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.
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