The meniscus is a specialized, crescent-shaped piece of fibrocartilage found within the knee joint. It is situated between the thigh bone (femur) and the shin bone (tibia). This positioning allows the meniscus to act as a dynamic gasket, improving the fit between the two bones and distributing forces across the joint surface. Its primary function is to provide cushioning and stability to the knee, protecting the smooth articular cartilage that covers the ends of the bones.
Molecular Building Blocks
The physical strength and resilience of the meniscus begin at the molecular level with its extracellular matrix (ECM). The composition of this matrix is dominated by water, which typically accounts for 70% to 80% of the total wet weight. This high level of hydration is fundamental to the tissue’s ability to resist compression and absorb shock.
The solid portion is primarily composed of the structural protein collagen, making up about 22% of the wet weight. This collagen is overwhelmingly Type I, constituting approximately 90% of the matrix’s dry weight, giving the meniscus its exceptional tensile strength. The predominance of Type I collagen distinguishes the meniscus from the hyaline cartilage on the bone surfaces, which is mostly Type II collagen.
The matrix also contains proteoglycans, such as aggrecan. These molecules attract and bind water due to their negative charge. This binding creates a swelling pressure that helps the tissue maintain its form and resist compressive loads, even though the overall proteoglycan content is lower than that found in articular cartilage.
Cellular Makeup and Fiber Arrangement
The maintenance and repair of the extracellular matrix are handled by the sparsely distributed cells within the tissue, known as fibrochondrocytes. These cells synthesize and regulate the collagen and proteoglycan components of the matrix. The low density of these cells contributes to the meniscus’s limited capacity for intrinsic healing following injury.
The mechanical strength of the meniscus is largely due to the highly organized arrangement of its collagen fibers. The majority of Type I collagen fibers are oriented circumferentially, running parallel to the outer edge of the C-shape. This specific organization allows the meniscus to resist the substantial pulling forces generated during knee movement.
Interwoven among these main circumferential bundles are radial “tie” fibers that run perpendicular to the circumference. These radial fibers act like sutures, preventing the larger, tension-bearing circumferential bundles from splitting apart when the tissue is under load. This complex fiber architecture gives the meniscus its structural integrity under multidirectional stress.
The meniscus also exhibits regional differences in its blood supply, organized into distinct zones. Only the outer 10% to 30% of the tissue, known as the “red-red zone,” receives a direct blood supply from the surrounding joint capsule. The inner two-thirds, the “white-white zone,” is avascular, a factor that significantly influences the tissue’s healing potential.
Composition’s Influence on Mechanical Role
The unique material composition and structural organization directly enable the meniscus’s mechanical functions. The high water content, restrained by the network of proteoglycans and collagen, gives the tissue viscoelastic properties essential for shock absorption. As the knee is loaded, the water is temporarily pressurized and displaced, which dissipates the energy of impact and protects the underlying bone and cartilage.
The circumferential arrangement of the Type I collagen fibers is adapted to manage the forces transmitted through the knee. When the femur presses down on the tibia, the axial force causes the meniscus to bulge radially outward. This deformation is resisted by the tensile strength of the circumferential fibers, which converts the vertical compressive load into a circumferential pulling force known as hoop stress.
The successful translation of axial load into hoop stress is the mechanism by which the meniscus stabilizes the joint and transmits load across the knee. If this fiber arrangement is compromised, such as by a radial tear, the meniscus loses its ability to sustain hoop stress and can extrude from the joint space. This loss of function dramatically increases the contact pressure on the joint cartilage, accelerating wear and degeneration.