Editing Synovial fluid

{{Redirect|Synovia|the plural of synovium|synovial membrane}}
{{Short description|Fluid found in the cavities of synovial joints}}
{{Infobox anatomy
|Name = Synovial fluid
|Latin = synovia
|Image = Joint.svg
|Caption = A typical joint
}}

'''Synovial fluid''', also called '''synovia''',<sup>[[#Etymology and pronunciation|[help 1]]]</sup> is a viscous, [[non-Newtonian fluid]] found in the cavities of [[synovial joint]]s. With its [[egg white]]–like consistency,<ref>{{Cite book|title = Rheumatology secrets|last = West|first = Sterling G.|publisher = Elsevier Mosby|year = 2015|isbn = 9780323037006|location = Philadelphia|pages = 19|oclc = 908716294|edition = 3rd|series = The secrets series}}</ref> the principal role of synovial fluid is to reduce friction between the [[articular cartilage]] of synovial joints during movement.<ref>{{Citation|last=Petty|first=Ross E.|title=Chapter 2 - Structure and Function|date=2016-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780323241458000028|work=Textbook of Pediatric Rheumatology (Seventh Edition)|pages=5–13.e2|editor-last=Petty|editor-first=Ross E.|place=Philadelphia|publisher=W.B. Saunders|language=en|doi=10.1016/b978-0-323-24145-8.00002-8|isbn=978-0-323-24145-8|access-date=2020-10-18|editor2-last=Laxer|editor2-first=Ronald M.|editor3-last=Lindsley|editor3-first=Carol B.|editor4-last=Wedderburn|editor4-first=Lucy R.|url-access=subscription}}</ref> Synovial fluid is a small component of the [[transcellular fluid]] component of [[extracellular fluid]].

==Structure==
The inner membrane of synovial joints is called the [[synovial membrane]] and secretes synovial fluid into the joints.<ref>{{Citation|last1=Bay-Jensen|first1=A. C.|title=Chapter 31 - Structural Biomarkers|date=2016-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780128098479000313|work=Biochemistry of Collagens, Laminins and Elastin|pages=203–233|editor-last=Karsdal|editor-first=Morten A.|publisher=Academic Press|language=en|doi=10.1016/b978-0-12-809847-9.00031-3|isbn=978-0-12-809847-9|access-date=2020-10-18|last2=Sand|first2=J. M. B.|last3=Genovese|first3=F.|last4=Siebuhr|first4=A. S.|last5=Nielsen|first5=M. J.|last6=Leeming|first6=D. J.|last7=Manon-Jensen|first7=T.|last8=Karsdal|first8=M. A.|url-access=subscription}}</ref> Synovial fluid is an ultrafiltrate from blood, and contains proteins derived from the [[blood plasma]] and proteins that are
produced by cells within the joint tissues.<ref>{{Cite journal|title = A Normative Study of the Synovial Fluid Proteome from Healthy Porcine Knee Joints|journal = Journal of Proteome Research|date = 2014-09-03|pmc = 4184458|pmid = 25160569|pages = 4377–4387|volume = 13|issue = 10|doi = 10.1021/pr500587x|language = EN|first1 = Tue|last1 = Bennike|first2 = Ugur|last2 = Ayturk|first3 = Carla M.|last3 = Haslauer|first4 = John W.|last4 = Froehlich|first5 = Benedikt L.|last5 = Proffen|first6 = Omar|last6 = Barnaby|first7 = Svend|last7 = Birkelund|first8 = Martha M.|last8 = Murray|first9 = Matthew L.|last9 = Warman}}</ref> The fluid contains [[hyaluronan]] secreted by fibroblast-like cells in the synovial membrane, lubricin (proteoglycan 4; [[PRG4]]) secreted by the surface [[chondrocytes]] of the [[Wiktionary:articular|articular]] cartilage and interstitial fluid filtered from the [[blood plasma]].<ref>{{cite journal |vauthors = Jay GD, Waller KA|title = The biology of lubricin: near frictionless joint motion|journal = [[Matrix Biol.|Matrix Biology]]|volume = 39|pages = 17–24|year = 2014|pmid = 25172828|doi = 10.1016/j.matbio.2014.08.008 |doi-access=free}}</ref>  This fluid forms a thin layer (roughly 50 [[micrometre|μm]]) at the surface of cartilage and also seeps into microcavities and irregularities in the articular cartilage surface, filling all empty space.<ref name="edwards2000">{{cite web
 |url=http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm 
 |title=Normal Joint Structure 
 |editor=Edwards, Jo 
 |year=2000 
 |work=Notes on Rheumatology 
 |publisher=[[University College London]] 
 |access-date=5 April 2013 
 |archive-url=https://web.archive.org/web/20121119080937/http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm 
 |archive-date=19 November 2012 
 |url-status=dead 
}}</ref>  The fluid in [[articular]] cartilage effectively serves as a synovial fluid reserve. During movement, the synovial fluid held in the cartilage is squeezed out [[mechanics|mechanically]] to maintain a layer of fluid on the cartilage surface (so-called ''weeping lubrication'').
The functions of the synovial fluid include:

