Knee Meniscus

Meniscus in the Knee

Anatomical Differences

Medial

• Semi-circular, asymmetric
• Posterior horn bigger than anterior horn
• Less mobile
• Completely peripherally fixed to and blends with deep MCL
• Triangular cross-section

Lateral

• Circular in shape
• Larger coverage of tibia
• More mobile
• Attached to tibia by coronary ligament
  • Interrupted by the popliteus tendon (popliteal hiatus)
• Not in contact with LCL
• Gives origin to other ligaments
  • Anterior (Humphrey’s) meniscofemoral ligament
  • Posterior (Wrisberg’s) meniscofemoral ligament
  • Lateral meniscus to MFC either side of PCL
• In alphabetical order: Humphrey’s > PCL > Wrisberg’s

Vascularity

• At birth, completely vascularised
• By age 10, a central avascular area develops
• In adults, the peripheral 10-30% is vascular (red zone)
• Blood supply: medial and lateral geniculate arteries
  • Perivascular Capillary Plexus (PCP) sends radial vessels into the meniscus, penetrating only 10-30% deep

Structure

Extracellular Matrix & Cells

• Extracellular Matrix
  • Water (70%)
  • Type 1 collagen (50% dry weight)
  • Proteoglycans, elastin, glycoproteins
• Cells
  • Fibrochondrocytes

Collagen Arrangement

• In 3 layers: superficial, surface, and middle
• Most fibres are longitudinal, running in the C-shaped direction of the meniscus, especially in the middle layer
• Some fibres are orientated radially, acting as ties to increase rigidity and prevent longitudinal splits

Function

1. Load transmission
2. Enhance joint conformity
3. Lubrication
4. Joint stability
5. Proprioception

Load Transmission

• Acts as shock absorbers
• 15% meniscectomy increases contact pressure by 300%
• Absence leads to rapid degeneration

Articular Conformity

• Menisci displace (lateral > medial) to maintain conformity
• Aids in load transmission and shock absorption in all positions

Lubrication

• High conformity promotes hydrodynamic lubrication

Joint Stability

• Displacement, conformity, and shape of menisci contribute to knee stability
• Medial meniscus is especially important for AP stability

Proprioception

• Via nerve fibres in the anterior and posterior horns in extreme ranges, they play a proprioceptive role

Anatomic and Developmental Anomalies

Medial Meniscus

• Anomalous attachment to the ACL or intercondylar notch

Hypoplastic or Absent Menisci

• May be part of a wider congenital knee problem

Discoid Meniscus

• Lateral much more than medial
• May or may not be symptomatic
• Symptoms include:
  • Snapping, locking, and pain in children

Classification

1. Type 1 - Incomplete
2. Type 2 - Complete
3. Type 3 - Wrisberg variant
   • Detached at posterior horn
   • Meniscofemoral ligaments are the only structures holding meniscus down – amenable to repair

Typical X-ray Findings

1. Increased lateral joint space
2. Squaring of LFC
3. Cupping of the lateral plateau
4. Hypoplastic lateral tibial spine

Management

• Saucerisation (shaping to accommodate LFC)
• Resection of tears
• Repair of Wrisberg variant (Type 3)
• Nothing if incidental finding

Meniscal Cysts

• Secondary to a tear and entrapment of synovial fluid

Meniscal Ganglions

• Similar etiology but more often associated with degenerative tears

Meniscal Tears

Classification Based On:

1. Position
2. Tear characteristics

Position

1. Peripheral (Red on red)
2. Intermediate (Red on white)
3. Central (White on white)

• Only peripheral tears have a good chance of healing and are amenable to repair

Tear Pattern

1. Vertical
2. Horizontal
3. Complex

• Vertical Orientation – longitudinal or radial
  • Longitudinal occurs in line with fibres (e.g., bucket handle)
  • Radial tears are usually peripheral, especially in the lateral meniscus
• Horizontal Plane Tears
  • Can be full or partial cleavage
  • Partial cleavage results in a flap
• Complex Tears
  • Combination of vertical and horizontal patterns
  • Often related to chronic degenerative tears

Meniscal Healing

• Fibrochondrocytes are responsible for healing
• Fibrovascular scar tissue is formed but not in inner zones

Deciding Whether to Repair or Resect

1. Location of tear - Vascularity matters
2. Tear pattern - Longitudinal tears are most amenable, >1cm ideal
3. Patient age - Younger patients do better
4. Timing - Earlier repair results in better outcomes