Joint cracking

Causes

For many decades, the physical mechanism underlying the cracking sound produced by bending, twisting, or compressing joints remained uncertain. Suggested causes included:

• Cavitation within the joint—small cavities of partial vacuum form in the synovial fluid and then rapidly collapse, producing a sharp sound.
• Rapid stretching of ligaments.
• Intra-articular (within-joint) adhesions being broken.
• Formation of bubbles of joint air as the joint is expanded.

There were several hypotheses to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it. When a spinal 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 the solution, making a bubble, or cavity (tribonucleation), which rapidly collapses upon itself, resulting in a "clicking" sound. The contents of the resultant gas bubble are thought to be mainly carbon dioxide, oxygen and nitrogen. The effects of this process will remain for a period of time known as the "refractory period", during which the joint cannot be "re-cracked", which lasts about 20 minutes, while the gases are slowly reabsorbed into the synovial fluid. There is some evidence that ligament laxity may be associated with an increased tendency to cavitate.

In 2015, research indicated that bubbles remained in the fluid after a joint cracked, suggesting that the cracking sound was produced when the bubble within the joint formed, rather than when it collapsed. In 2018, a team in France developed a mathematical simulation to investigate what occurs in a joint just before it cracks. This team concluded that the sound is caused by the collapse of bubbles, while the bubbles observed in the fluid result from a partial collapse. Due to the theoretical nature of this conclusion and the absence of physical experimentation, the scientific community remains unconvinced. The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.

Relation to arthritis

The common old wives' tale that cracking one's knuckles causes arthritis is without scientific evidence. A study published in 2011 examined the hand radiographs of 215 people (aged 50 to 89). It compared the joints of those who regularly cracked their knuckles to those who did not. The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles. This early study has been criticized for not taking into consideration the possibility of confounding factors, such as whether the ability to crack one's knuckles is associated with impaired hand functioning rather than being a cause of it.

The medical doctor Donald Unger cracked the knuckles of his left hand every day for more than sixty years, but he did not crack the knuckles of his right hand. No arthritis or other ailments formed in either hand, and for this, he was awarded 2009's Ig Nobel Prize in Medicine.

References

References

1. Richard Boggs, ''Hammaming in the Sham: A Journey Through the Turkish Baths of Damascus, Aleppo and Beyond'', 2012, {{isbn
2. Dvorsky, George. "Simulation May Finally Explain Why Knuckle Cracking Makes That Awful Sound". Gizmodo.
3. (15 April 2015). "Why knuckle cracking makes a popping sound, and why it might be beneficial". [[The Daily Telegraph]].
4. (15 April 2015). "Cracked it! Scientists solve puzzle of why knuckles pop when pulled". The Guardian.
5. (2015). "Real-Time Visualization of Joint Cavitation". PLOS ONE.
6. Brodeur R.. (1995). "The audible release associated with joint manipulation". J Manipulative Physiol Ther.
7. (September 2003). "Mechanism of action of spinal manipulative therapy". Joint Bone Spine.
8. (1971). "'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint.". Ann Rheum Dis.
9. (2002). "The Effect of Talocrural Joint Manipulation on Range of Motion at the Ankle". [[Journal of Manipulative and Physiological Therapeutics]].
10. "Why Does Cracking Your Knuckles Make So Much Noise? Science Finally Has an Answer".
11. (2018-03-29). "A Mathematical Model for the Sounds Produced by Knuckle Cracking". Scientific Reports.
12. (1 May 2002). "Joint cracking and popping: understanding noises that accompany articular release.". J Am Osteopath Assoc.
13. (August 2018). "Let's get a hand on this: Review of the clinical anatomy of "knuckle cracking"". Clinical Anatomy.
14. (2011). "Knuckle cracking and hand osteoarthritis". J Am Board Fam Med.
15. Simkin, Peter. (November 1990). "Habitual knuckle cracking and hand function.". Annals of the Rheumatic Diseases.
16. (August 2006). "2009 Winners of the Ig® Nobel Prize".