*reduction of friction — synovial fluid lubricates the articulating joints<ref>{{Cite book|title = New Atlas of Human Anatomy|publisher = MetroBooks|year = 2000|isbn = 9781586630973|location = China|oclc = 850877694|editor-last = McCracken|editor-first = Thomas}}</ref>{{Page needed|date=April 2013}}
*shock absorption — as a [[dilatant]] fluid, that possesses [[rheopectic]] properties,<ref>{{cite journal |author=Christorpher, GF |title=The role of protein content on the steady and oscillatory shear rheology of model synovial fluids |journal=  Soft Matter|volume=2014 |issue=10 |pages=5965–5973 |pmid=  24989639|doi=10.1039/C4SM00716F|date=2014-07-24 |bibcode=2014SMat...10.5965Z }}</ref> becoming more viscous under applied pressure; the synovial fluid in [[Wiktionary:diarthrosis|diarthrotic]] joints becomes thick the moment shear is applied in order to protect the joint and subsequently, thins to normal viscosity instantaneously to resume its lubricating function between shocks.<ref>{{cite web|url=http://www.orthopaedicsone.com/display/Main/Synovial%20fluid|title=Synovial fluid - OrthopaedicsOne Articles - OrthopaedicsOne|access-date=20 September 2016}}</ref> {{Dubious|date=December 2015}}
*nutrient and waste transportation — the fluid supplies oxygen and nutrients and removes carbon dioxide and metabolic wastes from the [[chondrocytes]] in the surrounding [[cartilage]]
*molecular sieving - pressure within the joint forces [[hyaluronan]] in the fluid against the synovial membrane forming a barrier against cells migrating into, or fluid migrating out of, the joint space. This function is dependent on the [[molecular weight]] of the [[hyaluronan]].<ref>{{cite journal |vauthors = Sabaratnam S, Arunan V, Coleman PJ, Mason RM, Levick JR|title = Size selectivity of hyaluronan molecular sieving by extracellular matrix in rabbit synovial joints|journal = [[J. Physiol.|The Journal of Physiology]]|volume = 567|issue = Pt 2|pages = 569–81|year = 2005|pmid = 15961430|doi = 10.1113/jphysiol.2005.088906 |doi-access=free |pmc=1474196}}</ref>

