Joints

Joints, or synovial connections (articulationes synoviales), are discontinuous connections of bones. Joints are characterized by the presence of cartilaginous articular surfaces, a joint capsule, a joint cavity and synovial fluid in it. Some joints additionally have formations in the form of articular discs, menisci or a glenoid labrum.

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

Articular cartilage (cartilago articularis) is usually hyaline. Only the temporomandibular and sternoclavicular joints have fibrous cartilage. The thickness of articular cartilage varies from 0.2 to 6 mm. Under mechanical load, articular cartilage flattens and 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 grows together with the periosteum, forming a closed joint cavity within which a pressure below atmospheric pressure is maintained. The capsule has two layers: a fibrous membrane on the outside and a synovial membrane on the inside. The fibrous membrane (membrana fibrosa) is strong and thick, formed by 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. Extracapsular ligaments are located outside the joint capsule. Some joints haveintracapsular ligaments in the joint cavity. Being inside the joint, intracapsular (intra-articular) ligaments are covered by a synovial membrane (for example, the cruciate ligaments of the knee joint). The synovial membrane (membrana synovialis) is thin, lines 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 fatty tissue (for example, at the knee joint).

The articular cavity (cavum articulare) is a closed slit-like space, limited by the articular surfaces and the capsule. The articular cavity contains synovial fluid (synovia), which, being mucus-like, moistens the articular surfaces and facilitates their sliding relative to each other. Synovial fluid participates in the nutrition of articular cartilage.

Articular discs and menisci (disci et menisci articulares) are intra-articular cartilaginous plates of various shapes that eliminate or reduce discrepancies (incongruence) of articular surfaces. Discs and menisci completely or partially divide the joint cavity into two floors. A disc in the form of a solid cartilaginous plate is found in the sternoclavicular, temporomandibular and some other joints. Menisci are typical for the knee joint. Discs and menisci are able to shift during movements, cushion shocks and concussions.

The glenoid labrum (labrum articulare) is present at the shoulder and hip joints. It is attached along the edge of the articular surface, increasing the depth of the glenoid fossa.

Classification of joints

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

According to biomechanical classificationJoints are divided depending on the number of axes of rotation. There are uniaxial, biaxial and multiaxial joints. Uniaxial joints have one axis of rotation around which flexion (flexio) and extension (extensio) or abduction (abductio) and adduction (adductio) occur. rotation outward (supination - supinatio), and inward (pronation - pronatio).

Uniaxial joints, based on the shape of the articular surfaces, include the humeroradial joint (block-shaped, ginglimus), proximal and distal radioulnar joints (cylindrical, art. cylindrica).

Biaxial joints have two axes of rotation, and therefore, for example, flexion and extension, abduction and adduction are possible in them. Such joints include the radiocarpal (ellipsoid, art. ellipsoidea), carpometacarpal joint of the first finger of the hand (saddle, art. sellaris), and also the atlanto-occipital (condylar, art. bicondylaris).

Triaxial (multiaxial) joints (shoulder, hip) have a spherical shape of articular surfaces (art. spheroidea). Various movements are performed in these joints: flexion - extension, abduction - adduction, 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 slight sliding of the articular surfaces relative to each other is possible. A variety of triaxial joints is a cup-shaped joint (art. cotylica), for example, the hip joint.

By the shape of the articular surfaces, joints resemble the surfaces of different geometric bodies (cylinder, ellipse, sphere). Therefore, cylindrical, spherical and other joints are distinguished. The shape of the articular surfaces is related to the number of axes of rotation carried out in this joint.

Biomechanics of joints

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

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

[ 1 ]