Biomechanics of Articular Cartilage

Biomechanics of Articular Cartilage

1. Introduction

Articular cartilage is a specialized connective tissue that covers the ends of bones in synovial joints, such as the knee, hip, and shoulder. It provides a smooth, low-friction surface for joint movement and distributes mechanical loads to protect underlying bone structures.

Understanding the biomechanics of articular cartilage is crucial for orthopedic research, sports medicine, joint replacement technology, and cartilage repair therapies.

2. Structure & Composition of Articular Cartilage

Articular cartilage is avascular, aneural, and alymphatic, meaning it has no blood vessels, nerves, or lymphatic supply. It relies on diffusion from synovial fluid for nutrition and waste removal.

a. Layered Structure

Articular cartilage has four distinct zones:

Zone	Collagen Fiber Orientation	Function
Superficial (Tangential) Zone	Parallel to surface	Reduces friction, resists shear forces
Middle (Transitional) Zone	Randomly arranged	Absorbs compressive forces
Deep Zone	Perpendicular to surface	Provides structural support
Calcified Cartilage Zone	Anchors to subchondral bone	Prevents cartilage separation

b. Composition of Articular Cartilage

• Water (65-80%): Maintains lubrication and load distribution.

• Collagen (15-22%): Primarily Type II collagen, providing tensile strength.

• Proteoglycans (4-7%): Trap water to resist compression (aggrecan is the main proteoglycan).

• Chondrocytes (<5%): Cartilage cells responsible for synthesis and maintenance.

3. Mechanical Properties of Articular Cartilage

a. Load-Bearing Function

• Supports joint movement by distributing forces over a large area.

• Acts as a shock absorber, preventing bone damage.

b. Stress-Strain Behavior

• Viscoelastic nature: Combines elasticity (immediate response) and viscosity (time-dependent response).

• Under sustained load, cartilage deforms slowly and recovers gradually.

c. Permeability & Fluid Flow

• Water movement in cartilage is crucial for its mechanical response.

• When compressed, fluid moves out, increasing stiffness.

• Once the load is removed, fluid re-enters, restoring cartilage shape.

4. Mechanical Forces Acting on Articular Cartilage

Force Type	Effect on Cartilage
Compression	Squeezes water out, increasing stiffness
Tension	Stretches collagen fibers, resisting deformation
Shear	Causes parallel sliding, which can damage superficial layers
Hydrostatic Pressure	Maintains structural integrity

5. Cartilage Wear & Degeneration

a. Causes of Cartilage Damage

• Aging: Natural wear and reduced collagen synthesis.

• Injury: Trauma (e.g., ACL tears) can lead to cartilage deterioration.

• Overloading: Excessive weight or repetitive impact damages the matrix.

• Osteoarthritis (OA): Breakdown of cartilage due to mechanical and biochemical changes.

b. Healing Limitations

• Lack of blood supply prevents cartilage from healing efficiently.

• Severe damage often requires surgical intervention or cartilage replacement therapies.

6. Clinical Applications & Treatments

Treatment	Biomechanical Basis
Viscosupplementation (Hyaluronic Acid Injections)	Restores lubrication and shock absorption
Autologous Chondrocyte Implantation (ACI)	Grows new cartilage cells for defect repair
Tissue Engineering & Biomaterials	Uses scaffolds to regenerate damaged cartilage
Joint Replacement (Arthroplasty)	Replaces cartilage with synthetic materials

7. Conclusion

Articular cartilage plays a vital role in joint biomechanics, allowing smooth motion and shock absorption. However, its limited healing capacity makes it vulnerable to injury and degeneration. Advances in biomechanics, regenerative medicine, and biomaterials aim to improve cartilage repair and develop effective treatments for joint disorders like osteoarthritis.