joint cracking

File:Real-Time-Visualization-of-Joint-Cavitation-pone.0119470.s002.ogv of a cracking finger joint depicting cavitation between the bones]]

File:Visualization of Joint Cavitation.png

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.{{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}}{{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}}
• Rapid stretching of ligaments.
• Intra-articular (within-joint) adhesions being broken.
• Formation of bubbles of joint air as the joint is expanded.{{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}}

There were several hypotheses to explain the cracking of joints. Synovial fluid cavitation has some evidence to support it.{{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}} 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.{{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}} The contents of the resultant gas bubble are thought to be mainly carbon dioxide, oxygen and nitrogen.{{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] 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.{{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 }}

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. 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.{{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}}{{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}}

The snapping of tendons or scar tissue over a prominence (as in snapping hip syndrome) can also generate a loud snapping or popping sound.{{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 }}

The common old wives' tale that cracking one's knuckles causes arthritis is without scientific evidence.{{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}} 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.{{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 }} 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.{{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}}

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 satirical Ig Nobel Prize in Medicine.{{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}}

• Crepitus—sounds made by joint

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