Joints

Joints, or synovial joints (articulationes synoviales), are discontinuous connections of bones. For joints is characterized by the presence of cartilage-covered articular surfaces, articular capsule, articular cavity and in it synovial fluid. Some joints additionally have formations in the form of articular disks, menisci or joint lip.

Articular surfaces (facies articulares) can correspond to each other in configuration (be congruent) or vary in shape and size (incongruent).

Articular cartilage (cartilago articularis), as a rule, is hyaline. Only the temporomandibular and sternocleid joints have cartilage fibrous. The thickness of the articular cartilage varies from 0.2 to 6 mm. Under the influence of mechanical load, the articular cartilage flattenes, springs due to its elasticity.

The joint capsule (capsula articularis) is attached to the edges of the articular cartilage or at some distance from it. It firmly fuses with the periosteum, forming a closed articular cavity within which the pressure is maintained below atmospheric pressure. The capsule is separated into two layers: the fibrous membrane from the outside and the synovial membrane from the inside. The fibrotic membrane (membrana fibrosa) is firm and thick, formed by a fibrous connective tissue. In some places it thickens, forming ligaments that strengthen the capsule. These ligaments are called capsular if they are located in the thickness of the fibrous membrane. Extra-capsular ligaments are located outside the articular capsule. Some joints in the joint cavity have intracapsular ligaments. Being inside the joint, the intracapsular (intraarticular) ligaments are covered by the synovial membrane (for example, the cruciate ligaments of the knee joint). The synovial membrane (membrana synovialis) is thin, lining the fibrous membrane from the inside, and also forms micro-growths - synovial villi, which significantly increase the area of the synovial membrane. The synovial membrane often forms synovial folds, which are based on accumulations of adipose tissue (for example, in the knee joint).

The articular cavity (cavum articulare) is a closed, slit-shaped space, bounded by articular surfaces and capsule. In the articular cavity there is a synovial fluid (synovia), which, being slimy, moistens the joint surfaces and facilitates their sliding relative to each other. The synovial fluid is involved in feeding the articular cartilage.

Joint discs and meniscuses (disci et menisci articulares) are intraarticular cartilaginous plates of various shapes, eliminating or reducing inconsistencies (incongruence) of articular surfaces. Discs and menisci completely or partially divide the joint cavity into two floors. A disk in the form of a solid cartilaginous plate is found in the sternoclavicular, temporomandibular and some other joints. Menisci are characteristic of the knee joint. Disks and menisci are able to shift during movement, absorb shock and tremors.

The labrum articulare is present in the shoulder and hip joints. It is attached along the edge of the articular surface, increasing the depth of the joint fossa.

Classification of joints

There are anatomical and biomechanical classifications. According to the anatomical classification, joints depending on the number of jointed bones are divided into simple and complex, as well as complex and combined. A simple joint (art. Simplex) is formed by two interlocking surfaces (humerus, hip, etc.). Complex joints (art composita) are formed by three or more articular surfaces of bones (radiocarpal, etc.). Complex joint (art complexa) has an intraarticular disc or meniscus (sternoclavicular, temporomandibular, knee joints). Combined joints (temporomandibular, etc.) are anatomically isolated, but they function together.

By biomechanical classification of joints are divided depending on the number of axes of rotation. Isolate uniaxial, biaxial and multiaxial joints. Uniaxial joints have one axis of rotation, around which flexion (flexion) and extension (extensio) or abduction (abduction) and reduction (adductio) occur. Rotation to the outside (supinatio - supinatio), and inside (pronation - pronatio).

Uniaxial joints in the form of articular surfaces include the humerus joint (block-shaped, ginglimus), proximal and distal radioclavicular joints (cylindrical, art cylindrica).

Biaxial joints have two axes of rotation, and therefore, for example, flexion and extension, withdrawal and reduction are possible. These joints include wrist (ellipsoid, art. Ellipsoidea), carpometacarpal joint of the first finger of the hand (saddle, art. Sellaris), as well as atlanto-occipital (condylar, art. Bicondylaris).

The triaxial (multi-axial) joints (humeral, hip) have a spherical shape of articular surfaces (art spheroidea). In these joints are various movements: flexion - extension, withdrawal - reduction, supination - pronation (rotation). Multiaxial joints also include flat joints (artt. Planae), the articular surfaces of which are, as it were, part of the surface of a large-diameter ball. In flat joints, only a slight sliding of the joint surfaces is possible with respect to each other. A variety of triaxial joints is a cup-shaped joint (art. Cotylica), for example, the hip joint.

In the form of articular surfaces, the joints resemble surfaces of different geometric bodies (cylinder, ellipse, ball). Therefore, the cylindrical, spherical and other joints are distinguished. The shape of the articular surfaces is in connection with the number of axes of rotation performed in this joint.

Biomechanics of joints

The range of movements in the joints is determined primarily by the shape and size of the joint surfaces, as well as by their correspondence to each other (congruence). The amount of mobility in the joints also depends on the tension of the joint capsule and ligaments that strengthen the joint, from individual, age and sexual characteristics.

The anatomical mobility of the joints is determined by the difference in the angular sizes of the surfaces of the connecting bones. So, if the size of the joint cavity is 140 °, and the joint head is 210 °, then the range of possible motion is 70 °. The greater the difference in the curvature of articular surfaces, the greater the range of motion in such a joint.

[1], [2], [3], [4], [5]