===Composition===
Synovial tissue is sterile and composed of vascularized connective tissue that lacks a basement membrane. Two cell types (type A and type B) are present: Type A is derived from blood monocytes, and it removes the wear-and-tear debris from the synovial fluid. Type B produces [[hyaluronan]]. Synovial fluid is made of [[hyaluronic acid]] and lubricin, proteinases, and collagenases. Synovial fluid exhibits [[non-Newtonian fluid|non-Newtonian flow]] characteristics; the viscosity coefficient is not a constant and the fluid is not linearly viscous.  Synovial fluid has anti-[[thixotropic]] characteristics; under conditions of sudden increase in pressure as in some kind of mechanical shock, its viscosity suddenly increases.<ref>< {{cite journal
 |title=Rheopexy of synovial fluid and protein aggregation |journal=Journal of the Royal Society Interface |author=Oates, Katherine |pmid=16849228 |doi=10.1098/rsif.2005.0086 |volume=3 |issue=6 |pmc=1618490 |year=2006 |pages=167–74}}</ref> Normal synovial fluid contains 3–4&nbsp;mg/ml [[hyaluronan]] (hyaluronic acid),<ref name=hui2012>{{cite journal
 |last1=Hui |first1=Alexander Y. |last2=McCart |first2=William J. |last3=Masuda |first3=Koichi |last4=Firestein |first4=Gary S. |last5=Sah |first5=Robert L.|date=Jan–Feb 2012|title=A Systems Biology Approach to Synovial Joint Lubrication in Health, Injury, and Disease |journal=Systems Biology and Medicine |series=Wiley Interdisciplinary Reviews |volume=4 |issue=1 |pages=15–7 |doi=10.1002/wsbm.157 |pmc=3593048 |pmid=21826801}}</ref> a polymer of [[disaccharide]]s composed of D-glucuronic acid and D-N-acetyl[[glucosamine]] joined by alternating beta-1,4 and beta-1,3 [[glycosidic bond]]s.<ref>{{cite journal |url=http://www.glycoforum.gr.jp/science/hyaluronan/HA01/HA01E.html |title=GlycoForum / Science of Hyaluronan |journal=Glycoforum |date=15 December 1997 |volume=1 |pages=A2 |last1=Hascall |first1=Vincent C. |last2=Laurent |first2=Torvard C. }}</ref>{{MEDRS|date=February 2013}} Hyaluronan is synthesized by the synovial membrane and secreted into the joint cavity to increase the viscosity and elasticity of articular cartilages and to lubricate the surfaces between [[synovium]] and cartilage.<ref>{{cite web |url=http://www.orthop.washington.edu/?q=patient-care/articles/arthritis/joints.html |publisher=University of Washington Medicine - Department of Orthopaedics and Sports Medicine |title=Joints |access-date=2013-02-04 |archive-date=2013-08-07 |archive-url=https://web.archive.org/web/20130807171425/http://www.orthop.washington.edu/?q=patient-care%2Farticles%2Farthritis%2Fjoints.html |url-status=dead }}</ref>{{MEDRS|date=February 2013}}

Synovial fluid contains [[PRG4|lubricin (also known as PRG4)]] as a second lubricating component, secreted by synovial [[fibroblast]]s.<ref name=jay2000>{{cite journal
 |last1=Jay |first1=GD |last2=Britt |first2=DE |last3=Cha |first3=CJ |date=March 2000 |title=Lubricin is a product of megakaryocyte stimulating factor gene expression by human synovial fibroblasts |journal=[[J Rheumatol]] |volume=27 |issue=3 |pages=594&ndash;600 |type=abstract |pmid=10743795 }}</ref>  Chiefly, it is responsible for so-called boundary-layer lubrication, which reduces friction between opposing surfaces of cartilage. There also is some evidence that it helps regulate synovial cell growth.<ref>{{cite journal |author=Warman M |journal=[[Arthritis Research & Therapy]] |year=2003 |volume=5 |issue=Suppl 3 |pages=5–7 |title=Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints |doi=10.1186/ar804 |doi-access=free |pmc=2833794 }}</ref>

It also contains [[Phagocyte|phagocytic cells]] that remove microbes and the debris that results from normal wear and tear in the joint.

==Clinical significance==

===Collection===
Synovial fluid may be collected by syringe in a procedure termed [[arthrocentesis]], also known as joint aspiration.

