Discoid Meniscus

A discoid meniscus is an anatomical variant in which the meniscus is abnormally thick, disc-shaped (covering most or all of the tibial plateau) rather than the normal crescent or C-shape. The lateral meniscus is affected in the vast majority of cases; medial discoid meniscus is exceedingly rare. Discoid meniscus occurs in approximately 3–5% of the Western population and up to 15% in Asian populations. It is a common incidental finding but can become symptomatic — presenting with mechanical symptoms (clicking, locking, snapping) or pain, particularly in children and young adults.

• Bilateral in approximately 15–20% of cases; the opposite knee should always be assessed or imaged when a discoid meniscus is identified
• Watanabe classification (the most widely used): Type I — complete (covers the entire lateral tibial plateau; normal peripheral attachments); Type II — incomplete (partial coverage, more than normal but not complete; normal peripheral attachments); Type III — Wrisberg variant (the posterior meniscofemoral ligament of Wrisberg is the only posterior attachment; the normal coronary ligament and posterior capsular attachments are absent; this type is hypermobile — it lacks the posterior capsular anchor — and is always symptomatic, producing the classic `snapping knee syndrome`)

• Many discoid menisci are asymptomatic and are discovered incidentally on MRI performed for other reasons; a stable discoid meniscus without a tear does not require treatment
• Snapping knee syndrome (Type III Wrisberg variant): the most dramatic presentation; an audible and palpable `clunk` or `snap` with knee flexion-extension, typically in a young child (aged 5–10 years); the unstable posterior aspect of the meniscus flips over the lateral femoral condyle as the knee moves through its range; the snap is often visible and felt by parents; the child may have difficulty fully extending the knee; this presentation is almost pathognomonic for Type III discoid meniscus; examination reveals a palpable lateral clunk at approximately 20–40° of extension
• Symptomatic stable discoid meniscus with tear: lateral knee pain (often in adolescents or young adults); mechanical symptoms (clicking, locking, catching); effusion; tenderness at the lateral joint line; the thicker, larger discoid meniscus is more vulnerable to tears — horizontal cleavage tears, radial tears, or complex tears are common; presentation may follow minor trauma or occur spontaneously
• Examination: lateral joint line tenderness; McMurray test (lateral compartment variant — valgus stress with external rotation and flexion-extension); Thessaly test; fixed flexion deformity in severe Type III cases (the flipped meniscus blocks full extension)

• MRI: the investigation of choice; diagnostic criteria on coronal MRI — a meniscus is defined as discoid if it has >3 consecutive sagittal slices with >5 mm of meniscal tissue (the `bow-tie sign`); normal menisci appear as bow-tie shapes on only 2 consecutive sagittal images; discoid menisci are larger and thicker; MRI identifies the type (complete, incomplete), any tears (horizontal cleavage, radial, complex), and the status of the posterior capsular attachments; the Wrisberg variant may be difficult to distinguish from complete on standard MRI — the absence of posterior capsular attachments is the key finding
• Plain radiographs: often normal; may show lateral joint space widening (due to the thicker lateral meniscus elevating the lateral tibial plateau relative to the femoral condyle); `squared-off` lateral femoral condyle; cupping of the lateral tibial plateau; these are subtle findings — X-rays are not diagnostic
• Arthroscopy: the gold standard for definitive classification and treatment; allows direct visualisation of the meniscal size, shape, attachments, and any tears

• Asymptomatic discoid meniscus (incidental finding): no treatment required; observe; counsel the patient and family about the higher risk of meniscal tears compared to a normal meniscus; avoid unnecessary arthroscopy
• Symptomatic discoid meniscus — saucerisation: the arthroscopic procedure of choice for a symptomatic stable discoid meniscus (Types I and II); the central portion of the meniscus is resected using arthroscopic shavers and biters, converting the disc into a normal crescent shape (approximately 6–8 mm peripheral rim is preserved); the goal is to preserve as much peripheral meniscus as possible while relieving symptoms; any associated tear is simultaneously debrided or repaired; saucerisation is superior to total meniscectomy — the preserved peripheral rim provides some load-bearing function
• Wrisberg variant (Type III) management: saucerisation alone is insufficient for the Type III Wrisberg variant — the underlying instability (absent posterior attachments) must also be addressed; after saucerisation, the posterior horn is stabilised by suturing it to the posterior capsule (peripheral repair); this restores the missing posterior capsular attachment and prevents recurrent instability; failure to address the posterior instability results in recurrent snapping and poor outcomes
• Meniscal repair: if a repairable tear is identified at the periphery of the discoid meniscus (within the vascular zone — outer one-third), repair should be performed after or concurrent with saucerisation; horizontal cleavage tears in the avascular zone are not suitable for repair and are debrided
• Total meniscectomy: historically performed but now abandoned — results in progressive lateral compartment OA; avoided in all current practice

• Long-term outcomes of discoid meniscus treatment: preservation of as much meniscal tissue as possible is the goal; saucerisation with peripheral preservation produces better long-term outcomes than total meniscectomy; studies show that extensive resection accelerates lateral compartment OA in proportion to the amount of meniscus removed; young patients with discoid menisci that require surgery are at long-term risk of lateral compartment OA — they require long-term follow-up; meniscal allograft transplantation may be an option in future years if severe lateral compartment OA develops in a young patient after prior meniscectomy
• Bilateral discoid meniscus: always consider the contralateral knee; if a symptomatic discoid is found in one knee, MRI of the opposite knee is warranted to identify a contralateral discoid before the patient becomes symptomatic; bilateral surgical procedures are staged (not simultaneous) to allow rehabilitation of one knee before treating the other
• Horizontal cleavage tears in discoid menisci: the most common tear pattern in discoid menisci; the thick meniscus is vulnerable to horizontal delamination through its substance; these tears extend from the free edge into the body; they are typically in the avascular zone and are not repairable; debridement is the treatment; if the peripheral rim is intact and well-vascularised, saucerisation preserving the peripheral rim can leave good tissue behind; excessively aggressive resection of the peripheral rim during treatment of a horizontal tear worsens lateral compartment loading

• Discoid meniscus: lateral (almost always); 3–5% Western, up to 15% Asian populations; Watanabe Types I (complete), II (incomplete), III (Wrisberg variant — unstable, no posterior attachments)
• Wrisberg variant (Type III): absent posterior coronary ligament; ALWAYS symptomatic; snapping knee syndrome — audible/palpable clunk with knee motion; typically children 5–10 years
• MRI bow-tie sign: >3 consecutive sagittal slices with >5 mm meniscal tissue = discoid; normal meniscus = 2 consecutive bow-tie slices
• X-ray: lateral joint space widening; squared-off lateral femoral condyle; cupping of lateral tibial plateau — subtle, not diagnostic
• Treatment: saucerisation — resect central disc, preserve 6–8 mm peripheral rim; converts disc to crescent shape; Type III MUST also have posterior capsular repair to address instability; saucerisation alone insufficient for Wrisberg variant
• Asymptomatic discoid meniscus: no treatment; observe; higher risk of future tears vs normal meniscus; counsel patient
• Total meniscectomy: abandoned — accelerates lateral compartment OA; never the treatment of choice in modern practice
• Horizontal cleavage tear: most common tear in discoid meniscus; avascular zone; debride — not repairable; preserve peripheral rim during saucerisation
• Bilateral in 15–20%: image the contralateral knee; stage bilateral surgical procedures separately
• Long-term OA risk: proportional to volume of meniscus removed; preserve as much peripheral rim as possible; long-term follow-up required in young patients