| Synovial joint | |
|---|---|
 Structure of synovial joint | |
 Types of synovial joints.Clockwise from top-right:ball and socket joint,condyloid joint,plane joint,saddle joint,hinge joint andpivot joint. | |
| Details | |
| Identifiers | |
| Latin | junctura synovialis |
| TA98 | A03.0.00.020 |
| TA2 | 1533 |
| FMA | 7501 |
| Anatomical terminology | |
Asynovial joint, also known asdiarthrosis, joins bones or cartilage with a fibrousjoint capsule that is continuous with theperiosteum 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.[1] The synovial cavity/joint is filled withsynovial fluid. The joint capsule is made up of an outer layer of fibrous membrane, which keeps the bones together structurally, and an inner layer, thesynovial membrane, which seals in the synovial fluid.
They are the most common and most movable type ofjoint in the body. As with most other joints, synovial joints achieve movement at the point of contact of the articulatingbones. They originated 400 million years ago in the first jawed vertebrates.
Synovial joints contain the following structures:
Many, but not all, synovial joints also contain additional structures:[2]
The bone surrounding the joint on the proximal side is sometimes called theplafond (French word for ceiling), especially in thetalocrural joint. Damage to this structure is referred to as aGosselin fracture.
The blood supply of a synovial joint is derived from the arteries sharing in theanastomosis around the joint.
There are seven types of synovial joints.[4] Some are relatively immobile, therefore more stable. Others have multiple degrees of freedom, but at the expense of greater risk of injury.[4] In ascending order of mobility, they are:
| Name | Example | Description |
|---|---|---|
| Plane joints (or gliding joint) | carpals of thewrist,acromioclavicular joint | These joints allow only gliding or sliding movements, are multi-axial such as the articulation between vertebrae. |
| Hinge joints | elbow (between thehumerus and theulna) | These joints act as adoorhinge does, allowing flexion and extension in just one plane, i.e. uniaxial. |
| Pivot joints | atlanto-axial joint,proximal radioulnar joint, anddistal radioulnar joint | One bone rotates about another |
| Condyloid joints (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;[5][6] these joints allow flexion, extension, abduction, and adduction movements (circumduction). |
| Saddle joints | Carpometacarpal or trapeziometacarpal joint ofthumb (between themetacarpal andcarpal -trapezium),sternoclavicular joint | Saddle joints, where the two surfaces are reciprocally concave/convex in shape, which resemble asaddle, permit the same movements as the condyloid joints but allows greater movement. |
| Ball and socket joints "universal Joint" | shoulder (glenohumeral) andhip joints | These allow for all movements except gliding |
| Compound joints[7][8] / bicondyloid joints[2] | knee joint | condylar joint (condyles of femur join with condyles of tibia) and saddle joint (lower end of femur joins with patella) |
Amultiaxial joint (polyaxial joint ortriaxial joint) is a synovial joint that allows for several directions of movement.[9] In the human body, theshoulder andhip joints are multiaxial joints.[10] 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.[11]
The movements possible with synovial joints are:
Thejoint space equals the distance between the involved bones of the joint. Ajoint space narrowing is a sign of either (or both)osteoarthritis and inflammatory degeneration.[12] The normal joint space is at least 2 mm in thehip (at the superioracetabulum),[13] at least 3 mm in theknee,[14] and 4–5 mm in theshoulder joint.[15] For thetemporomandibular joint, a joint space of between 1.5 and 4 mm is regarded as normal.[16] Joint space narrowing is therefore a component of severalradiographic classifications of osteoarthritis.
Inrheumatoid arthritis, the clinical manifestations are primarily synovial inflammation and joint damage. Thefibroblast-like synoviocytes, highly specialized mesenchymal cells found in thesynovial membrane, have an active and prominent role in the pathogenic processes in the rheumatic joints.[17] Therapies that target these cells are emerging as promising therapeutic tools, raising hope for future applications in rheumatoid arthritis.[17]
Synovial joints has been found in earliest jawed vertebrates (gnathostomes) 400 million years ago during theSilurian andDevonian. This finding overturns an earlier view that these joints first evolved in earlytetrapods forterrestrial locomotion. Comparative studies find that synovial joints in all major groups of jawed vertebrates, includingcartilaginous fishes (sharks, skates, and rays),bony fishes, and tetrapods. They are however absent injawless vertebrates such aslampreys andhagfish. Cartilaginous fishes have true synovial joints with clear synovial cavities, articular cartilage lined by flattened chondrocytes, and express key developmental signaling molecules includinggrowth differentiation factor-5 (Gdf5) andβ-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 jawlessosteostracans had pectoral fin connections filled with canals incompatible with fluid-filled joint cavities, while early jawedplacoderms 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.[18][19]
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This article incorporates text from afree content work. Licensed under CC BY 4.0. Text taken fromAnatomy and Physiology, J. Gordon Bettset al, Openstax.
