Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

Special Section
Arthrology, artrologia [the study of bone joints] — General Overview
Interrupted bone joints — diarthroses (synovial joints)

Interrupted bone connections—diarthroses—evolved from continuous connections. During embryonic development, a syndesmosis is formed, which subsequently differentiates into a joint, or synovial connection (articulatio s. junctura synovialis).

Joints are movable, interrupted connections of the skeletal bones that allow bone levers to move relative to one another, facilitate body movement in space, and maintain posture. They are characterized by the presence of the following anatomical elements (Fig. 75A):

- articular surfaces covered with Cartilage;

- articular capsule that encloses the articular ends of the bones and is secured by ligaments;

- articular cavity, located between the connected bone surfaces enclosed by the articular capsule.

Articular surfaces (facies articulares) are covered with articular cartilage (cartilage articularis). As a rule, one of the articulating surfaces of a joint is convex, while the other is concave. Structurally, the cartilage may be hyaline or, less frequently, fibrocartilage. Their free surface facing the joint cavity is smooth, which facilitates the movement of bones relative to each other. The inner surface of the cartilage is firmly fused with the bone, through which it receives nourishment. Convex articular surfaces (for example, the heads of long tubular bones) are covered with a thicker layer of hyaline cartilage at their most convex central part and a thinner layer at the periphery. Conversely, articular fossae on bones have a thin layer of cartilage in the center and a thicker layer at the periphery. The thickness of the articular cartilage ranges from 0.5 to 2–3 mm. The elasticity of hyaline cartilage helps cushion impacts. In addition, the cartilages smooth out all irregularities of the articulating surfaces, giving them the appropriate shape and increasing their congruence (fit).

The articular capsule (capsula articularis) encloses the articular surfaces of the bones, attaching along their margins or slightly away from them to form a hermetically sealed joint cavity. The articular capsule consists of two layers: an outer fibrous layer (stratum fibrosum) and an inner synovial layer (stratum synoviale). The outer layer is composed of Fibrous Connective Tissue, with fibers running longitudinally On the surface and obliquely or transversely in the deeper portions. In some areas, the fibrous layer thins out, leaving the Joint Capsule formed solely by the synovial layer. In other areas, conversely, it becomes denser and significantly thicker due to external ligaments interweaving into the outer fibrous layer of the capsule. Where Muscles or tendons lie adjacent to the capsule, it is thinner, whereas in other areas it is thicker. In joints with a wide range of motion, the capsule is thinner than in strictly limited-mobility joints. The synovial layer consists of loose connective tissue covered by a layer of cylindrical epithelial Cells. It exhibits two structural types: smooth and folded. The synovial layer terminates at the margins of the articular cartilages, where specialized outgrowths—synovial villi (villi synoviales)—are present, which participate in The production of synovial fluid (sinovia). Synovial fluid ensures the adhesion of articulating surfaces, facilitates the gliding of one surface over another, and reduces friction.

In addition to villi, the synovial layer forms synovial folds (plicae synoviales) that project into the joint cavity; adipose tissue may be deposited within them, in which case they are referred to as adipose folds (plicae adiposae). When the synovial layer projects outward, synovial bursae (bursae synoviales) are formed, located in areas of maximum friction, such as beneath muscles or tendons. Furthermore, in regions housing large joints, the synovial layer may form more or less closed spaces known as synovial recesses (recessus synoviales). Such recesses are found within the articular capsule of the knee joint.

The articular cavity (cavitas articularis) is a slit-like space bounded by the articular surfaces of the bones and the synovial membrane. It is filled with a small amount of synovial fluid. Its shape and dimensions depend on the size of the articular surfaces and the attachment sites of the capsule.

In addition to the aforementioned primary components present in every joint, auxiliary elements may also be present: the labrum, intra-articular discs, menisci, intracapsular and extracapsular ligaments, sesamoid bones, as well as synovial and fat folds.

The articular labrum (labrum articulare) is composed of fibrocartilage attached along the rim of the articular surface. It increases the contact area of the articulating surfaces and promotes a more even distribution of pressure from one bone to another. For example, an articular labrum is present in the shoulder and hip joints.

Intra-articular discs (disci articulares) and articular menisci (menisci articulares; meniscus meaning crescentic sickle) are flat, intra-articular cartilaginous structures. When the cartilage completely divides the joint cavity into two compartments, it is called a disc (e.g., in the temporomandibular joint). When the division of the joint cavity is partial, it is referred to as a meniscus (e.g., in the knee joint). Biomechanically, menisci ensure the congruence of articulating surfaces; additionally, they act as buffers during the transmission of pressure from one bone to another and increase the contact area. During movement, the menisci in the knee joint shift within a range of 1 cm on the upper articular surfaces of the tibial condyles. They are not covered by the synovial membrane. Menisci are elastic structures, and their elasticity remains constant throughout life, unaffected by age or pathology (Schweitzer, 1931).

Intracapsular ligaments (ligg. intracapsularia) consist of fibrous tissue and connect bones while residing within the joint cavity (for example, the ligament of the HEAD of the Femur in the hip joint). Ligaments that reinforce the articular capsule are called extracapsular ligaments (ligg. extracapsularia). By strengthening the joints, ligaments also perform a braking role, limiting the range of motion of the bones' articular surfaces. An example is the iliofemoral ligament, which restricts extension in the hip joint.

Sesamoid bones (ossa sesamoidea) are located within the joint capsule or embedded in tendons. Their inner surface, facing the joint cavity, is covered with hyaline cartilage, while the outer surface fuses with the fibrous layer of the capsule. An example of a sesamoid bone is the Patella, situated within the capsule of the knee joint. The biomechanical Significance of the patella lies in its ability to increase the angle of insertion of the quadriceps Muscle, thereby enhancing the muscle's rotational leverage while reducing its compressive effect (A.P. Nikolaiev, 1947). Joint stabilization is largely maintained by factors such as synovial bursae, accessory ligaments, muscle tension, and the adhesion of one articular surface to another. The close contact between articular surfaces is governed by three factors: atmospheric pressure, muscle tone, and the active contraction of periarticular muscles (A.P. Nikolaiev, 1947). If the articular capsule is damaged, air enters the cavity, causing the articular surfaces to separate. Ligaments also help keep the articular surfaces in contact, but only when the joint is under tension. Their primary function, however, is to restrict specific movements (A.P. Nikolaiev, 1947).

Blood supply to the joints is provided by major arterial trunks running adjacent to the joint, which send vascular branches toward it. Occasionally, a joint receives nourishment not from one or two branches, but via a vascular network originating from several Arteries. An example is the arterial network of the elbow joint. Venous blood drainage from the joints flows into venous vessels accompanying the corresponding arteries.

Lymphatic drainage is directed into regional Lymph Nodes.

Innervation of the joints is supplied by nerves passing close to the joint. They dispatch nerve branches into the articular capsule, forming a series of ramifications with terminal nerve endings (receptors).