The term “articular” is a fundamental concept in anatomy, referring to anything related to a joint within the body. The articular system provides a seamless interface between bones, allowing for both stability and a wide range of motion. This system involves specialized tissues designed to minimize friction and absorb mechanical forces. This article will define “articular” and explore the specific structures that facilitate smooth, protected movement.
What “Articular” Means in Anatomy
The word “articular” is an adjective that means “pertaining to a joint” or “relating to an articulation.” An articulation, or joint, is defined as any location where two or more bones meet in the skeletal system. These meeting points are necessary for the skeleton to function as a mobile structure rather than a single, rigid frame.
The human body contains various types of articulations, ranging from the immovable sutures of the skull to the freely movable joints of the limbs, such as the knee or shoulder. An “articular surface” specifically refers to the region of the bone that engages with a neighboring bone at the joint. The entire apparatus surrounding this connection, including the capsule, ligaments, and cartilage, is referred to as the articular system.
The presence of an articulation determines the potential for movement, from small shifts to large, sweeping motions. Joints that allow for free movement are known as synovial joints, where the articular term is most frequently applied. These joints are characterized by a joint cavity, which is a small space between the bones that allows for greater mobility.
Key Structures That Make Up Articular Surfaces
The surface where bones meet is protected by a layer of specialized connective tissue called articular cartilage. This cartilage is a form of hyaline cartilage, which is avascular and aneural, meaning it lacks direct blood vessels or nerves. The extracellular matrix is highly hydrated, consisting of approximately 74% water, which is crucial for its mechanical properties.
The dry weight of the cartilage is primarily composed of Type II collagen fibers and large molecules called proteoglycans. Type II collagen provides the tissue with tensile strength and a fibrous scaffold. The proteoglycans, particularly aggrecan, are highly negatively charged, trapping water within the matrix to resist compressive forces.
Beneath the cartilage lies the subchondral bone, a specialized layer that supports the overlying cartilage. This bone is structured into two main parts: the compact subchondral bone plate adjacent to the cartilage, and the more porous trabecular bone below it. This structure provides a firm foundation while also allowing for a degree of elasticity.
The entire movable joint is enclosed by a joint capsule, lined with the synovial membrane. This membrane secretes synovial fluid, a viscous, non-Newtonian fluid that fills the joint cavity. Synovial fluid contains hyaluronic acid for lubricating viscosity and lubricin (Proteoglycan 4) for boundary layer lubrication.
How Articular Tissues Facilitate Movement
The function of articular tissues is to create a bearing surface that allows for motion with low friction and high load tolerance. The combination of articular cartilage and synovial fluid results in one of the most efficient lubrication systems found in nature. The coefficient of friction in a healthy joint can be as low as 0.005 to 0.025, which is far lower than ice sliding on ice.
During movement, the cartilage employs a mechanism known as “weeping lubrication.” When pressure is applied, water retained within the proteoglycan matrix is squeezed out onto the surface, creating a thin, pressurized fluid film between the articulating bones. This fluid film prevents the cartilage surfaces from making direct contact during heavy load-bearing activities.
The structural components work to manage and distribute mechanical stresses. The water-retaining capacity of the proteoglycans allows the articular cartilage to absorb compressive forces, acting as a hydrostatic shock absorber. Simultaneously, the underlying trabecular bone provides elasticity that helps to dissipate energy from impacts, preventing damage to the cartilage or deeper bone structure.