===Classification===
Synovial fluid may be classified into normal, noninflammatory, inflammatory, septic, and hemorrhagic:
{| class="wikitable" border="1" style="border-collapse:collapse"
|+Classification of synovial fluid in an adult [[knee]] joint.
|- bgcolor="#cccccc"
|bgcolor="#ffffff"| || Normal || Non-inflammatory || Inflammatory || Septic || Bleeding
|-
|bgcolor="#cccccc"| Volume (ml) || <3.5   || >3.5             || >3.5         || >3.5   || >3.5
|-
|bgcolor="#cccccc"| [[Viscosity]]   || High   || High             || Low          || Mixed  || Low
|-
|bgcolor="#cccccc"| Clarity     || Clear  || Clear            || Cloudy       || Opaque || Mixed
|-
|bgcolor="#cccccc"| Color       || Colorless/straw || Straw/yellow || Yellow  || Mixed  || Red
|-
|bgcolor="#cccccc"| [[White blood cell|WBC]]/mm<sup>3</sup> || <200 || <2,000<ref name=agabegi2nd6-6/> || 5,000<ref name=agabegi2nd6-6/>-75,000 || >50,000<ref name=agabegi2nd6-6/> || Similar to blood level
|-
|bgcolor="#cccccc"| [[Polysegmented neutrophil|Polys]] (%)  || <25    || <25<ref name=agabegi2nd6-6/>              || 50<ref name=agabegi2nd6-6/>-70<ref name=agabegi2nd6-6>Table 6-6 in: {{cite book |author1=Elizabeth D Agabegi |author2=Agabegi, Steven S. |title=Step-Up to Medicine (Step-Up Series) |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year=2008 |isbn=978-0-7817-7153-5 |url-access=registration |url=https://archive.org/details/stepuptomedicine0000agab }}</ref>          || >70<ref name=agabegi2nd6-6/>    || Similar to blood level
|-
|bgcolor="#cccccc"| [[Gram stain]] || None || None   || None    || + (S aureus & Streptococcus) - (Neisseria gonorrhoeae)|| None
|}
Glucose (mg/dl) concentration in synovial fluid is nearly equal to serum.
;Synovial fluid viscosity
''Normal'':
*Normal
*Traumatic arthritis
*Degenerative (Osteo) arthritis
* Pigmented villonodular synovitis
''Normal or decreased'':
*Systemic lupus erythematosus
''Decreased'':
*Rheumatic fever
*Rheumatoid arthritis
*Gout
*Pyogenic (Septic) arthritis
* Tubercular arthritis
* Less of lubrication in joints

===Pathology===
[[Image:Illu synovial joint.jpg|right|Synovial joint]]
Many synovial fluid types are associated with specific diagnoses:<ref name=fpnlupus>{{cite web
 |url=http://www.fpnotebook.com/Rheum/Lab/LpsAntcglnt.htm 
 |title=Lupus Anticoagulant 
 |publisher=Family Practice Notebook 
 |archive-url=https://web.archive.org/web/20130118110623/http://www.fpnotebook.com/Rheum/Lab/LpsAntcglnt.htm 
 |archive-date=18 January 2013 
 |url-status=live 
 |access-date=7 April 2013 
}}</ref><ref>[http://www.rheumatology.org/publications/primarycare/number6/hrh0033698.asp?aud=mem American College of Rheumatology]  {{webarchive |url=https://web.archive.org/web/20040905224401/http://www.rheumatology.org/publications/primarycare/number6/hrh0033698.asp?aud=mem |date=September 5, 2004 }}</ref>

