Knee Cartilage: Composition, Types, and Importance

What is knee cartilage made of?

Knee cartilage is primarily composed of specialized cells called chondrocytes embedded within an extensive extracellular matrix, which itself is largely made of water, collagen fibers (predominantly Type II for articular cartilage and Type I for meniscal cartilage), and proteoglycans.

Introduction to Knee Cartilage

The knee joint, a marvel of biomechanical engineering, relies heavily on its cartilaginous structures for smooth, pain-free movement, shock absorption, and load distribution. These resilient tissues, though often taken for granted until damaged, are crucial for the long-term health and function of the joint. Understanding their composition is fundamental to appreciating their mechanics, unique properties, and why they are so challenging to repair once injured.

Types of Cartilage in the Knee

The knee joint contains two primary types of cartilage, each with a distinct structure, composition, and function tailored to its specific role:

Articular Cartilage (Hyaline Cartilage)

Location: This smooth, glistening tissue covers the ends of the femur (thigh bone), the top of the tibia (shin bone), and the back of the patella (kneecap) where they articulate. Function: Articular cartilage provides an extremely low-friction surface, allowing the bones to glide effortlessly over one another during movement. It also acts as a shock absorber, distributing forces across the joint surface during activities like walking, running, and jumping. Composition: Articular cartilage is a form of hyaline cartilage. It is predominantly composed of:

• Chondrocytes: These specialized cells make up a small percentage (typically 2-5%) of the cartilage volume. They are responsible for synthesizing and maintaining the extracellular matrix (ECM).
• Extracellular Matrix (ECM): This non-cellular component constitutes the vast majority (95-98%) of the cartilage and gives it its unique properties. The ECM consists primarily of:
  • Water: Accounts for 65-80% of the wet weight, making it the largest component. Water is crucial for shock absorption and nutrient transport.
  • Collagen Fibers: Primarily Type II collagen, which forms a intricate, highly organized network providing tensile strength and structural integrity.
  • Proteoglycans: Large molecules, mainly aggrecan, which are composed of a protein core with numerous attached glycosaminoglycan (GAG) chains (like chondroitin sulfate and keratan sulfate). These molecules are highly hydrophilic, attracting and trapping large amounts of water within the matrix, enabling cartilage to resist compressive forces.
  • Non-collagenous Proteins and Glycoproteins: Minor components that play roles in matrix organization and cell signaling.

Articular cartilage is unique in that it is avascular (lacks blood vessels), aneural (lacks nerves), and alymphatic (lacks lymphatic vessels). This explains its limited capacity for self-repair after injury, as it cannot readily receive nutrients or inflammatory cells to initiate healing.

Meniscal Cartilage (Fibrocartilage)

Location: The menisci are two C-shaped, wedge-like structures (medial and lateral menisci) located between the femur and tibia within the knee joint. Function: Meniscal cartilage serves multiple critical roles:

• Shock Absorption: It significantly increases the surface area for load distribution, reducing stress on the articular cartilage.
• Load Distribution: It helps distribute body weight evenly across the joint, preventing concentrated stress points.
• Joint Stability: It acts as a "wedge" to improve the congruity between the rounded femoral condyles and the relatively flat tibial plateau, enhancing joint stability.
• Lubrication and Nutrition: It assists in spreading synovial fluid, contributing to joint lubrication and nutrient delivery to the articular cartilage. Composition: Meniscal cartilage is a form of fibrocartilage. Its composition differs from hyaline cartilage:
• Chondrocytes and Fibroblasts: Contains a mix of chondrocyte-like cells and fibroblasts, which synthesize the matrix.
• Extracellular Matrix (ECM):
  • Water: Accounts for 60-70% of the wet weight.
  • Collagen Fibers: Primarily Type I collagen, which is organized into a robust, dense network providing significant tensile strength and resistance to tearing.
  • Proteoglycans: Present in smaller amounts than in hyaline cartilage, contributing less to the water-binding capacity.
  • Non-collagenous Proteins: Similar to articular cartilage, but with differences reflecting its fibrous nature.

Unlike articular cartilage, the outer third of the menisci has a limited blood supply, which allows for some healing potential if injuries occur in this vascularized zone. The inner two-thirds are avascular, similar to hyaline cartilage, and thus have very poor healing capacity.

Key Components of Cartilage Explained

To fully grasp what knee cartilage is made of, it's essential to understand the roles of its primary constituents:

• Chondrocytes: These are the sole cells found within healthy cartilage. They reside in small spaces called lacunae within the ECM. Chondrocytes are metabolically active, constantly synthesizing and degrading components of the ECM to maintain tissue homeostasis. Their health and activity are paramount to cartilage integrity.
• Water: The most abundant component by weight, water gives cartilage its ability to deform under compression and then return to its original shape. It is essential for the transport of nutrients and waste products within the avascular tissue.
• Collagen Fibers: These fibrous proteins provide the structural framework and tensile strength to cartilage.
  • Type II Collagen: Found predominantly in articular (hyaline) cartilage, it forms a fine, mesh-like network that resists swelling and provides resilience.
  • Type I Collagen: The primary collagen in meniscal (fibrocartilage), it forms thicker, more robust bundles, providing superior resistance to tearing and shear forces.
• Proteoglycans: These large, complex molecules are crucial for cartilage's ability to withstand compressive loads. Their highly negatively charged glycosaminoglycan chains attract and trap water molecules, creating a high osmotic pressure that resists compression. Aggrecan is the most abundant proteoglycan in articular cartilage.

Function and Importance of Knee Cartilage

The unique composition of knee cartilage allows it to perform its vital functions:

• Low Friction Movement: The smooth, hydrated surface of articular cartilage reduces friction between bones, allowing for effortless joint motion.
• Shock Absorption: Both articular and meniscal cartilages act as natural shock absorbers, protecting the underlying bone from impact forces.
• Load Distribution: They distribute mechanical loads evenly across the joint surfaces, preventing localized stress and wear.
• Joint Stability: The menisci, in particular, enhance the fit between the femoral and tibial bones, contributing to knee stability.

Cartilage Health and Maintenance

Given its limited healing capacity, maintaining cartilage health is critical. Factors influencing cartilage integrity include:

• Nutrition: Adequate intake of vitamins (e.g., Vitamin C for collagen synthesis), minerals, and amino acids.
• Hydration: Sufficient water intake supports the hydration of the cartilage matrix.
• Appropriate Loading: Regular, moderate physical activity provides the necessary mechanical stimuli for chondrocytes to maintain the matrix, while excessive or improper loading can lead to degeneration.
• Injury Prevention: Avoiding traumatic injuries and managing chronic conditions that affect joint health.

Conclusion

Knee cartilage, whether the smooth articular hyaline cartilage or the robust meniscal fibrocartilage, is a sophisticated biological material. Its unique composition of specialized cells (chondrocytes), a rich water content, and a meticulously organized extracellular matrix of collagen and proteoglycans enables it to withstand immense mechanical stresses while facilitating fluid movement. Understanding "what knee cartilage is made of" provides critical insight into its remarkable function and the challenges associated with its repair and regeneration in the face of injury or degenerative conditions like osteoarthritis.