Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Musculoskeletal system
Skeletal structure
Types of bone joints

Classification of articulations. There are two MAIN TYPES OF Bone Articulations: continuous and discontinuous, or joints (Fig. 1.7, Atl., Fig. 7-17). Continuous articulations are present in all lower vertebrates and during the embryonic Selection/3.html">Stages of development in higher ones. When the bone primordia form in the latter, their precursor material (Connective Tissue, Cartilage) is preserved between them. This material fuses the bones together, forming a continuous articulation. Discontinuous articulations develop at later Stages of Ontogeny in terrestrial vertebrates and are more advanced, as they provide more differentiated mobility of skeletal parts. They develop due to the appearance of a cleft in the precursor material remaining between the bones. In this case, remnants of cartilage cover the articulating surfaces of the bones. There is also a third, intermediate type of articulation—the semi-joint (amphiarthrosis).

Continuous articulations. A continuous articulation—synarthrosis, or fusion—occurs when bones are linked together by connecting tissue. In this case, movement is extremely limited or completely absent. Depending on The Nature of the binding tissue, a distinction is made between fibrous articulations, or syndesmoses (Fig. 1.5, A), cartilaginous articulations, or synchondroses (Fig. 1.5, B), and bony fusions, or synostoses.

Syndesmoses are of three types: 1) interosseous membranes, for example, between the BONES OF THE forearm or lower leg (Atl., Figs. 9, 15); 2) ligaments connecting bones (but not associated with joints), such as ligaments between vertebral processes or their arches (Atl., Fig. 7); 3) sutures between the bones of the Skull (Atl., Fig. 19).

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Fig. 1.5. Types of bone articulations (diagram):

A — syndesmosis; B — synchondrosis; C — joint; 1 — periosteum; 2 — bone; 3 — Fibrous connective tissue; 4 — cartilage; 5 — synovial and 6 — fibrous layers of the Joint Capsule; 7 — articular cartilages; 8 — joint cavity

Interosseous membranes and ligaments allow for some movement of the bones. In sutures, the layer of connective tissue between the bones is extremely thin, making movement impossible.

An example of a synchondrosis is the Articulation of the first rib with the Sternum via costal cartilage, the elasticity of which allows for some mobility of these bones.

Synostoses develop from syndesmoses and synchondroses with age, when the connective tissue or cartilage between the ends of certain bones is replaced by Bone tissue. Examples include the fusion of the sacral vertebrae and fused cranial sutures. Naturally, no movement occurs here.

Discontinuous articulations. A discontinuous articulation—diarthrosis, articulation, or joint (Fig. 1.5, C)—is characterized by a small space (cleft) between the ends of the articulating bones. Joints are classified as simple, formed by only two bones (e.g., the shoulder joint) (Atl., Fig. 8); compound, when the articulation involves a larger number of bones (e.g., the elbow joint) (Atl., Fig. 9); and combined, which allow movement only simultaneously with movement in other anatomically separate joints (e.g., the proximal and distal radioulnar joints). A joint consists of: articular surfaces, a joint capsule, and a joint cavity.

The articular surfaces of the connecting bones correspond more or less to each other (are congruent). On one of the bones forming the joint, the articular surface is usually convex and is called the HEAD. On the other bone, a corresponding concavity develops—a fossa or cavity. Both the head and the cavity can be formed by two or more bones. The articular surfaces are covered with hyaline cartilage, which reduces friction and facilitates movement in the joint.

The joint capsule attaches to the margins of the articular surfaces of the bones, forming a sealed joint cavity. The joint capsule consists of two layers. The superficial fibrous layer, composed of fibrous connective tissue, merges with the periosteum of the articulating bones and serves a protective function. The inner, or synovial, layer is rich in Blood Vessels. It forms projections (villi) that secrete a viscous fluid—synovia (synovial fluid)—which lubricates the articulating surfaces and facilitates their gliding. In normally functioning joints, there is very little synovial fluid; for example, in the largest of them—the knee joint—there is no more than 3.5 cm³. In some joints (such as the knee), the synovial membrane forms folds where fat is deposited, serving a protective function. In other joints, such as the shoulder joint, the synovial membrane forms external protrusions over which the fibrous layer is almost absent. These protrusions, in the form of synovial bursae, are located in the areas of tendon attachment and reduce friction during movement.

The joint cavity is a hermetically sealed, slit-like space bounded by the articulating surfaces of the bones and the joint capsule. It is filled with synovial fluid. There is negative pressure (below atmospheric pressure) within the joint cavity between the articular surfaces. The atmospheric pressure exerted on the capsule helps stabilize the joint. Therefore, in certain diseases, the sensitivity of the joints to fluctuations in atmospheric pressure increases, and such patients can "predict" weather changes. The tight compression of the articular surfaces against each other in A number of joints is due to Muscle tone, or active muscular tension.

In addition to the essential components, accessory structures may be found in a joint. These include articular ligaments and labra, intra-articular discs, menisci, and sesamoid (from the Arabic sesamo — grain) bones.