{{columns-list|
* Noninflammatory (Group I)
** [[Osteoarthritis]], [[degenerative joint disease]]
** Trauma
** [[Rheumatic fever]]
** Chronic [[gout]] or [[pseudogout]]
** [[Scleroderma]]
** [[Polymyositis]]
** [[Systemic lupus erythematosus]]
** [[Erythema nodosum]]
** [[Neuropathic arthropathy]] (with possible hemorrhage)
** [[Sickle-cell disease]]
** [[Hemochromatosis]]
** [[Acromegaly]]
** [[Amyloidosis]]
* Inflammatory (Group II)
** [[Rheumatoid arthritis]]
** [[Reactive arthritis]]
** [[Psoriatic arthritis]]
** Acute rheumatic fever
** Acute gout or pseudogout
** Scleroderma
** Polymyositis
** Systemic lupus erythematosus
** [[Ankylosing spondylitis]]
** Inflammatory bowel disease arthritis
** [[Infection]] (viral, fungal, bacterial) including [[Lyme disease]]
** Acute [[crystal synovitis]] (gout)
* Septic (Group III)
** Pyogenic bacterial infection
** [[Septic arthritis]]
* Hemorrhagic
** Trauma
** [[Tumor]]s
** [[Hemophilia]]/[[coagulopathy]]
** [[Scurvy]]
** [[Ehlers-Danlos syndrome]]
** Neuropathic arthropathy
}}

==Analysis==
Glucose (mg/dl) concentration in synovial fluid is nearly equal to [[Serum (blood)|serum]].

The cytological and biochemical analysis of human synovial fluid began around 1940 using cadaver-derived fluid and comparing characteristics to those of, for instance, bovine synovial fluid.<ref>{{Citation
 |last1=Ropes |first1=Marian W. |last2=Rossmeisl |first2=Elsie C. |last3=Bauer |first3=Walter |date=November 1940 |title=The Origin and Nature of Normal HUman Synovial Fluid |journal=[[J Clin Invest]] |volume=19 |issue=6 |pages=795&ndash;799 |pmid=16694795 |doi=10.1172/JCI101182 |pmc=435014}} ''and references therein''</ref>

=== Chemistry ===
The mucin clot test is a very old approach to determining if an inflammatory infiltrate is present.  In this test, acetic acid is added to the synovial fluid specimen.  In a normal specimen, this should lead to a congealing of the hyaluronic acid, forming a 'mucin clot.'  If inflammation is present, a mucin clot is not formed (the hyaluronic acid is degraded).<ref name=demais2009>{{Citation |last=De Mais |first=Daniel |year=2009 |title=Quick Compendium of Clinical Pathology |edition=2nd |publisher=[[ASCP Press]] |location=Chicago |isbn=9780891895671 |oclc=692198047 |url-access=registration |url=https://archive.org/details/quickcompendiumo0000mais }}{{pages needed|date=April 2013}}</ref>

Lactate is elevated in septic arthritis, usually above 250&nbsp;mg/dL.

Complement factors are decreased in rheumatoid arthritis and lupus arthritis.

=== Microscopy ===
Microscopic analysis of synovial fluid is performed to evaluate for cell count and crystals.  Crystals include [[monosodium urate crystals]], [[calcium pyrophosphate]], [[hydroxyapatite]] and [[corticosteroid]] crystals.<ref name=demais2009/>

Monosodium urate crystals are seen in [[gout]] or gouty arthritis and appear as needle-shaped negatively birefringent crystals varying in length from 2 to 20&nbsp;[[μm]]. With negative birefringence, the crystals appear yellow in parallel light and blue with perpendicular light.

[[Calcium pyrophosphate]] crystals are seen in [[pseudogout]] (also known as calcium pyrophosphate deposition disease or, CPPD). These crystals are rod-shaped or rhomboids varying in length from 2 to 20&nbsp;μm and with positive birefringence (blue with parallel light, yellow with perpendicular light).

[[Hydroxyapatite]] crystals are small and negatively birefringent. They are usually only detectable with an [[Alizarin Red S]] stain.

