Osteoarthrosis: how synovial joints are arranged?

Osteoarthritis is a disease of synovial joints (diarthrosis). The main functions of diarthrosis are motor (movement of the joints of the joints along certain axes) and support (load during standing, walking, jumping). The synovial joint consists of articulating bone surfaces covered with cartilage, an articular cavity containing synovial fluid, and an articular capsule. Unreliable anatomical elements of diarthrosis are ligaments located on the outside or, less often, inside the articulation, and cartilaginous menisci.

According to the form of the articulating bone surfaces, the diarrtroses are divided into the following types:

1. flat joints (for example, some wrists and tarsal joints);
2. Spherular joints in which one articular end has the shape of a ball or a part of a sphere, and the other is a concave surface congruent with a spherical articulating end; an example of a spherical joint is the shoulder joint, in which greater freedom of movement of all kinds is possible - flexion, extension, retraction and reduction, circular motions;
3. ellipsoid joints in which one of the articulating ends has the form of an ellipse and the other is a congruent hollow, as a result of this anatomical structure the volume of movements in these joints is limited in comparison with spherical ones and in them are impossible, for example, circular motions; distinguish simple ellipsoidal joints and complex joints with several pairs (for example, wrist bands);
4. block joints in which one articular end is in shape a block resembling a coil, a bobbin, and the other concave articular end covers the part of the block and corresponds to it in shape; a typical block-shaped joint is the interphalangeal joint of the hand and foot; movements in such junctions can occur only in one plane - flexion and extension; To the block joints also belongs the elbow joint - it consists of three joints - the brachiolar, the brachial and the proximal radiolucent, as a result of which, in addition to flexion and extension, supination and pronation are possible in this complex articulation, i.e. Rotational movements;
5. rotational (wheel-like) joints, an example of which is the median atlanto-joint joint consisting of a ring formed by the front arch of the atlas and a transverse ligament and a dentate process of the 2nd cervical vertebra that enters the ring and serves as a kind of axis around which the atlant ring rotates; in the elbow joint, the radiolucent articulation should also be attributed to the rotational type of the joints, since the radial head rotates in the annular sheath surrounding the head of the ray and attached to the ulnar cut;
6. Saddle joints, an example of such joints is the carpometacarpal joint of the thumb of the hand; the articulated surface in the form of a saddle has a trapezoid bone, and the shape of the concave saddle is the first metacarpal bone; Such anatomical structure allows for circular movements in the sagittal and frontal planes, circular axial movements in this joint are impossible;
7. condylar joints, the anatomical feature of which are the pair condyles - convex and concave, in which friendly movements are possible; an example of a condylar joint can be a knee consisting of three components forming a single biomechanical system - patellofemoral and internal and external tibiofemoral articulations; the not completely perfect congruence of the condyles of the tibia is replaced by the outer and inner menisci; powerful lateral ligaments prevent lateral and swinging movements of the shin around the thigh, and also protect the shin from subluxation forward and backward during joint movements; In this condylar joint, flexion and extension are possible, external and internal rotation in the bent position of the joint; with flexion-extensor movements, the femoral condyles rotate with respect to the condyles of the tibia and their simultaneous slip due to the displacement of the rotation axes; Thus, the knee joint is multi-axis or polycentric, during full extension, the lateral ligaments and tendons woven into the joint capsule are maximally stressed, which creates the conditions for the greatest stability and joint resilience in this position.

The joint is surrounded by a fibrous capsule attached to the bone near the periphery of the articular cartilage and passing into the periosteum. The synovial joint capsule consists of two layers - external fibrous and internal - synovial. The fibrous layer consists of a dense fibrous tissue, in some places the fibrous layer of the capsule is thinned with the formation of twists or burs, in other places it is thickened, performing the function of the joint ligament. The thickness of the fibrous layer of the capsule is determined by the functional load on the joint.

Thickening capsules form bundles consisting of dense parallel bundles of collagen fibers that serve to stabilize and strengthen the joint and restrict certain movements. Among the features of the capsule, in addition to performing the function of support for the synovial membrane and the connection with the ligaments, it should be noted a large number of nerve endings in it, in contrast to synovia, which has a small number of such endings, and articular cartilage that does not contain them at all. It is believed that together with the nerves of the muscles, the nerves of the capsule participate in the control of the position, and also respond to painful effects.

The synovial membrane is the smallest in mass and volume, but the most important part of the synovial joint, since most of the rheumatic diseases proceed with the inflammation of the synovial membrane, which is commonly referred to as synovitis. The synovial membrane lining all intra-articular structures except articular cartilage, its thickness is 25-35 microns. Histologically, it is a layer of connective tissue, consisting of the integument, collagen and elastic layers. The synovial membrane normally has a known number of folds and fingerlike villi and forms a thin synovial layer (sometimes called a cover layer); it consists of a layer of integumentary cells that forms the lining of non-articular surfaces of the joint, and a subsynoviral supporting layer consisting of a fibrous-fatty connective tissue of various thicknesses that connects to the capsule. The synovial layer often merges with the subnavial tissue through a smooth transition from the avascular inner cover containing a multitude of cells to a vascularized subconventional connective tissue with fewer cells, which becomes more and more saturated with collagen fibers as it approaches the connection to the fibrous capsule. From the blood vessels of the subunovial connective tissue, the cells and nutrients emerge into the synovial fluid due to the absence of a morphological separation of the synovial and subsidonovial layers (absence of the basal membrane, the presence of gaps between the cover cells).

The synovial membrane is normally lined with 1-3 layers of synovitocytes - synovial cells located in the matrix (the main substance), rich in microfibrils and aggregates of proteoglycans. Synovitocytes are divided into two groups - type A (macrophage-like) and type B (fibroblast astodus). Synovitis of type A has an uneven cell surface with a large number of outgrowths, they have a well developed Golgi complex, many vacuoles and vesicles, but the ribosomal endoplasmic reticulum is poorly expressed. Macrophage synovitis may also contain a large amount of phagocytosed material. In Type B synovvites, the surface is relatively smooth, the ribosomal endoplasmic reticulum is well developed, they contain only a small number of vacuoles. Classical division of synovvits into A cells carrying out a phagocytizing function and B cells, the main function of which is the production of synovial fluid components, primarily hyaluronic acid, does not reflect all the functions of the synovitis. Thus, synovitis of type C, which by their ultrastructural characteristics occupy an intermediate position between cells of type A and B, has been described. In addition, it has been established that macrophage-like cells are capable of synthesizing hyaluronic acid, and fibroblast-like cells have the capacity for active phagocytosis.

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