DefinitionAbnormal development of the meniscus that leads to a formation of a disc-shaped (and hypertrophic) meniscus rather than the normal crescent-shaped meniscus.
- 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.
- 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.
- 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.
- 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.
- The patient may have pain and mechanical block with clicking/snapping, catching or locking.
- It often becomes symptomatic in adolescence.
SignsPalpation: May have a palpable click at the knee.
Movement/ROM: The mechanical symptoms are most pronounced in extension.
X-rayProjections: AP and lateral of the knee.
- 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.
OverviewShould 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 TreatmentIndication: Asymptomatic discoid meniscus that is not torn.
Method: It should be observed.
Operative TreatmentPartial 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.
- Indication: If the meniscus is detached (Wrisberg variant).