Editing Synovial joint

{{Short description|Articulation which admits free motion in the joint; the most common type of articulation}}
{{Infobox anatomy
|Name = Synovial joint
|Latin = junctura synovialis
|Image = 907 Synovial Joints.jpg
|Caption = Structure of synovial joint
|Image2 = 909 Types of Synovial Joints.jpg
|Caption2 = Types of synovial joints.

Clockwise from top-right: [[ball and socket joint]], [[condyloid joint]], [[plane joint]], [[saddle joint]], [[hinge joint]] and [[pivot joint]].
}}
A '''synovial joint''', also known as '''diarthrosis''', joins bones or cartilage with a fibrous [[joint capsule]] that is continuous with the [[periosteum]] of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulating surfaces. This joint unites long bones and permits free bone movement and greater mobility.<ref>The Musculoskeletal System. In: Dutton M. eds. ''Dutton's Orthopaedic Examination, Evaluation, and Intervention, 5e''. McGraw-Hill;  Accessed January 25, 2021. https://accessphysiotherapy-mhmedical-com.libaccess.lib.mcmaster.ca/content.aspx?bookid=2707&sectionid=224662311</ref> The synovial cavity/joint is filled with [[synovial fluid]]. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, the [[synovial membrane]], which seals in the synovial fluid.

They are the most common and most movable type of [[joint]] in the body. As with most other joints, synovial joints achieve movement at the point of contact of the articulating [[bone]]s. They originated 400 million years ago in the first jawed vertebrates. 

== Structure ==
Synovial joints contain the following structures:
* Synovial cavity: all diarthroses have the characteristic space between the bones that is filled with [[synovial fluid]].
* [[Joint capsule]]: the fibrous capsule, continuous with the periosteum of articulating bones, surrounds the diarthrosis and unites the articulating bones; the joint capsule consists of two layers - (1) the outer [[Fibrous connective tissue|fibrous]] membrane that may contain ligaments and (2) the inner [[synovial membrane]] that secretes the lubricating, shock absorbing, and joint-nourishing synovial fluid; the joint capsule is highly innervated, but without blood and lymph vessels, and receives nutrition from the surrounding blood supply via either [[diffusion]] (slow), or via [[convection]] (fast, more efficient), induced through exercise.
* [[Articular cartilage]]: the bones of a synovial joint are covered by a layer of [[hyaline cartilage]] that lines the [[Epiphysis|epiphyses]] of the joint end of the bone with a smooth, slippery surface that prevents [[adhesion]]; articular cartilage functions to absorb shock and reduce [[friction]] during movement.

Many, but not all, synovial joints also contain additional structures:<ref name="Graysp20"/>
* [[Articular disc]]s or menisci - the [[fibrocartilage]] pads between opposing surfaces in a joint
* Articular fat pads - [[adipose tissue]] pads that protect the articular cartilage, as seen in the infrapatellar fat pad in the knee
* [[Tendons]]<ref name="Graysp20"/> - cords of [[dense regular connective tissue]] composed of parallel bundles of [[collagen fibers]]
* Accessory ligaments (extracapsular and intracapsular) - the fibers of some fibrous membranes are arranged in parallel bundles of dense regular connective tissue that are highly adapted for resisting strains to prevent extreme movements that may damage the articulation{{Citation needed|date=October 2012}}
* [[Bursae]] - sac-like structures that are situated strategically to alleviate friction in some joints (shoulder and knee) that are filled with fluid similar to synovial fluid<ref name="Tortora12thEd"/>{{Page needed|date=October 2012}}

The bone surrounding the joint on the proximal side is sometimes called the ''plafond'' (French word for ceiling), especially in the [[talocrural joint]]. Damage to this structure is referred to as a [[Gosselin fracture]].

===Blood supply===
The blood supply of a synovial joint is derived from the arteries sharing in the [[anastomosis]] around the joint.

