Editing Joint cracking (section)

==Causes==
[[File:Real-Time-Visualization-of-Joint-Cavitation-pone.0119470.s002.ogv|thumb|[[Magnetic resonance imaging|MRI]] of a cracking finger joint depicting cavitation between the bones]]
[[File:Visualization of Joint Cavitation.png|thumb|right|Static images of the hand in the resting phase before cracking (left). The same hand following cracking with the addition of a post-cracking distraction force (right). Note the dark, interarticular void (yellow arrow).]]

For many decades, the physical mechanism that causes the cracking sound as a result of bending, twisting, or compressing joints was 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.<ref name="telegraph">{{cite news|url=https://www.telegraph.co.uk/news/science/science-news/11539913/Why-knuckle-cracking-makes-a-popping-sound-and-why-it-might-be-beneficial.html|title=Why knuckle cracking makes a popping sound, and why it might be beneficial|last1=Knapton|first1=Sarah|date=15 April 2015|work=[[The Daily Telegraph]]|access-date=17 December 2016|location=London}}</ref><ref name="guardian">{{cite web|url=https://www.theguardian.com/science/2015/apr/15/cracked-it-scientists-solve-puzzle-of-why-knuckles-crack-when-pulled|title=Cracked it! Scientists solve puzzle of why knuckles pop when pulled|first1=Ian|last1=Sample|date=15 April 2015 |access-date=20 September 2016|work=The Guardian |location=London}}</ref>
* Rapid stretching of ligaments.<ref name="Protopapas" />
* Intra-articular (within-joint) [[Adhesion (medicine)|adhesions]] being broken.<ref name="Protopapas" />
*Formation of bubbles of [[pneumatosis|joint air]] as the joint is expanded.<ref name="kawchuk">{{cite journal|author1=Gregory N. Kawchuk|author2=Jerome Fryer|author3=Jacob L. Jaremko|author4=Hongbo Zeng|author5=Lindsay Rowe|author6=Richard Thompson|year=2015|title=Real-Time Visualization of Joint Cavitation|journal=PLOS ONE|volume=10|issue=6|pages=384–390|bibcode=2015PLoSO..1019470K|doi=10.1371/journal.pone.0119470|pmc=4398549|pmid=25875374|doi-access=free}}</ref>

There were several hypotheses to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it.<ref>{{cite journal |author=Brodeur R. |title=The audible release associated with joint manipulation |journal=J Manipulative Physiol Ther |volume=18 |issue=3 |pages=155–64 |year=1995 |pmid=7790795}}</ref> 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 [[gas]]es that are dissolved in the synovial fluid (which are naturally found in all bodily fluids) leave the solution, making a [[Bubble (physics)|bubble]], or [[Cavitation|cavity]] ([[tribonucleation]]), which rapidly collapses upon itself, resulting in a "clicking" sound.<ref>{{cite journal|last1=Maigne|first1=Jean-Yves|last2=Vautravers|first2=Philippe|title=Mechanism of action of spinal manipulative therapy|journal=Joint Bone Spine|date=September 2003|volume=70|issue=5|pages=336–341|doi=10.1016/S1297-319X(03)00074-5|pmid=14563460}}</ref> The contents of the resultant gas bubble are thought to be mainly [[carbon dioxide]], [[oxygen]] and [[nitrogen]].<ref>{{cite journal |vauthors=Unsworth A, Dowson D, Wright V |year=1971 |title='Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. |journal=Ann Rheum Dis |volume=30 |issue=4 |pages=348–58 |doi=10.1136/ard.30.4.348 |pmc=1005793 |pmid=5557778}}[https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1005793/pdf/annrheumd00004-0008.pdf]</ref> The effects of this process will remain for a period of time known as the "[[Refractory period (physiology)|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 [[Ligamentous laxity|ligament laxity]] may be associated with an increased tendency to cavitate.<ref name="Fryer">{{cite journal |author1=Fryer, Gary |author2= Jacob Mudge |author3=McLaughlin, Patrick |name-list-style=amp |year=2002 |title=The Effect of Talocrural Joint Manipulation on Range of Motion at the Ankle |journal=[[Journal of Manipulative and Physiological Therapeutics]] |issue=6 |pages=384–390 |pmid=12183696 |doi=10.1067/mmt.2002.126129 |volume=25|url= http://vuir.vu.edu.au/504/1/Ankle_manipulation_-_revised.pdf |archive-url=https://web.archive.org/web/20170921215401/http://vuir.vu.edu.au/504/1/Ankle_manipulation_-_revised.pdf |archive-date=2017-09-21 |url-status=live }}</ref>

In 2015, research showed that bubbles remained in the fluid after cracking, suggesting that the cracking sound was produced when the bubble within the joint was formed, not when it collapsed.<ref name="kawchuk" /> In 2018, a team in France created a mathematical simulation of what happens in a joint just before it cracks. The team concluded that the sound is caused by bubbles' collapse, and bubbles observed in the fluid are the result of a partial collapse. Due to the theoretical basis and lack of physical experimentation, the scientific community is still not fully convinced of this conclusion.<ref name="dvorsky" /><ref>{{cite magazine|url=https://time.com/5220275/knuckles-crack-science-why-reason/|title=Why Does Cracking Your Knuckles Make So Much Noise? Science Finally Has an Answer|magazine=Time|language=en|access-date=2018-03-30}}</ref><ref>{{Cite journal|last1=Chandran Suja|first1=V.|last2=Barakat|first2=A. I.|date=2018-03-29|title=A Mathematical Model for the Sounds Produced by Knuckle Cracking|journal=Scientific Reports|language=En|volume=8|issue=1|page=4600|doi=10.1038/s41598-018-22664-4|pmid=29599511|pmc=5876406|issn=2045-2322|bibcode=2018NatSR...8.4600C}}</ref>

The snapping of tendons or scar tissue over a prominence (as in [[snapping hip syndrome]]) can also generate a loud snapping or popping sound.<ref name="Protopapas">{{cite journal |vauthors=Protopapas M, Cymet T, Protapapas M |title=Joint cracking and popping: understanding noises that accompany articular release. |journal=J Am Osteopath Assoc |volume=102 |issue=5 |pages=283–7 |date=1 May 2002 |pmid=12033758 |url=http://www.jaoa.org/cgi/reprint/102/5/283 |access-date=2 March 2007 |archive-url=https://web.archive.org/web/20070927214025/http://www.jaoa.org/cgi/reprint/102/5/283 |archive-date=27 September 2007 |url-status=dead }}</ref>