Articular Cartilage: Biomechanics, Structure, and Properties

What is the biomechanics of articular cartilage?

Articular cartilage is a remarkable connective tissue that lines the ends of bones within synovial joints, acting as a low-friction, load-bearing surface crucial for smooth, pain-free movement and efficient force transmission.

Understanding Articular Cartilage

Articular cartilage is a specialized type of hyaline cartilage, typically 2-4 mm thick, found at the articulating surfaces of bones. Unlike most tissues, it is avascular (no blood supply), aneural (no nerves), and alymphatic (no lymphatic drainage), meaning it has very limited capacity for self-repair after injury. Its unique biomechanical properties are derived from its specific composition and structural organization, enabling it to withstand immense compressive, shear, and tensile forces during daily activities and high-impact movements.

Structure and Composition

The mechanical behavior of articular cartilage is directly attributable to its intricate extracellular matrix (ECM), which is synthesized and maintained by specialized cells called chondrocytes. The primary components of the ECM include:

• Water (60-80%): The largest component by weight, water is crucial for the tissue's ability to resist compression and facilitate fluid flow.
• Collagen (10-30%): Primarily Type II collagen, these fibers provide tensile strength and form a resilient framework. They are organized in distinct zones (tangential, transitional, radial) that optimize load distribution and resistance to shear forces.
• Proteoglycans (5-10%): Large macromolecules, predominantly aggrecan, that are highly hydrophilic. They trap water within the collagen network, creating a swollen, hydrated gel that resists compression and contributes significantly to the tissue's viscoelastic properties.
• Glycoproteins and other proteins (<5%): Contribute to matrix organization and cell-matrix interactions.

This unique composition allows articular cartilage to function as a sophisticated biphasic material, behaving as both a solid (collagen-proteoglycan matrix) and a fluid (interstitial water).

Key Biomechanical Properties

The biomechanics of articular cartilage are defined by several critical properties:

• Viscoelasticity: This is the most important property. Articular cartilage exhibits both viscous (fluid-like) and elastic (solid-like) characteristics.
  • Creep: When subjected to a constant load, the cartilage deforms gradually over time as water is slowly squeezed out of the matrix.
  • Stress Relaxation: When deformed to a constant strain, the internal stress within the cartilage decreases over time as the fluid redistributes within the matrix.
  • This viscoelastic behavior allows the cartilage to absorb shock effectively and distribute loads over a larger area and longer duration, protecting the underlying bone.
• Compressibility: The ability of the tissue to deform under compressive loads, primarily due to the movement of interstitial fluid. This deformation is temporary and reversible.
• Permeability: The ease with which fluid can flow through the porous matrix. Permeability is low, which helps to trap water and maintain the tissue's turgor, but also allows for controlled fluid exudation under pressure.
• Low Friction Coefficient: Articular cartilage boasts one of the lowest friction coefficients in biological systems (approaching that of ice on ice), significantly reducing wear and tear between articulating bone surfaces. This is achieved through sophisticated lubrication mechanisms.
• High Compressive Strength: Despite its deformable nature, articular cartilage can withstand substantial compressive forces due to the interaction between the fluid phase and the solid matrix.

How Articular Cartilage Manages Load

The biomechanical strategies employed by articular cartilage to manage forces are complex and highly efficient:

• Fluid Pressurization (Poroelasticity): When a joint is loaded, the interstitial fluid within the cartilage bears a significant portion of the immediate load. This fluid pressurization acts as a hydrostatic cushion, protecting the solid matrix from excessive deformation. As the load persists, fluid slowly seeps out, and the solid matrix gradually takes on more of the load. This mechanism is critical for shock absorption.
• Stress Distribution: The curved surfaces and deformable nature of articular cartilage allow it to distribute concentrated forces over a wider area of the underlying subchondral bone, thereby reducing peak stresses and preventing damage to the bone.
• Lubrication:
  • Fluid-Film Lubrication: Under dynamic loading, a thin layer of synovial fluid is trapped between the articulating surfaces, preventing direct contact. This is highly effective during movement.
  • Weeping Lubrication: As the cartilage is compressed, fluid is exuded from the porous matrix onto the surface, further enhancing lubrication. When the load is removed, fluid is reabsorbed, carrying nutrients into the cartilage.
  • Boundary Lubrication: Specific molecules in the synovial fluid (e.g., lubricin, hyaluronic acid) adhere to the cartilage surface, providing a low-friction layer even under high loads or static conditions.

Factors Influencing Cartilage Biomechanics

The biomechanical properties of articular cartilage are not static and can be influenced by several factors:

• Loading Patterns: Moderate, intermittent loading (e.g., walking, running) is essential for maintaining cartilage health and stimulating chondrocyte activity. Excessive, repetitive, or impact loading can lead to breakdown and degeneration. Conversely, prolonged immobilization or disuse can also compromise cartilage integrity.
• Age: With aging, there are changes in cartilage composition (e.g., decreased proteoglycan content, altered collagen network), which can lead to reduced elasticity, increased stiffness, and diminished capacity to withstand loads.
• Injury and Disease: Acute injuries (e.g., meniscal tears, ligamentous injuries) can alter joint mechanics, leading to abnormal loading patterns on the cartilage. Degenerative diseases like osteoarthritis are characterized by progressive breakdown of articular cartilage, leading to pain, stiffness, and loss of joint function, fundamentally altering its biomechanical behavior.
• Nutrition and Hydration: Proper nutrition and hydration are vital for maintaining the health and integrity of the cartilage matrix.

Clinical Significance and Preservation

Understanding the biomechanics of articular cartilage is paramount for clinicians and fitness professionals. It informs strategies for:

• Injury Prevention: Designing exercise programs that promote optimal joint loading without overstressing cartilage.
• Rehabilitation: Guiding rehabilitation protocols after joint injuries to restore function and prevent secondary cartilage damage.
• Osteoarthritis Management: Developing interventions that aim to preserve remaining cartilage, manage symptoms, and slow disease progression.
• Surgical Interventions: Guiding the development of cartilage repair techniques and joint replacement prostheses that mimic natural joint mechanics.

Conclusion

Articular cartilage is a marvel of biological engineering, exquisitely adapted to its biomechanical role. Its unique viscoelastic, compressible, and low-friction properties, derived from its intricate biphasic structure, enable it to efficiently absorb and distribute forces, facilitate smooth joint movement, and protect underlying bone. Maintaining its health through appropriate loading and lifestyle choices is fundamental to lifelong joint function and mobility.