Case of the Month: September, 2010 Page 3

Answer

C. The most likdsely diagnosis is discoid lateral meniscus.

Findings

  • Frontal radiograph of the right knee demonstrates lateral joint space widening.
  • MRI of the right knee identifies meniscal tissue as the cause of lateral joint space widening. This is demonstrated on coronal MR image and confirmed on sagittal MR images demonstrating continuity of the anterior and posterior horns of the lateral meniscus on greater than three consecutive images.

Discoid Meniscus

  • Large congenitally dysplastic meniscus with loss of normal semi-lunar shape of  meniscus
  • Results from failure of fetal discoid form to involute
  • Meniscus is derived from mesenchyme that is initially disc shaped and subsequently regresses to assume semilunar configuration

Epidemiology

  • 1.5-4.5% prevalence in the United States
  • Up to 15% prevalence in Asia, with highest incidence in Japan
  • Lateral greater than medial (7:1)

Presentation-Children

  • Most commonly asymptomatic
  • Increased incidence of meniscal tear
  • Can be symptomatic with or without associated tear
  • Pain, clicking, snapping

Presentation-Adults

  • Most commonly symptomatic
  • Affects females in second decade and males in third decade
  • Peak incidence for injury or associated tear
  • Highest incidence in Asians

Imaging--Discoid Meniscus

  • Normal lateral meniscus should measure less than 12 mm from the capsular attachment to the free edge on central coronal image
  • Look for lack of meniscal tapering medially with or without extension of meniscus into notch
  • Meniscus measures 2 mm greater in craniocaudal dimension than medial meniscus

Complications

  •  Frequently demonstrates meniscal tear or intra-meniscal tear seen as increased signal intensity within the substance of the meniscus
  • Horizontal tear is most common
  • More prone to complex tears

Treatment

  • Conservative management
  • Partial meniscectomy with saucerization
  • Capsular reattachment of Wrisberg variant

Pearls

  • Continuity of anterior and posterior horns of meniscus on three or more consecutive sagittal MR images (5 mm slices)
  • Coronal imaging demonstrates slab like meniscus extending into intercondylar notch
  • Commonly associated with meniscal tear
  • Can be symptomatic with or without tear

 

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