Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Skeleton
Articulations of the Skeletal Bones

The BONES OF THE Skeleton in a living human are interconnected by various TYPES OF JOINTS. All connections of the human skeletal bones can be divided into continuous, hemarthroses (symphyses), and discontinuous (synovial joints).

Continuous joints are formed by various types of Connective Tissue. Depending on the type of tissue forming the connection, they are subdivided into fibrous, cartilaginous, and bony joints.

Fibrous joints (named after the coarse fibrous tissue), or syndesmoses, are characterized by the presence of connective tissue between the articulating bones. These joints are strong and more or less mobile. Examples include the sutures between the bones of the Skull (serrate, squamous, and plane), ligaments, and interosseous membranes. The connections between tooth roots and the dental alveoli of the upper and lower jaws are also classified as fibrous joints, since a thin layer of connective tissue exists between the tooth ROOT and the alveolar walls. Interosseous membranes, stretched between the bones of the forearm and leg and serving as an origin site for many Muscles, are likewise considered fibrous joints. Ligaments, which connect adjacent bones, hold them together, and reinforce the joints, also belong to fibrous joints. Ligaments consist of bundles of dense fibrous tissue.

Cartilaginous joints (synchondroses) are characterized by strength, elasticity, resilience, and limited mobility. Cartilaginous joints include intervertebral discs, cartilaginous attachments to the Sternum, and cartilaginous layers between parts of young, not yet fused bones of the same skeletal element. These cartilaginous layers are called temporary cartilaginous joints because at a certain age (usually adolescence or Puberty) they are replaced by Bone tissue.

Bony joints refer to areas of bone tissue that have developed in place of preceding Cartilage (for example, at the junction of the pubis, ilium, and ischium into a single pelvic bone, or at the epiphyseal-diaphyseal junctions of long bones).

Hemarthroses, or symphyses, are connections of two bones via cartilage (a cartilaginous layer) that contains a narrow cleft with a small amount of fluid. A complete Separation between the two articulating bones does not yet exist; the cavity is only beginning to form. An example of such a connection is the Pubic Symphysis, the joint between the two pubic bones that form the anterior pelvic ring.

Synovial (discontinuous) joints, or diarthroses, are distinguished by high mobility, a wide range of movements, and structural complexity. Each joint features several mandatory elements (Fig. 17): 1) the articular surfaces of the articulating bones; 2) the articular cartilage covering the articular surfaces; 3) the articular capsule, which surrounds the ends of the articulating bones like a sleeve; 4) the joint cavity, bounded by the articular cartilages and the inner surface of the articular capsule; 5) the synovial fluid, present in a small amount within the cavity of every joint. This fluid internally lubricates the articular cartilages and also contributes to their nourishment.

The articular surfaces of bones are covered with articular cartilage. Its thickness is directly dependent on the load experienced by the joint: the greater the load, the thicker the cartilage.

Articular cartilage possesses high spring-like properties. This is because its chondrocytes and connective tissue fibers deep within the cartilage are oriented perpendicularly to the free surface of the cartilage, countering pressure forces, while in the superficial layers they run parallel to the surface, countering frictional forces. The resilient articular cartilage not only dampens impacts during movement, walking, and running, but also evenly distributes pressure across the articular surfaces of the joined bones.

The articular capsule of each joint consists of two layers. The outer layer—the fibrous membrane—is dense, coarse-fibrous, and quite thick. It attaches to the bones near the edges of the articular surfaces and transitions into the periosteum. The inner, thin layer of the articular capsule is the synovial membrane, which lines the fibrous membrane from the inside. Facing the joint cavity, the synovial membrane is covered with flat epithelial Cells that produce the joint fluid (synovia).

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Fig. 17. Structure of a joint:

1 — periosteum; 2 — bone; 3 — articular capsule; 4 — articular cartilage; 5 — joint cavity

Synovial fluid (synovia), which enters the joint cavity from the Vessels of the inner layer of the articular capsule, facilitates the gliding of the cartilage-covered articular surfaces. This fluid lubricates the contacting surfaces of the articular cartilages, eliminating friction between them.

The articular capsule is reinforced by ligaments—thick bundles of dense Fibrous connective tissue whose ends attach to the articulating bones. Ligaments not only strengthen the joints, but also guide and restrict movements, preventing joint hyperextension.