Articular ligaments are bundles of dense fibrous tissue. They are located within or on top of the joint capsule, representing localized thickenings of its fibrous layer. Spanning across the joint and attaching to the bones, the ligaments strengthen the articulation. However, their primary role is to limit the range of motion, preventing it from exceeding certain limits. Most ligaments are non-elastic but highly durable. Some joints, such as the knee joint, contain intra-articular ligaments.

Articular labra consist of fibrocartilage that forms a ring around the margins of the articular cavities, supplementing and increasing their surface area. Articular labra provide the joint with greater stability but reduce the range of motion (for example, in the shoulder joint).

Discs and menisci are cartilaginous pads—either solid or perforated. They are located inside the joint between the articular surfaces, and fuse with the joint capsule along their margins. The surfaces of the discs and menisci conform to the shape of the articular surfaces of the bones adjacent to them on both sides. Discs and menisci contribute to the variety of movements in the joint. They are found in the knee and temporomandibular joints.

Sesamoid bones are small and located near certain joints. Some of these bones are embedded within the joint capsule and, by increasing the surface area of the articular fossa, articulate with the articular head (for example, in the joint of the big toe); others are embedded in the tendons of Muscles that span across the joint (for example, the Patella, which is enclosed in the tendon of the quadriceps femoris muscle). Sesamoid bones are also classified as Accessory structures of muscles.

In athletes, joint mobility increases under METABOLISM/18.html">The Influence of training. In children, most joints are generally more mobile than in adults or elderly people.

The Classification of Joints is based on comparing the shape of the articular surfaces with segments of various geometric solids of revolution resulting from the movement of

a straight or curved line (the so-called generatrix) around a fixed imaginary axis. Different forms of movement of the generating line produce different solids of revolution. For example, a straight generatrix rotating parallel to the axis describes a cylindrical shape, while a semicircular generatrix produces a sphere (Fig. 1.6). Articular surfaces of a specific geometric shape allow movements only along the axes characteristic of that shape. Consequently, joints are classified as uniaxial, biaxial, and triaxial (or practically multiaxial).

Uniaxial joints can be cylindrical or hinge joints.

A cylindrical joint has articular surfaces in the shape of cylinders, with the convex surface enclosed by a concave cavity (Fig. 1.6, A). The axis of rotation is vertical, parallel to the long axis of the articulating bones. It provides movement along a single vertical axis. In a cylindrical joint, rotation inward and outward along the axis is possible. Examples include the Articulations between the radius and ulna, and the joint between the dens of the axis and the atlas.

Fig. 1.6. Shape of joints:

A — pivot (proximal radioulnar); B — hinge (interphalangeal); C — saddle (carpometacarpal of the thumb); D — ellipsoid (radiocarpal); E — ball-and-socket (shoulder); F — plane (between the articular processes of the vertebrae)

A hinge joint is a variation of the pivot joint, differing in that its axis of rotation runs perpendicular to the long axis of the rotating bone and is referred to as the transverse or frontal axis. Flexion and extension are the movements permitted in this joint. Interphalangeal joints are prime examples (Fig. 1.6, B).

Biaxial joints (Fig. 1.6, C, D) can be saddle-shaped (where the articular surface is concave in one direction and convex in the other, perpendicular direction) or ellipsoidal (having ellipsoidal articular surfaces). An ellipsoid, as a solid of revolution, has only one axis. Movement around the second axis in an ellipsoid joint is possible due to the incomplete fit of the articular surfaces. Biaxial joints permit movement around two mutually perpendicular axes lying in the same plane: flexion and extension around the frontal axis, and adduction (toward the median plane) and abduction around the sagittal axis. The radiocarpal joint is an example of an ellipsoid joint, while the carpometacarpal joint of the thumb is an example of a saddle joint.

Triaxial (multiaxial) joints include ball-and-socket and plane joints.

Ball-and-socket joints are the most mobile articulations (Fig. 1.6, E). Movements in these joints occur around three primary mutually perpendicular axes that intersect at the center of the joint head: the frontal axis (flexion and extension), the vertical axis (internal and external rotation), and the sagittal axis (adduction and abduction). However, an infinite number of axes can pass through the center of the articular head, making the joint functionally multiaxial. The shoulder joint is a classic example.

A variation of the ball-and-socket joint is the cotyloid joint (enarthrosis), in which a significant portion of the spherical articular head is enclosed by the deep socket, thereby restricting the range of motion. The hip joint is a prime example. While movement can occur in any plane, the range of motion is limited.

A plane joint represents a segment of a sphere with an extremely large radius, meaning the curvature of the articulating surfaces is minimal, and no distinct head or socket can be identified. This joint has limited mobility, permitting only slight gliding of the articulating surfaces in various directions. An example is the joint between the articular processes of the thoracic vertebrae (Fig. 1.6, F).

In addition to the described movements, biaxial and triaxial joints also allow a movement known as circumduction. During this movement, the end of the bone opposite the joint describes a circle, while the bone as a whole traces the shape of a cone.

A symphysis (hemiarthrosis) is characterized by bones joined by a cartilaginous disc that contains a slit-like cavity. A joint capsule is absent. Thus, this type of articulation represents a transitional form between a synchondrosis and a diarthrosis (such as The connection between the pubic bones of the pelvis).