=== Types ===
There are seven types of synovial joints.<ref name="umich2010couse"/> Some are relatively immobile, therefore more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury.<ref name="umich2010couse"/> In ascending order of mobility, they are:

{| class="wikitable"
! Name
! Example
! Description
|- 
| [[Plane joint]]s<br/>(or gliding joint) 
| [[carpal]]s of the [[wrist]], [[acromioclavicular joint]]
| These joints allow only gliding or sliding movements, are multi-axial such as the articulation between vertebrae.
|- 
| [[Hinge joint]]s
| [[Elbow-joint|elbow]] (between the [[humerus]] and the [[ulna]])
| These joints act as a [[door]] [[hinge]] does, allowing flexion and extension in just one plane, i.e. uniaxial.
|-
| [[Pivot joint]]s
| [[atlanto-axial joint]], [[proximal radioulnar joint]], and [[distal radioulnar joint]]
| One bone rotates about another
|- 
| [[Condyloid joint]]s<br/>(or ellipsoidal joints)
| [[wrist joint]] ([[radiocarpal joint]])
| A condyloid joint is a modified ball and socket joint that allows primary movement within two perpendicular axes, passive or secondary movement may occur on a third axes. Some classifications make a distinction between condyloid and ellipsoid joints;<ref name="Rogers2010p157"/><ref name="Sharkey2008p33"/> these joints allow flexion, extension, abduction, and adduction movements (circumduction).  
|- 
| [[Saddle joint]]s
| [[Carpometacarpal]] or trapeziometacarpal joint of [[thumb]] (between the [[metacarpal]] and [[carpal]] - [[trapezium (bone)|trapezium]]), [[sternoclavicular joint]]
| Saddle joints, where the two surfaces are reciprocally concave/convex in shape, which resemble a [[horse tack|saddle]], permit the same movements as the condyloid joints but allows greater movement.
|- 
| {{nowrap|[[Ball and socket joint]]s}}<br/>"universal Joint"
| [[shoulder]] ([[glenohumeral]]) and [[hip]] joints
| These allow for all movements except gliding
|- 
| Compound joints<ref name="Moini2011p231"/><ref name="Abernethy2005p331"/><br/>/ bicondyloid joints<ref name="Graysp20"/>
| [[knee]] joint
| condylar joint (condyles of femur join with condyles of tibia) and saddle joint (lower end of femur joins with patella)
|}

====Multiaxial joints====
[[File:903 Multiaxial Joint.jpg|thumb|A multiaxial joint, such as the hip joint, allows for three types of movement: anterior-posterior, medial-lateral, and rotational.]]
A '''multiaxial joint''' ('''polyaxial joint''' or '''triaxial joint''') is a synovial joint that allows for several directions of movement.<ref>{{cite web |last1=Miles |first1=Linda |title=LibGuides: BIO 140 - Human Biology I - Textbook: Chapter 41 - Classification of Joints |url=https://guides.hostos.cuny.edu/bio140/12-41 |website=guides.hostos.cuny.edu |publisher=Hostos Community College Library |access-date=21 May 2023 |language=en}}</ref> In the human body, the [[shoulder joint|shoulder]] and [[hip joint]]s are multiaxial joints.<ref>{{cite book |last1=Lawry |first1=George V. |title=Chapter 1 - Anatomy of Joints, General Considerations, and Principles of Joint Examination |chapter=Anatomy of Joints, General Considerations, and Principles of Joint Examination |chapter-url=https://www.sciencedirect.com/science/article/abs/pii/B9780323030038500052 |website=Musculoskeletal Examination and Joint Injection Techniques |publisher=Mosby |access-date=21 May 2023 |pages=1–6 |language=en |date=1 January 2006|doi=10.1016/B978-0-323-03003-8.50005-2 |isbn=978-0-323-03003-8 }}</ref> They allow the upper or lower limb to move in an anterior-posterior direction and a medial-lateral direction. In addition, the limb can also be rotated around its long axis. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.<ref name="ana&physio">{{cite book |last1=Betts |first1=J. Gordon |title=Anatomy & physiology |date=2013 |publisher=OpenStax |chapter=9.1 Classification of joints|location=Houston, Texas |isbn=978-1-947172-04-3 |url=https://openstax.org/books/anatomy-and-physiology/pages/9-1-classification-of-joints |access-date=14 May 2023}}</ref>

==Function==
{{Main|Anatomical terms of motion}}
The movements possible with synovial joints are:
* [[Abduction (kinesiology)|abduction]]: movement away from the mid-line of the body
* [[adduction]]: movement toward the mid-line of the body
* [[Extension (kinesiology)|extension]]: straightening limbs at a joint
* [[flexion]]: bending the limbs at a joint
* [[rotation]]: a circular movement around a fixed point

