Discoid Meniscus (Pediatric)

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).