Week 5 Basic Biomechanics of the Musculoskeletal System, Chapter 3 Discussion of the biomechanics of cartilage.

Basic Biomechanics of the Musculoskeletal System: Cartilage

Composition and Structure of Articular Cartilage

• Articular cartilage is discussed, focusing on its biomechanics.
• Key aspects include composition, structure, and biomechanical behavior.

Functions of Articular Cartilage

• The two primary functions are:
  • Distributing joint loads over a wide area.
  • Allowing relative movement of opposing joint surfaces with minimal friction and wear.

Collagen

• Collagen is the most abundant protein in the body.
• It serves as the "My CRiB" (likely a metaphor for structural support).
• Tensile strength varies:
  • Single fibril: 860 MPa
  • Tendon: 50 MPa
  • ADL (Activities of Daily Living) peak stress in hip and knee: 5 MPa

Articular Cartilage Zones

• Superficial tangential zone:
  • Sheets of fine, densely packed fibrils randomly woven parallel to the surface.
• Middle zone:
  • Greater distances between randomly oriented and homogeneously dispersed fibrils.
• Deep zone:
  • Larger, radially oriented bundles.
• Tidemark:
  • Interface between articular cartilage and calcified cartilage.

Tensile Strength of Articular Cartilage

• Tensile strength ranges from 3 to 100 MPa.
• It is time-dependent:
  • 3 MPa at 0.001 Hz
  • 16 MPa at 1 Hz
  • Typically, 5-10 MPa.
• Stress-strain curve:
  • Toe region: Fibril pull out and realignment.
  • Fibrillation can lead to Osteoarthritis (OA).

Proteoglycans

• Populations of aggrecans exist.
• First population:
  • Present throughout life.
  • Rich in chondroitin sulfate (CS).
• Second population:
  • Only present in adults.
  • Rich in keratan sulfate (KS).
• Changes with maturation:
  • Water content, carbohydrate/protein ratio progressively decrease.
  • CS decreases while KS increases.
  • CS:KS ratio: 10:1 at birth, 2:1 in adults.

Water Content

• Most concentrated near the articular surface.
• Avascular: Gas, nutrient, and waste product exchange occur through water.
• Donnan osmotic pressure theory contributes to swelling.
• Cations: Na^+, K^+, Ca^{++}

Role of Water in Compression

• Fixed negative charges of KS and CS provide a repulsive force contributing to stiffness.
• When compressed:
  • Water is forced out.
  • Charge density increases.
  • Osmotic pressure increases.
  • Repulsive force increases.
  • PG in the collagen network enables compression resistance.

Permeability

• Porosity:
  • Ratio of pore volume to total volume.
  • Interconnected pores allow permeability.
  • Ease of flow is inversely proportional to frictional drag.
• Cartilage:
  • High porosity (~80%).
  • Low permeability: Fluid flows very slowly (< 1 micron per second).
  • With deformation, pores shrink, decreasing permeability.

Chondrocytes

• Normal joint motion generates stimuli to promote maintenance.
• Compromised cartilage leads to abnormal stimuli, abnormal remodeling, and debilitating function.

Degeneration

• Limited ability to repair.
• Failure is related to:
  • Magnitude of the stress.
  • Total number of stress peaks.
  • Changes in the intrinsic molecular and microscopic structure of the collagen-PG matrix.
  • Changes in the intrinsic mechanical properties of the tissue.
  • "Loosening" of the collagen network allows PG expansion and tissue swelling.
  • Decrease in stiffness.
  • Increased permeability.

Lubrication and Wear

• Extremely low friction due to lubricin (superficial zone protein).
• Interfacial wear:
  • Bearing surfaces are in direct contact without a lubricant film.
  • Involves adhesion or abrasion.
• Fatigue wear:
  • Accumulation of microscopic subsurface damage under repetitive stressing.
  • Caused by:
    • Repeated high loads over a short period.
    • Repeated low loads over an extended period.
  • Leads to fissure and erosion.
  • Delamination of the superficial zone.
  • Fibrillation.