==Clinical significance==
{{anchor|space}}The ''joint space'' equals the distance between the involved bones of the joint. A ''joint space narrowing'' is a sign of either (or both) [[osteoarthritis]] and inflammatory degeneration.<ref name="JacobsonGirish2008">{{cite journal|last1=Jacobson|first1=Jon A.|last2=Girish|first2=Gandikota|last3=Jiang|first3=Yebin|last4=Sabb|first4=Brian J.|title=Radiographic Evaluation of Arthritis: Degenerative Joint Disease and Variations|journal=Radiology|volume=248|issue=3|year=2008|pages=737–747|issn=0033-8419|doi=10.1148/radiol.2483062112|pmid=18710973}}</ref> The normal joint space is at least 2&nbsp;mm in the [[hip joint|hip]] (at the superior [[acetabulum]]),<ref name="Lequesne2004">{{cite journal|last1=Lequesne|first1=M|title=The normal hip joint space: variations in width, shape, and architecture on 223 pelvic radiographs|journal=Annals of the Rheumatic Diseases|volume=63|issue=9|year=2004|pages=1145–1151|issn=0003-4967|doi=10.1136/ard.2003.018424|pmc=1755132|pmid=15308525}}</ref> at least 3&nbsp;mm in the [[knee]],<ref>{{cite book|title=Osteoarthritis: Diagnosis and Medical/surgical Management, LWW Doody's all reviewed collection|author=Roland W. Moskowitz|publisher=Lippincott Williams & Wilkins|year=2007|isbn=9780781767071|page=[https://books.google.com/books?id=YfFj8Gbq5H0C&pg=PA6 6]}}</ref> and 4–5&nbsp;mm in the [[shoulder joint]].<ref>{{cite web|url=http://radref.org/ref.php?id=361|title=Glenohumeral joint space|website=radref.org}}, in turn citing: {{cite journal|last1=Petersson|first1=Claes J.|last2=Redlund-Johnell|first2=Inga|title=Joint Space in Normal Gleno-Humeral Radiographs|journal=Acta Orthopaedica Scandinavica|volume=54|issue=2|year=2009|pages=274–276|issn=0001-6470|doi=10.3109/17453678308996569|pmid=6846006|doi-access=free}}</ref> For the [[temporomandibular joint]], a joint space of between 1.5 and 4&nbsp;mm is regarded as normal.<ref name="Massilla ManiSivasubramanian2016">{{cite journal|last1=Massilla Mani|first1=F.|last2=Sivasubramanian|first2=S. Satha|title=A study of temporomandibular joint osteoarthritis using computed tomographic imaging|journal=Biomedical Journal|volume=39|issue=3|year=2016|pages=201–206|issn=2319-4170|doi=10.1016/j.bj.2016.06.003|pmc=6138784|pmid=27621122}}</ref> Joint space narrowing is therefore a component of several [[radiographic classifications of osteoarthritis]].

In [[rheumatoid arthritis]], the clinical manifestations are primarily synovial inflammation and joint damage. The [[fibroblast-like synoviocytes]], highly specialized mesenchymal cells found in the [[synovial membrane]], have an active and prominent role in the pathogenic processes in the rheumatic joints.<ref name=nygaard>{{cite journal |doi=10.1038/s41584-020-0413-5 |title=Restoring synovial homeostasis in rheumatoid arthritis by targeting fibroblast-like synoviocytes |year=2020 |last1=Nygaard |first1=Gyrid |last2=Firestein |first2=Gary S. |journal=Nature Reviews Rheumatology|volume=16 |issue=6 |pages=316–333 |pmid=32393826 |pmc=7987137 }}</ref> Therapies that target these cells are emerging as promising therapeutic tools, raising hope for future applications in rheumatoid arthritis.<ref name=nygaard />

