into relatively unusual positions, that their own muscles are unable to achieve, and which are not commonly experienced in everyday use. For example, bending a finger backwards away from the palm (into extension), pulling them away from the hand (distraction), or compressing a finger knuckle toward the palm (into flexion).
When a manipulation is performed, the applied force separates the articular surfaces of a fully encapsulated synovial joint, which in turn creates a reduction in pressure within the joint cavity. In this low pressure environment, some of the gases that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave solution creating a bubble or cavity, which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of this gas bubble are thought to be mainly carbon dioxide.[1] The effects of this process will remain for a period of time known as the "refractory period", which can range from a few minutes to some hours while it is slowly reabsorbed back into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.[2]
Cracking within the body may also be caused by a bone being broken.
In many early motion pictures and subsequently parodied in animated cartoons, the gesture of cracking knuckles was associated with a "tough guy" image, especially when accompanied by the implicit or explicit threat of violence.
The physical mechanism is unknown, but possibilities that have been suggested include:
1. cavitation within the joint – small cavities of partial vacuum form in the fluid then rapidly collapse, producing a sharp sound (hypothesis in a medical journal)
2. the sudden stretching of ligaments
3. release of gas from the joints being adjusted (this applies to the popping that can occur in any joint such as during chiropractic manipulation)
4. adhesions being broken, which simply means that as two cartilage surfaces are pressed together, they form adhesions, and when the joints are separated this makes the popping or cracking sound.
The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.[3]
A single event is not enough to cause damage to the joint, although there is a hypothesis that prolonged joint stress due to cracking knuckles may eventually lead to a higher risk of joint damage. The long-term consequences of this practice have not been studied thoroughly, and the scientific evidence is inconclusive. However, the common parental advice "cracking your knuckles gives you arthritis" is not supported by any evidence, but habitual knuckle crackers are more likely to have hand swelling and lower grip strength.[4]
While some people find cracking to be meditative, others find the practice to be disgusting and annoying to listen to.
A chiropractic perspective:
“ As a joint is being distracted, the capsule invaginates inward and as the stress on the capsule reaches a certain threshold, it suddenly snaps back from the synovial fluid, increasing the volume of the capsule (and decreasing pressure) and causing the audible sound. The sudden increase in the volume causes the tension to drop, allowing the joint to increase in movement. Eventually, the elastic limit of the capsule is reached and the process stops. The time elapsed during all this is shorter than that required for completion of the stretch reflex, so it can occur without muscular resistance. The sudden jerk on the capsule and the other periarticular tissues is theorized to cause firing of the high-threshold mechanoreceptors.[5]
Another alternative description can be found in the e-book The Missing Owners Manual which describes the process of adhesions forming between cartilage surfaces[citation needed], and how chiropractic releases those adhesions.[6]
References
1. ^ Unsworth A, Dowson D, Wright V. (1971). "'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint.". Ann Rheum Dis 30 (4): 348-58. PMID 5557778.
2. ^ Fryer, Gary and Jacob, Mudge and McLaughlin, Patrick (2002) The Effect of Talocrural Joint Manipulation on Range of Motion at the Ankle. Journal of Manipulative and Physiological Therapeutics, 25. pp.384 to 390. PMID: 12183696
3. ^ Protopapas M, Cymet T, Protapapas M (2002). "Joint cracking and popping: understanding noises that accompany articular release.". J Am Osteopath Assoc 102 (5): 283-7. PMID 12033758. Available Online
4. ^ Castellanos J. Axelrod D. Effect of habitual knuckle cracking on hand function. Annals of the Rheumatic Diseases. 49(5):308-9, 1990
5. ^ Murphy, Donald R. (1996). Mechanisms Involved in Joint Manipulation. Chiropractic OnLine Today. Retrieved on 2006-09-08.
6. ^ Klein, David (2006). Breaking The Adhesion. Seaside Chiropractic. Retrieved on 2006-09-08.
* What makes your knuckles pop?. HowStuffWorks. Retrieved on 2006-09-08.
* Joint Crackers Community – Community of Joint Crackers
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Kraaken