Discoid Meniscus (Pediatric)

Synonyms
Popping knee syndrome

INTRODUCTION

Definition

Abnormal development of the meniscus that leads to a formation of a disc-shaped (and hypertrophic) meniscus rather than the normal crescent-shaped meniscus.

Epidemiology

  • Most common form is lateral discoid meniscus. Medial discoid meniscus is extremely rare. 
  • The incidence is 3-5% of population, but only 1 % of these are symptomatic.
  • The prevalence may be higher in Asia.
  • The condition is bilateral in 25 % of cases.

Pathoanatomy

  • The discoid meniscus may range from complete disc to a ring shaped meniscus.
  • The discoid variant is not normally found in the fetus, even though a congenital etiology for discoid menisci has been proposed.
  • Discoid snapping knee is usually caused by a discoid meniscus with a deficient menisco-tibial ligament.

Classification

Watanabe Classification:
  • Type 1: Incomplete covering of tibial plateau.
  • Type 2: Complete covering of tibial plateau.
  • Type 3 (Wrisberg variant): Lack of posterior meniscotibial attachment to tibia. This type is unstable. 
Classification based on stability:
  • Stable:
    • The complete type has normal peripheral attachments and normal mobility.
    • The meniscus is stable due to the presence of a posterior menisco-femoral ligament.
    • Symptoms are caused by either a tear or a posterior menisco-femoral detachment is usually present.
    • The treatment is arthroscopic partial menisectomy and meniscoplasty.
  • Unstable (wrisberg):
    • The meniscus is unstable and hypermobile due to lack of the posterior tibio-meniscal ligaments.
    • It has only one attachment posperiorly, the posterior meniscofemoral ligament.
    • The abnormal meniscus is pulled posteromedially into the intercondylar notch, instead of gliding forward, with knee extension. This is due to the action of the meniscofemoral ligaments.
    • The recommended treatment is total menisectomy since there is a lack of the posterior meniscal tibial attachments and there is an unstable posterior horn.

CLINICAL FEATURES

Symptoms

  • The patient may have pain and mechanical block with clicking/snapping, catching or locking. 
  • It often becomes symptomatic in adolescence.

Signs

Palpation: May have a palpable click at the knee.
Movement/ROM: The mechanical symptoms are most pronounced in extension.

DIAGNOSIS

X-ray

Projections: AP and lateral of the knee.
Findings
  • Might show widening (up to 11 mm) of the cartilage space (joint space) on the affected side.
  • Squaring of condyles (lateral condyle) with cupping of lateral tibial plateau may be visible.
  • The lateral intercondylar spine is often hypoplastic.

Magnetic Resonance Imaging (MRI)

Indication: Suspected symptomatic meniscal pathology.
Findings: MRI will show the meniscal body. The discoid meniscus is larger than usual. 
  • There are ≥ 3 sagittal images (5 mm) with meniscal tissue/continuity (“bow-tie sign”).
  • Sagittal MRI shows abnormally thick and flat meniscus. 
  • Coronal MRI shows a thick and flat meniscal tissue extending across entire lateral compartment. 
  • Observe that a false negative can occur with an unstable (Wrisberg) type of discoid meniscus which has a relative semilunar shape.

TREATMENT

Overview

Should be operated if torn and symptomatic. An asymptomatic discoid meniscus does not require treatment.
  • Pain, swelling, and a history of trauma are relative indications for arthroscopy.

Nonoperative Treatment

Indication: Asymptomatic discoid meniscus that is not torn.
Method: It should be observed.

Operative Treatment

Partial meniscectomy and saucerization
  • Indication: If the meniscus is symptomatic (pain and mechanical symptoms) and torn.
  • Method: Obtain anatomic looking meniscus with debridement.
    • Meniscoplasty: Resection of the discoid meniscus but leaving the peripheral rim intact. May be indicated in tears of a stable meniscus.
    • Complete menisectomy: Required wtih an unstable discoid meniscus.
Meniscal repair
  • Indication: If the meniscus is detached (Wrisberg variant).
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References