== Evolutionary origins ==
Synovial joints has been found in earliest jawed vertebrates ([[Gnathostomata|gnathostomes]]) 400 million years ago during the [[Silurian]] and [[Devonian]]. This finding overturns an earlier view that these joints first evolved in early [[Tetrapod|tetrapods]] for [[terrestrial locomotion]]. Comparative studies find that synovial joints  in all major groups of jawed vertebrates, including [[Chondrichthyes|cartilaginous fishes]] (sharks, skates, and rays), [[Osteichthyes|bony fishes]], and tetrapods. They are however absent in [[Agnatha|jawless vertebrates]] such as [[Lamprey|lampreys]] and [[hagfish]]. Cartilaginous fishes have true synovial joints with clear synovial cavities, articular cartilage lined by flattened chondrocytes, and express key developmental signaling molecules including [[GDF5|growth differentiation factor-5 (Gdf5)]] and [[Catenin beta-1|β-catenin]], and require muscle contraction for proper joint cavitation. In contrast, cyclostomes have joints filled with tissue rather than fluid-filled cavities, with proteoglycans uniformly distributed across cartilages. Fossil evidence finds jawless [[Osteostraci|osteostracans]] had pectoral fin connections filled with canals incompatible with fluid-filled joint cavities, while early jawed [[Placoderm|placoderms]] have reciprocally articulating surfaces separated by joint cavities. Synovial joints, it have been suggested, arose due to the high mechanical loads associated with predation and feeding and as a result allowed for the evolution of the complex skeletons of modern jawed vertebrates.<ref>{{Cite journal |last1=Sharma |first1=Neelima |last2=Haridy |first2=Yara |last3=Shubin |first3=Neil |date=2025-02-25 |editor-last=Cooper |editor-first=Kimberly L. |title=Synovial joints were present in the common ancestor of jawed fish but lacking in jawless fish |journal=PLOS Biology |language=en |volume=23 |issue=2 |pages=e3002990 |doi=10.1371/journal.pbio.3002990 |doi-access=free |issn=1545-7885 |pmc=11856278 |pmid=39999036}}</ref><ref>{{Cite journal |last=Crump |first=J. Gage |date=2025-02-26 |title=Evolutionary origin of lubricated joints at the dawn of jawed vertebrates |journal=PLOS Biology |language=en |volume=23 |issue=2 |pages=e3003044 |doi=10.1371/journal.pbio.3003044 |doi-access=free |issn=1545-7885}}</ref>

== References ==
<references>
<ref name="Graysp20">{{Cite book|author1=Drake, Richard L.|author2=Vogl, Wayne|author3=Mitchell, Adam W. M.|author4=Gray, Henry|date=2015|title=[[Gray's Anatomy for Students]]|edition= 3rd|chapter=Skeletal system|page=20|publisher=Bukupedia |isbn=978-0-7020-5131-9|oclc=881508489}}</ref> 
<ref name="Tortora12thEd">Tortora & Derrickson () ''Principles of Anatomy & Physiology'' (12th ed.). Wiley & Sons</ref>
<ref name="umich2010couse">{{cite web|last=Umich|date=2010|work=Learning Modules - Medical Gross Anatomy|url= http://www.med.umich.edu/lrc/coursepages/m1/anatomy2010/html/modules/joints_module/joints_01.html|title= Introduction to Joints|publisher= University of Michigan Medical School|url-status=dead|archive-url= https://web.archive.org/web/20111122101527/http://www.med.umich.edu/lrc/coursepages/m1/anatomy2010/html/modules/joints_module/joints_01.html|archive-date=2011-11-22}}</ref>
<ref name="Rogers2010p157">Rogers, Kara (2010) ''Bone and Muscle: Structure, Force, and Motion'' [https://books.google.com/books?id=wDIjRrMdlBgC&pg=PA157 p.157]</ref>
<ref name="Sharkey2008p33">Sharkey, John (2008) ''The Concise Book of Neuromuscular Therapy'' [https://books.google.com/books?id=GiL3PeUMEJsC&pg=PA33 p.33]</ref>
<ref name="Moini2011p231">Moini (2011) ''Introduction to Pathology for the Physical Therapist Assistant'' [https://books.google.com/books?id=RTRv18iTn8YC&pg=PA231 pp.231-2]</ref>
<ref name="Abernethy2005p331">Bruce Abernethy (2005) ''The Biophysical Foundations Of Human Movement'' pp.23, [https://books.google.com/books?id=w-FlXA8p7vQC&pg=PA331 331]</ref>
</references>

===Sources===
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{{Commons category|Synovial joints}}

{{Joints}}
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{{DEFAULTSORT:Synovial Joint}}
[[Category:Joints]]