MENISCI AND DISCS

General

Composition
- Fibrocartilagenous structure
- Interlacing network collagen fibres
Consists of
- Water
- Cells (Fibrochondrocytes)
- Matrix
  - Matrix consists of
    - Collagen Type I & VI
    - Ground substance (PG)
    - Elastin < 1%

Menisci

Structure
- Semilunar in shape
- Roughly triangular in cross-section
- Layered appearance
  - Three Collagen Layers
    - Superficial Layer
      - Femoral articulating surface
        
        Composed of mesh-like collagen fibrils parallel to surface
    - Surface layer
      - Just beneath superficial layer
      - Irregular aligned fibrils
    - Middle Layer
      - Large coarse collagen fibres
      - Major orientation of fibres is parallel, circumferential direction
      - Some radial fibres resist splitting
- Peripheral 2/3rd innervated by type I & II nerve endings
  - Most at horns
  - Few fibres in middle
Blood Supply
- From Geniculate Arteries
- Circumferentially arranged plexus
- Supplies outer 1/4 (Red Zone)
- Able to stimulate vascular repair
Function
- Increase surface area joint to ↓ unit load
  - 50% weight bearing in extension
  - 85% weight bearing in flexion
  - Meniscectomy - 50% reduction surface area
  - 30% Partial meniscectomy = 350% Increase Unit pressure
- Shock Absorption
  - Outward or radial force that pushes menisci out from between bones
  - Generated on weight bearing
  - These forces converted to hoop tension in circumferential fibres
  - Hoop stresses transmitted to tibia
- Increased Stability
- Lubrication
- Proprioception

Intervertebral Disc

Structure
- Upper & lower surfaces of vertebral body
- Covered by thin plate of hyaline cartilage
Annulus Fibrosis
- Type 1 collagen
- Consists of concentric lamellae
  - At 45° to vertebral body
  - At 45° to each other
- Withstands high bending & torsional loads
Nucleus Pulposis
- Derived from Notochord
- Inner portion of disc
- Semi-liquid gelatinous substance
- Accounts for 15% of disc
- Rich in Proteoglycans
- Some Type II collagen
- High water content
  - 90% in child
  - 70% in adult
- Lies in posterior part of disc
- Especially in lumbar spine
  - Herniates posteriorly more easily
- Is under pressure
  - 10 N/ cm2
  - Due to pre-stress from longitudinal ligaments
  - Acts hydrostatically during loading of spine
  - Distributes pressure throughout disc
- Hydrophilic
  - Imbibes water at night
  - Water squeezed out during day
  - Explains 1 cm height loss with standing
Function
- Movement
  - Prevent excessive movement
- Load
  - Is load-bearing
  - Load causes ↑
  - Compressive stress in nucleus pulposis
  - Tensile stress / hoop stress in annulus fibrosis