Diagram of a typical synovial (diarthrosis) joint
A joint or articulation (or articular surface) is the location at which bones connect. They are constructed to allow movement (except for skull, sacral, sternal, and pelvic bones) and provide mechanical support, and are classified structurally and functionally.
- Structural classification (binding tissue) 1.1
- Functional classification (movement) 1.2
- Biomechanical classification 1.3
- Anatomical 1.4
- Joint disorders 2
- Joint 3.1.1
- Articulation 3.1.2
- Etymology 3.1
- See also 4
- References 5
- External links 6
Joints are mainly classified structurally and functionally. Structural classification is determined by how the bones connect to each other, while functional classification is determined by the degree of movement between the articulating bones. In practice, there is significant overlap between the two types of classifications.
Terms ending in the suffix -sis are singular and refer to just one joint, while -ses is the suffix for pluralization.
An articulate facet is generally seen as a small joint, especially used when speaking of the joints of the ribs.
Structural classification (binding tissue)
Structural classification names and divides joints according to the type of binding tissue that connects the bones to each other. There are three structural classifications of joints:
- fibrous joint – joined by dense regular connective tissue that is rich in collagen fibers 
- cartilaginous joint – joined by cartilage
- synovial joint – not directly joined – the bones have a synovial cavity and are united by the dense irregular connective tissue that forms the articular capsule that is normally associated with accessory ligaments.
Functional classification (movement)
Joints can also be classified functionally according to the type and degree of movement they allow:
- synarthrosis – permits little or no mobility. Most synarthrosis joints are fibrous joints (e.g., skull sutures).
- amphiarthrosis – permits slight mobility. Most amphiarthrosis joints are cartilaginous joints (e.g., intervertebral discs).
- diarthrosis – freely movable. All diarthrosis joints are synovial joints (e.g., shoulder, hip, elbow, knee, etc.), and the terms "diarthrosis" and "synovial joint" are considered equivalent by Terminologia Anatomica. Diarthroses can in turn be classified into six groups according to the type of movement they allow: arthrodia, enarthrosis, ginglymus, rotary diarthrosis, condyloid articulation and articulation by reciprocal reception.
Joints can also be classified, according to the number of axes of movement they allow, into nonaxial (gliding, as between the proximal ends of the ulna and radius), monoaxial (uniaxial), biaxial and multiaxial. Another classification is according to the degrees of freedom allowed, and distinguished between joints with one, two or three degrees of freedom. A further classification is according to the number and shapes of the articular surfaces: flat, concave and convex surfaces. Types of articular surfaces include trochlear surfaces.
Joints can also be classified based on their anatomy or on their biomechanical properties. According to the anatomic classification, joints are subdivided into simple and compound, depending on the number of bones involved, and into complex and combination joints:
- Simple joint: two articulation surfaces (e.g. shoulder joint, hip joint)
- Compound joint: three or more articulation surfaces (e.g. radiocarpal joint)
- Complex joint: two or more articulation surfaces and an articular disc or meniscus (e.g. knee joint)
The joints may be classified anatomically into the following groups:
- Articulations of hand
- Elbow joints
- Wrist joints
- Axillary articulations
- Sternoclavicular joints
- Vertebral articulations
- Temporomandibular joints
- Sacroiliac joints
- Hip joints
- Knee joints
- Articulations of foot
A joint disorder is termed an arthropathy, and when involving inflammation of one or more joints the disorder is called an arthritis. Most joint disorders involve arthritis, but joint damage by external physical trauma is typically not termed arthritis.
Arthropathies are called polyarticular (multiarticular) when involving many joints and monoarticular when involving only a single joint.
Arthritis is the leading cause of disability in people over the age of 55. There are many different forms of arthritis, each of which has a different cause. The most common form of arthritis, osteoarthritis (also known as degenerative joint disease), occurs following trauma to the joint, following an infection of the joint or simply as a result of aging. Furthermore, there is emerging evidence that abnormal anatomy may contribute to early development of osteoarthritis. Other forms of arthritis are rheumatoid arthritis and psoriatic arthritis, which are autoimmune diseases in which the body is attacking itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint that results in subsequent inflammation. Additionally, there is a less common form of gout that is caused by the formation of rhomboidal-shaped crystals of calcium pyrophosphate. This form of gout is known as pseudogout.
The English word joint is a past participle of the verb join, and can be read as joined. Joint is derived from Latin iunctus, past participle of the Latin verb iungere, join together, unite, connect, attach. In classical Latin, a similar name, derived from the verb iungere, exists that clearly is related to the meaning of the (anatomical) joint in the English language. Iunctura as joint is attested in the writings of Celsus and Ovidius. Celsus used besides the term iunctura also articulus and rarely nodus to refer to joints. The term iunctura was in classical Latin not a very precise designation as it could refer to all kinds of connections between bones as the sutures of the skull were also referred to as the iuncturae serratae. Ovidius refers to the knee joint as the iunctura genuum  and to the cervical joints as the iuncturae verticis. Besides the ascribed anatomical meaning, iunctura could also refer to a relationship or to consanguinity. In English, jointure and juncture  are both derived from classical Latin iunctura. In the Romance languages, French jointure, Italian giuntura  and Spanish juntura are used to refer to a joint with the first similarly derived from Latin iunctura.
In the anatomic Latin of the Nomina Anatomica, renamed in 1998 as Terminologia Anatomica, numerous expressions can be found that utilize the noun junctura, including junctura ossium  to refer to connection between bones (in general), and translated as joining of bones  in the English translation of the Basle Nomina Anatomica, junctura fibrosa  to refer to a fibrous joint, junctura cartilaginea  for a cartilaginous joint, juncturae ossium thoracicae  for the joints of the arms, and the juncturae ossium pelvinae  for the joints of the legs.
The English term articulation is derived from Latin articulatio. The usage of this term, akin to its equivalents in Romance languages, like French articulation, Spanish articulación, Italian articolazione, and Portuguese articulação, that are similarly derived from the same Latin word, can be considered as a barbarism, in the sense that an incorrect meaning is ascribed to this word. The term articulatio can be found in the writings of Pliny the Elder as "the putting forth of new joints or knots"  or could refer to a "disease of the vine at the joints óf the tendrils". Articulatio as articulation or joint is not attested in classical Latin. As mentioned earlier, in Roman antiquity, various terms like iunctura, nodus and articulus were used to refer to a joint or articulation.
The first edition of the official Latin nomenclature, Basle Nomina Anatomica, used in 1895 the incorrect form articulatio. In the subsequent edition, authorized in 1935 in Jena, hence the name Jena Nomina Anatomica, the editor clearly objected to the use of the incorrect form articulatio and thought it was self-evident that this barbarism had to be replaced by the classical Latin articulus. The edition of the Nomina Anatomica approved by the nomenclature committee in 1955 in Paris however reverted this correction to the incorrect form articulatio. In each subsequent edition, the incorrect form articulatio was favored.
Although not common, English article can also be found to refer to a joint. In the Romance languages, similar words with the same meaning, like French article, also derived from Latin articulus, Spanish articulo  and Italian articolo, can also be found.
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- Synovial joints Illustrations and Classification