Discoid Meniscus (Pediatric)

Imaging Gallery

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Discoid Lateral Meniscus: A congenital morphological variant where the lateral meniscus is discoid (disc-shaped) rather than the normal crescent (C-shape), covering a larger portion of the tibial plateau. It is thicker, hypercellular, and has disorganized collagen.

Watanabe Classification:

• Type I (Complete): Covers the entire plateau. Block shaped.
• Type II (Incomplete): Covers less than 100% but more than 80% (or significantly wider than normal).
• Type III (Wrisberg): Normal shape but LACKS posterior coronary ligaments. Unstable.

Saucerization: The surgical procedure of resecting the central portion of a discoid meniscus to create a functional peripheral rim (recreating the C-shape).

Wrisberg Ligament: The Posterior Meniscofemoral Ligament (PMFL). It originates from the posterior horn of the lateral meniscus and inserts on the medial femoral condyle. In Type III discoid meniscus, it is often hypertrophied and acts as the only posterior restraint.

Coronary Ligaments (Meniscotibial): Fibrous bands connecting the inferior edge of the meniscus to the tibial plateau. Their absence defines the Type III variant.

Discoid Meniscus is a morphological variant of the meniscus, predominantly affecting the lateral side.

• Etiology Theories:
  • Smillie's Theory (1948): Proposed that the meniscus starts as a disc in the embryo and the center resorbs. Failure of resorption = Discoid. (Disproven. Embryonic meniscus is C-shaped from start).
  • Kaplan (1955): Suggested absence of posterior attachment causes hypermobility, leading to shape change. (Partially true for Type III).
  • Modern View: Genetic congenital malformation. Familial clusters reported.
• Histopathology:
  • Disorganized collagen bundles (loss of hoop stress pattern).
  • Intrameniscal mutinous degeneration.
  • Cyst formation (Parameniscal cysts are common).
  • Decreased vascularity in the central portion compared to normal meniscus.

Genetic Factors:

• Familial cases suggested autosomal dominant inheritance with variable penetrance.
• Bilateral cases are more likely in familial presentations.
• Genes involved? Unknown, possibly Collagen type I alpha 2 (COL1A2).

Associated Conditions:

• Fibular hemimelia? (Rare).
• Hypoplasia of the lateral femoral condyle (Result or Cause?).
• High riding fibular head.

Normal vs Discoid:

• Normal: C-shaped. Attached to tibia via coronary ligaments. Attached to femur via Meniscofemoral ligaments (Wrisberg/Humphry).

• Discoid: Block-like. Fills the lateral compartment.

  • Thickened (up to 8mm vs normal 3-4mm peripheral height).
  • Poor blood supply to the central 'block'.

• Vascular Zones:

  • Red-Red Zone: Peripheral 10-25%. Good healing potential. (Target for repair).
  • Red-White Zone: Middle third. Variable healing.
  • White-White Zone: Central avascular. No healing. (Target for saucerization).
  • In Discoid meniscus, the central "slab" is entirely White-White and prone to degeneration.
  • Inferior Lateral Genicular Artery: Supplies the peripheral rim. Preservation of this vessel during dissection for repair is crucial for healing.

Nerve Relations:

• Common Peroneal Nerve (CPN):
  • Runs posterior to the Biceps Femoris tendon at the joint line level.
  • Risk of injury during Inside-Out suturing if the retractor is not placed correctly (between Biceps and Capsule).
  • Risk during Outside-In technique? Less common but possible.
• Saphenous Nerve:
  • Risk during medial repairs (not applicable here unless concomitant pathology).

Ligamentous Attachments:

• Coronary Ligaments (Meniscotibial): Connect the inferior edge of the meniscus to the tibia. Prevent superior migration. Absent in Type III.
• Meniscofemoral Ligaments:
  • Ligament of Wrisberg: Posterior to PCL. Connects posterior horn of lateral meniscus to medial femoral condyle.
  • Ligament of Humphry: Anterior to PCL.
  • In Type III, Wrisberg is often hypertrophied and is the ONLY posterior restraint.

Imaging Gallery

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Radiographic Signs of Degeneration (Fairbank Changes): Fairbank (1948) described 3 classic signs of post-meniscectomy arthritis:

1. Ridge formation: An osteophyte ridge on the femoral condyle.
2. Flattening: Squaring of the femoral condyle.
3. Narrowing: Loss of joint space.

Long Term Prognosis:

• Symptomatic Relief: Excellent in the short/medium term.
• Degeneration: Even with saucerization, the remaining meniscus is ultrastructurally abnormal. Long term osteoarthritis rates are higher than normal population, but much lower than total meniscectomy.
• Total Meniscectomy: Results in severe lateral compartment overload and rapid degeneration (Fairbank changes). Avoid at all costs.
• Re-operation Rate: Approximately 10-20% (for recurrent tear or instability).

Epidemiology:

• Higher prevalence in Asian populations (Japan/Korea/China).
• Australia has diverse population, see it frequently.

Referral:

• Paediatric Orthopaedic Surgeon or Sports Knee Surgeon.
• Arthroscopy in children requires smaller instruments (2.7mm or 4mm scope depending on size).

Public System Coverage:

• Diagnostic knee arthroscopy covered under public hospital system
• Arthroscopic meniscectomy, repair, and saucerization fully funded
• Paediatric cases managed at tertiary children's hospitals