[[Corticosteroid]] crystals may be seen following therapeutic corticosteroid injection into the joint space. They appear blunt, jagged, and show variable birefringence.<ref name=demais2009/>

===Cracking joints===
{{main|Cracking joints}}
When the two articulating surfaces of a synovial joint are separated from one other, the volume within the joint capsule is increased and a negative pressure results. The volume of synovial fluid within the joint is insufficient to fill the expanding volume of the joint and gases dissolved in the synovial fluid (mostly [[carbon dioxide]]) are liberated and quickly fill the empty space, leading to the rapid formation of a bubble.<ref>{{cite journal |vauthors=Unsworth A, Dowson D, Wright V | title = 'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. | journal = Ann Rheum Dis | volume = 30 | issue = 4 | pages = 348–58 | year = 1971 | pmid = 5557778 | doi = 10.1136/ard.30.4.348 | pmc=1005793}}</ref> This process is known as [[cavitation]]. Cavitation in synovial joints results in a high frequency 'cracking' sound.<ref>Watson P, Kernoham WG, Mollan RAB. A study of the cracking sounds from the metacarpophalangeal joint. Proceedings of the Institute of Mechanical Engineering [H] 1989;203:109-118.</ref><ref>{{cite web|url=http://health.howstuffworks.com/question437.htm|title=What makes your knuckles pop?|date=3 August 2000|access-date=20 September 2016}}</ref>

==Etymology and pronunciation==
The term ''synovia'' ({{IPAc-en|s|ɪ|ˈ|n|oʊ|v|i|ə}}) came to English around 1640 (the anglicized form ''synovial'' is first recorded in the mid 18th century) from [[Neo-Latin]], where it was coined perhaps by [[Paracelsus]] from Greek ''[[wikt:συν-|συν-]]'' "with" and Latin ''[[wikt:ovum#Latin|ovum]]'' "egg" and ''[[wikt:-ia#Latin|-ia]]'' because it resembles egg white in consistency and external appearance.<ref name=collins>[https://www.wordreference.com/definition/synovia "synovia"] in the Collins Concise English Dictionary</ref><ref>[https://www.dictionary.com/browse/synovia "synovia"] in the Random House Unabridged Dictionary</ref><ref name="AHD">[https://www.ahdictionary.com/word/search.html?q=synovia "synovia"] in the American Heritage Dictionary</ref><ref name="OxfordDictionaries">[https://web.archive.org/web/20180901151212/https://en.oxforddictionaries.com/definition/synovial "synovial"] in the Oxford Dictionaries Online</ref><ref>[http://encyclopedia2.thefreedictionary.com/synovia "synovia"] in the Great Soviet Encyclopedia</ref>

The term ''synovium'' is a much more recent pseudo-Latin coinage for what is less confusingly called the [[synovial membrane]]. It is not recorded in general dictionaries, and medical dictionaries only explain its meaning, not its etymology, but it is apparently derived from the term ''synovia'', i.e. the obfuscated etymology of mixed Greek and Latin elements of the singular term ''synovia'' was misunderstood and the word was erroneously reinterpreted as the plural of the previously non-existent term ''synovium'' (perhaps in analogy to other plural terms for liquids such as "waters" for [[amniotic fluid]]). If one insists on using this pseudo-Latin term ''synovium'' for the synovial membrane, the non-Latinate plural ''synoviums'' is better and less confusing than ''synovia''.

==References==
{{Reflist|30em}}

==Further reading==
* {{cite journal |last1=Warman |first1=M. |year=2003 |title=Delineating biologic pathways involved in skeletal growth and homeostasis through the study of rare Mendelian diseases that affect bones and joints |journal=Arthritis Research & Therapy |volume=5 |pages=5 |doi=10.1186/ar804|doi-access=free |pmc=2833794 }}

==External links==
* [http://www.glycoforum.gr.jp/science/hyaluronan/HA01/HA01E.html Hyaluronan: structure and properties]
* [https://web.archive.org/web/20121119080937/http://www.ucl.ac.uk/~regfjxe/NORMALJOINT.htm Normal joint structure] from the [[University College London]]

{{Joints}}

{{Authority control}}

[[Category:Body fluids]]
[[Category:Joints]]