Discoid meniscus

Discoid menisci are those that have a body that is too wide, usually affecting the lateral meniscus. They are incidentally found in 3-5% of knee MRI examinations. 

Epidemiology

Discoid menisci are congenital, frequently bilateral (up to 50%) and have been reported in twins, although no genetic locus has been identified ². There is a higher prevalence in Asians without any gender predilection ⁷. Lateral discoid meniscus is far more common than medial discoid meniscus, with the latter being rare. 

Clinical presentation

Although frequently asymptomatic, discoid menisci are prone to cystic degeneration with subsequent tear. Clinical presentation may therefore be either incidental or with pain, locking or a 'clunk'.

Pathology

Discoid menisci have decreased collagen fibers and loss of normal collagen orientation, which predisposes them to intrameniscal mucoid degeneration ⁷. 

Classification

Classification is based on the degree of peripheral attachments to the tibial plateau, and the shape of the meniscus itself:

• complete vs incomplete
  • 80% coverage of the tibial plateau is often used as the cut-off between incomplete and complete ⁷
• stable vs unstable
  • stable: normal peripheral attachments with an intact posterior meniscofemoral ligament
  • unstable (also known as a Wrisberg variant): lack or tear of a posterior meniscocapsular (in particular meniscopopliteal) ligaments with an attachment only from the meniscofemoral ligament of Wrisberg ⁴
• anterior or posterior mega horn

Radiographic features

Plain radiograph

Radiographs may well suggest the diagnosis with the widening of the lateral joint space and cupping of the lateral tibial plateau, which is normally flat or even slightly convex. Additionally, there may be associated hypoplasia of the lateral tibial spine.

MRI

On coronal imaging, a meniscal body width of 15 mm or more is typically considered diagnostic of discoid morphology. Alternatively, a ratio of the minimal meniscal width to the maximal tibial width of more than 20% may be used ^7,8. 

On sagittal imaging, the body of the lateral meniscus is normally only seen on two adjacent slices. A discoid meniscus is usually present if the meniscal body is seen on three or more standard sagittal slices - the opposite of the absent bow tie sign.

Treatment and prognosis

Ideally, the meniscus is preserved with conservative management. If this is unsuccessful, then partial or total resection may be carried out. 

Complications

• meniscal tears
• intrasubstance mucinous degeneration
• early bony degenerative change