Discoid Meniscus

Epidemiology

  • Congenitally abnormal meniscus
  • Affects 1% of all lateral menisci
  • Only 3% of all discoid menisci are medial
  • Caused by abnormal formation of fibrocartilage in the mesenchyme

Pathology

  • Different types, variety of pathology, combinations of:
    • Lateral meniscotibial ligaments are absent, meniscus isn’t attached to tibial plateau
    • Meniscofemoral fibres are present
    • Ligament of Wrisberg attaches to PCL
    • Peripheral meniscus is thickened
    • Covers all or part of the plateau

History

  • often presents as a “snapping knee”
  • bilateral lateral discoid menisci in 20 %
    • Usually presents around age 8
    • Snapping, shift of tibial plateau
    • Visible/palpable mass, prominent anterolaterally with flexion, reduces with extension
    • May become associated with osteochondritis dissicans
  • Complete and incomplete types (see below)
    • Present as per meniscal tear
    • Often “young” for a meniscal tear – 14-16

Investigations

  • may have widened joint space
  • MRI indicated most of the time

Classification

Watanabe:

  • Type 1 – Complete type (stable) – most common
    • Discoid meniscus covers tibial plateau
    • Lateral ligaments are in tact, therefore stable
    • Treatment of symptomatic tears
      • Saucerization to stable peripheral rim of 6-8 mm
  • Type 2 – Incomplete type:
    • Similar to complete, covers less of the plateau
    • Treatment of symptomatic tears
      • Saucerization to stable peripheral rim of 6-8 mm
  • Type 3 – Wrisberg Ligament type (unstable):
    • Deficiency of posterior horn meniscal tibial ligaments
    • unstable and hypermobile posterior horn
    • on knee extension, abnormal meniscus is pulled posteromedially into the intercondylar notch (instead of gliding forward) due to the action of the meniscofemoral ligaments
    • probably responsible for the true “snapping knee;”
    • Treatment
      • try to repair to reattach the posterior horn
      • menisectomy may be needed for since lacks posterior meniscal tibial attachments & has unstable posterior horn
      • meniscal transplant? – not yet shown to be effective in skeletally immature patient

Treatment

  • If asymptomatic, incidental finding, not recommended to treat
  • Total meniscectomy is associated with 75% rate of degenerative tear
  • Contouring or sculpting of meniscal tissue is hopeful procedure, but no long term studies to advocate
  • Minor tears may heal, may need partial excision, repair
  • Treatment of the Wrisberg type is most challenging:
    • Thickness of cartilage rim can lead to recurrent snapping
    • Techniques for reattachment, contouring are still evolving
    • Meniscal transplant as a teenager may be best solution