Knee Menisci: Composition, Structure, and Functional Importance

What are the menisci of the knee composed of?

The menisci of the knee are primarily composed of fibrocartilage, a specialized connective tissue characterized by a dense network of Type I collagen fibers, a high water content, and a sparse population of chondrocytes and fibroblasts within a proteoglycan-rich extracellular matrix.

Introduction to the Menisci

The knee joint, a complex hinge joint, relies on several structures for its stability, mobility, and shock absorption. Among the most critical are the menisci – two C-shaped wedges of cartilage located between the femoral condyles and the tibial plateau: the medial meniscus and the lateral meniscus. Far from being simple spacers, these crescent-shaped structures play vital roles in distributing forces, enhancing joint stability, aiding lubrication, and proprioception. Understanding their unique composition is key to appreciating their remarkable mechanical properties and their susceptibility to injury.

Primary Composition: Fibrocartilage

The defining characteristic of the menisci's composition is their make-up from fibrocartilage. This tissue is distinct from hyaline cartilage (found on the articular surfaces of bones) and elastic cartilage. Its unique blend of components allows it to withstand the immense compressive, shear, and tensile forces experienced during daily activities and athletic endeavors.

The key components of meniscal fibrocartilage include:

• Water (65-75%): Water is the most abundant component by weight. It is crucial for the tissue's viscoelastic properties, allowing it to deform under pressure and then return to its original shape. The interaction between water and the proteoglycans is fundamental to the menisci's ability to resist compression.
• Collagen Fibers (15-25%): Collagen is the primary solid component, providing the menisci with their impressive tensile strength.
  • Type I Collagen: This is the predominant type, accounting for over 90% of the collagen content. Type I collagen fibers are robust and highly resistant to stretching, forming the main structural framework.
  • Type II Collagen: Present in smaller amounts, Type II collagen is more common in hyaline cartilage and provides some resistance to compression.
  • Minor Collagens: Other types (e.g., Type III, V, VI) are present in trace amounts, contributing to the overall matrix organization and integrity.
• Proteoglycans (1-2%): These are large molecules consisting of a protein core with attached glycosaminoglycan (GAG) chains (e.g., aggrecan, decorin, biglycan). While less abundant than in articular cartilage, proteoglycans in the menisci are negatively charged and attract water, contributing to the tissue's osmotic swelling pressure and its ability to resist compressive loads.
• Elastin (trace amounts): Provides a small degree of elasticity, allowing the tissue to deform and recoil.
• Cells (1-2%): The cellular component of the menisci is sparse and includes:
  • Fibrochondrocytes: These are specialized cells that exhibit characteristics of both fibroblasts (producing collagen) and chondrocytes (producing proteoglycans). They are responsible for synthesizing and maintaining the extracellular matrix of the menisci.

Structural Organization of Collagen Fibers

The arrangement of collagen fibers within the menisci is highly organized and crucial for their mechanical function. This intricate architecture allows the menisci to effectively convert compressive forces into tensile hoop stresses, protecting the articular cartilage.

• Circumferential Fibers (Longitudinal): The vast majority of Type I collagen fibers are oriented circumferentially, running parallel to the C-shape of the menisci. These fibers act like the hoops on a barrel, resisting outward expansion when the menisci are compressed. This arrangement is fundamental to their ability to absorb and distribute axial loads across the knee joint.
• Radial Fibers (Tie Fibers): A smaller number of collagen fibers are oriented radially, extending from the inner edge to the outer periphery. These fibers act as "tie fibers," connecting and stabilizing the circumferential bundles, preventing them from splaying apart under load. They also help anchor the menisci to the tibial plateau.

Regional Variations in Composition and Structure

While the general composition holds true, there are subtle but important regional variations within the menisci:

• Vascularity: The outer one-third (or periphery) of the menisci is vascularized, meaning it receives a blood supply. This region contains more fibroblasts and is capable of some healing after injury. The inner two-thirds are largely avascular, receiving nutrition primarily through diffusion from the synovial fluid, which significantly limits their intrinsic healing capacity.
• Cell Density: Cell density tends to be higher in the outer, vascularized zone and decreases towards the inner, avascular zone.
• Collagen Fiber Orientation: The highly organized circumferential and radial patterns are most pronounced in the main body of the meniscus, while the anterior and posterior horns (attachments) have a more interwoven, irregular fiber arrangement to facilitate their anchoring.

Why this Composition Matters: Functional Implications

The unique fibrocartilaginous composition and the sophisticated architectural arrangement of its collagen fibers directly underpin the menisci's critical functions:

• Shock Absorption: The high water content and proteoglycans allow the menisci to deform and absorb impact forces, protecting the underlying articular cartilage and subchondral bone from excessive stress.
• Load Distribution: By conforming to the shapes of the femoral condyles and tibial plateau, the menisci increase the contact area between these bones, effectively distributing weight-bearing forces over a wider surface. This reduces peak stresses on the articular cartilage, preventing premature wear and tear.
• Joint Stability: The menisci deepen the relatively flat tibial plateau, creating a more congruent fit with the rounded femoral condyles. This "cushioning" effect enhances the stability of the knee joint, particularly during flexion, extension, and rotational movements.
• Joint Lubrication: The menisci contribute to the distribution of synovial fluid across the articular surfaces, aiding in joint lubrication and reducing friction.

Conclusion

The menisci of the knee are marvels of biological engineering, crafted from a specialized fibrocartilaginous tissue. Their primary composition of Type I collagen, substantial water content, and strategic fiber orientation enable them to withstand immense mechanical stresses, distribute loads, absorb shock, and stabilize the knee joint. This intricate design is a testament to the body's ability to create highly specialized tissues adapted to specific functional demands, highlighting why understanding their composition is fundamental to comprehending knee biomechanics and pathology.