Joint

JOINT (lat. articulatio), a special case of the connection of bones or cartilage with each other, having a certain degree of mobility. According to the degree of mobility, one can distinguish more or less movable joints (diarthrosis) and immovable joints (synarthrosis). Different forms of connection of bones or cartilage can be considered as different stages of the phylogenetic process of their formation, which occur in the process of ontogenetic development under the influence of hereditary genetic causes on the one hand, and individual social-labor and biological factors on the other. The material for the formation of parts included in the connections is mesenchyme and in exceptional cases entoderm (notochord). Thus, individual types of connections of bones and cartilage are connected with each other by transitional forms. The connection of both with each other in the simplest case occurs with the help of fibrous connective tissue - syndesmosis (syndesmosis). Such syndesmoses are observed in a number of cartilages of the larynx: between the thyroid cartilage and the hyoid bone (membrana hyothyreoidea), thyroid and cricoid (membrana thyreo-cricoidea), cricoid and trachea (membrana crico-tra-chealis), thyroid cartilage and epiglottis (lig. thyreo-epiglotticum), hyoid bone and epiglottis (lig. hyo-epiglotti-cum), arytenoid cartilage and corniculate (lig. ary-corniculatum). Numerous syndesmoses are also observed between bones. Here one can mention the syndesmosis between the sternum and the second and subsequent ribs (lig. sterno-costalia interarticularia), between the proximal ends of both clavicles (lig. interclaviculare), connections between the arches of the vertebrae (lig. interarcualia, s. intercruralia, s. flava), proper ligaments of the scapula (lig. trans-versum scapulae superius et inferius), connections between the bones of the wrist and tarsus. All these syndesmoses are characterized either by complete immobility or relatively small mobility. Among syndesmoses, the connection of the dens of the second cervical vertebra with the occipital bone (ligamentum apicis dentis, s. ligamentum suspensorium dentis) stands apart; this connection is a remnant of the notochord. Much more often, connections between bones occur with the help of cartilage - these are synchondroses (synchondrosis). These include connections between almost all primordial bones of the skull. For a long time they retain remnants of that cartilage in the mass of which these bones developed from their centers of ossification. In most cases, these synchondroses ossify over time, but some remain for life. Such are also the synchondroses between the individual bones forming the innominate bone and the synchondroses between the epiphyses and diaphyses of long bones, which also ossify over time, i.e. turn into synostosis (synostosis).- A special form is the connection of the bodies of the vertebrae with each other. The sacral vertebrae represent true synchondroses, which then turn into synostoses. The lumbar and thoracic vertebrae are connected to each other by means of fibrocartilage, which forms the intervertebral discs, and a gelatinous nucleus - a remnant of the notochord (fibrocartilago intervertebralis and nucleus pulposus). The cervical vertebrae represent true joints. The transition between synarthroses and diarthroses is served by half-joints, or flat joints (amphiarthrosis, s. hemiartrosis, s. symphysis), in which bones are connected by their flat articular surfaces by means of fibrous connective tissue or fibrocartilage, but in which rudiments of a fissure or joint cavity with a joint capsule (cavum articulare, s. cavitas glenoidalis and capsula articularis) may be present in each given case to varying degrees of development. Such are the pubic joint (symphysis ossium pubis, s. synchondrosis pubis, s. articulatio pubis), sacro-coccygeal joint (synchondrosis sacro-coccygea) and sacro-iliac joint (symphysis, s: articulatio sacro-iliaca), joints of the heads of the ribs and transverse processes (articulationes costo-transversariae), acromioclavicular joint (art. acromio-clavicularis), sternoclavicular joint (art. sterno-clavicularis); upper and lower joints of the tibia (art. tibio-fibularis superior et inferior) and some others. In flat joints, the degree of mobility can be very different both during the life of a given individual and in different individuals. True joints, i.e. movable joints, can be both between cartilages and between bones. In the human body, true joints between cartilages are quite rare. These include joints between the arytenoid and cricoid cartilages (art. crico-arytaenoidea) and between the thyroid and cricoid cartilages (art. crico-thyreoidea lateralis). A true joint between bones can be simple (art. simplex) and complex (art. composita), when there are more or less separate joints between several bones forming one common joint. In joints of the type of true joints, the forms of the contacting bones are considered, - joint cavities, joint capsules, menisci, synovial villi, synovial folds, fat folds, synovial bursae, synovial fluid, free bodies, 'joint mice'. The parts of the bones entering the joint are lined with hyaline cartilage. On its free surface, this cartilage is covered with perichondrium, the surface cells of which become flat and resemble epithelial tissue. In some joints, the cartilage is fibrous (sternoclavicular and temporomandibular joints). According to the shape of their surface, the articulating bones can coincide or more precisely represent the reverse image of each other - congruent surfaces or not coincide, more precisely - not represent the correct reverse image of each other - incongruent surfaces. Incongruity is leveled by cartilagous layers - articular cartilages, or menisci (discus, s. meniscus articularis). The articular capsule, or capsule (capsula articularis), consists of two layers: