{{short description|Bending a person's joints to produce a distinct cracking or popping sound}} {{Use dmy dates|date=August 2020}} {{Multiple image |direction = vertical |width = 250 |image1 = Cracking knuckles.jpg |caption1 = Cracking finger joints makes a distinct cracking or popping sound. |image2 = Knuckle crack.ogg }} '''Joint cracking''' is the manipulation of [[joint]]s to produce a sharp popping sound and a related, subjective sensation. It is sometimes performed by [[Physical therapy|physical therapist]]s, [[chiropractor]]s, and [[osteopath]]s,<ref>Richard Boggs, ''Hammaming in the Sham: A Journey Through the Turkish Baths of Damascus, Aleppo and Beyond'', 2012, {{isbn|1859643256}}, [https://books.google.com/books?id=uoMH_psyizoC&pg=PT161 p. 161]</ref> in pursuit of a variety of outcomes.
The cracking mechanism involves nitrogen gas that is dissolved in synovial fluid. When the pressure inside the joint cavity is quickly reduced due to mechanical expansion, nitrogen gas is released from the solution, leading to the formation of cavitation bubbles. When these nitrogen bubbles collapse, they create a cracking noise. It takes about 20 minutes for the nitrogen to fully dissolve back into the synovial fluid, which allows the joint to produce another set of cavitation bubbles and generate the cracking sound again.<ref name="dvorsky">{{Cite news|url=https://gizmodo.com/now-we-know-why-knuckle-cracking-makes-that-awful-sound-1824154323|title=Simulation May Finally Explain Why Knuckle Cracking Makes That Awful Sound|last=Dvorsky|first=George|work=Gizmodo|access-date=2018-03-30|language=en-US}}</ref>
Voluntary joint cracking is sometimes viewed as a type of "tic" and has been linked to the spectrum of obsessive-compulsive disorder. Two relevant studies highlight this connection:
* Abouhendy and Jawad (2013) discuss compulsive joint clicking and its relation to the obsessive-compulsive spectrum in their case report published in ''The Primary Care Companion for CNS Disorders''. * Johnson, Linse, and Novoa (2022) address diagnostic, ethical, and legal considerations in treating individuals with compulsive neck cracking in their article published in ''Cureus''.
These studies provide insights into the complexities of this behavior and its implications for treatment.
==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 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.<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 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.<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 }}</ref>
==Relation to arthritis== The common [[old wives' tale]] that cracking one's knuckles causes [[arthritis]] is without scientific evidence.<ref>{{Cite journal|last1=Rizvi|first1=Asad|last2=Loukas|first2=Marios|last3=Oskouian|first3=Rod J.|last4=Tubbs|first4=R. Shane|date=August 2018|title=Let's get a hand on this: Review of the clinical anatomy of "knuckle cracking"|journal=Clinical Anatomy|language=en|volume=31|issue=6|pages=942–945|doi=10.1002/ca.23243|pmid=30080300|s2cid=51920825|issn=0897-3806}}</ref> 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.<ref name="Deweber">{{cite journal |vauthors=Deweber K, Olszewski M, Ortolano R | title = Knuckle cracking and hand osteoarthritis | journal = J Am Board Fam Med | pages = 169–174 | year = 2011 | doi = 10.3122/jabfm.2011.02.100156| volume = 24 | pmid=21383216 | issue = 2| doi-access = free }}</ref> The study concluded that knuckle-cracking did not cause hand osteoarthritis, no matter how many years or how often a person cracked their knuckles.<ref name="Deweber"/> 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.<ref>{{cite journal|last=Simkin|first=Peter|title=Habitual knuckle cracking and hand function.|journal=Annals of the Rheumatic Diseases|date=November 1990 |volume=49 |issue=11 |page=957 |doi=10.1136/ard.49.11.957-b|pmid=2256753|pmc=1004281}}</ref>
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.<ref>{{cite web|url=http://improbable.com/ig/winners/#ig2009 | title=2009 Winners of the Ig® Nobel Prize| date=August 2006|access-date=27 November 2011}}</ref>
==See also== *[[Crepitus]]—sounds made by a joint
==References== {{Reflist}}
[[Category:Articles containing video clips]] [[Category:Habits]] [[Category:Joints]]