external and internal. The outer layer (stratum fibrosum capsulae articularis) consists of coarser parallel connective tissue fibers with individual elastic fibers. In this layer, individual bundles of denser tendon fibers can be distinguished in the form of auxiliary ligaments (lig. accessoria). The inner layer of the capsule (stratum synoviale capsulae articularis, s. intima) represents loose connective tissue masses with fibrocytes, lined inside with flat epithelial-like cells, hence the name endothelium of joints. The deep layers contain fat cells, are rich in blood and lymph vessels and sensory nerves. This synovial layer forms synovial villi (villi synoviales), synovial folds (plicae synoviales), as well as fat folds (plicae adiposae). Synovial folds and villi, increasing in size, can sometimes partially detach and form free bodies - 'joint mice' (corpora libera), which can be fibrous and even cartilaginous. Inside the joint, there can also be connective tissue bundles, which are actually formed outside and then move into the joint; they connect the articulating bones, as well as the bones and the articular discs between them. At the edges of some depressions there are cartilaginous lips (labrum glenoidale). Both the articular lips and the articular menisci consist of fibrous connective tissue or fibrocartilage, covered on the free surface with flat cells. Articular discs can divide the joint cavity into two more or less independent cavities, resulting in two-chambered joints. Articular lips represent cartilaginous masses that are located along the edge of the articular cavity, thereby increasing the articular surface and thus the grip of the articular head, which entails a decrease in the volume of movement in this joint. Synovial bursae (bursae synoviales) are thin-walled sacs with a small cavity, which normally contains a small amount of synovial fluid. Synovial bursae are located between bones on one side and muscles, tendons and skin on the other, as well as between tendons and skin. They serve as padding to reduce friction, to transmit pressure to a larger surface, and to protect soft structures from protruding and sharp parts of bone. Synovial bursae may communicate with the joint cavity, but may also be separated from it. All synovial membranes secrete synovial fluid or synovia (liquor synovialis, s. synovia) into the joint cavity. It is a yellowish fluid of alkaline reaction, protein and mucous in nature, with wandering cells and products of decay of the surrounding tissue. According to the nature of possible movement in joints, one-axis, two-axis and three-axis (multi-axis) joints are distinguished, which differ from each other accordingly and by the degrees of freedom of movement.

Finally, based on the shape of the articulating bone parts, and consequently on the nature of the movements they allow and the degree of such freedom, the following forms of joints are distinguished: ball-and-socket, pear-shaped, egg-shaped or elliptical, cylindrical, block-shaped or hinge-like, screw-shaped, and saddle-shaped. Ball-and-socket Joint (arthrosis, s. articulatio spherica, s. spheroidea, s. articulus sphericus) with a concave spherical surface of the articular fossa and a convex spherical surface of the head allows rotational movement, theoretically occurring around many axes, but practically having three axes and three forms of movement: around the sagittal axis - adduction and abduction (adductio and abductio), around the frontal axis - flexion and extension (flexio and extensio) and around the vertical (longitudinal) axis - rotation (rotatio). The range of movement in this joint depends on the difference in size of the articulating surfaces (directly proportional to this difference) and on the nature of the soft and hard parts surrounding the joint. Pear-shaped Joint (enarthrosis) represents a special case of a ball-and-socket joint, when the articular cavity covers more than half of the articular head. The range of this theoretically multi-axial joint is in practice very limited. Egg-shaped, or elliptical Joint (condylarthrosis, s. articulatio elliptica, s. articulus ellipticus) is characterized by the elliptical shape of the articulating bone parts, two axes (major and minor) and two movements of the nature of flexion and extension around the major and minor axes of the joint and two degrees of freedom. Cylindrical Joint (art. trochoidea, s. trocharthrosis, s. trochus) represents a connection of two mutually fitting cylindrical surfaces, allowing only one movement - rotation of the cylinders around the longitudinal axis of the cylinders and partly sliding with a small range of movement and with one degree of freedom. Block-shaped, or hinge Joint (ginglymus, door hinge) represents a special case of a cylindrical joint, in which the head has the shape of a block, and the articular cavity - the block's housing; it has one longitudinal axis of the block with the possibility of only one movement and with one degree of freedom. It may have a guiding ridge and a guiding groove. Screw-shaped Joint (ginglymarthrosis cochlearis) also represents a special form of cylindrical joint, in which the guiding ridge and groove have a spiral course. Saddle-shaped Joint (art. sellaris) represents a connection of two oppositely arranged saddle-shaped surfaces; each of them represents a combination of two surfaces, of which one is convex in one direction, and the other is concave in a direction perpendicular to the previous one. Movement here is possible in two mutually perpendicular directions around two axes. A more complex joint is the ginglymoarthrosis, part of the surface of which is a cylinder, and the other - a ball. It should also be added that the limbs of the body, due to the connection of several joints, actually have in total a greater number of degrees of freedom of movement than the individual forms of articulation allow.