The Discoid Meniscus

Courtesy: Dr Guri Ranum Ekas, Associate Professor, University of Oslo, Norway

General Overview

• Discoid meniscus is a congenital anatomical variation of the meniscus.

• It most commonly involves the lateral meniscus of the knee.

• The lateral meniscus normally exhibits greater mobility during knee flexion and extension.

• Blood supply to the meniscus is limited and confined primarily to the peripheral zone.

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CONGENITAL DISCOID MENISCUS

• A developmental variation present from birth.

• May account for up to 25 percent of meniscal surgeries in children.

• More prone to:

  • Meniscal tears

  • Mechanical symptoms

  • Locking of the knee joint

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WHAT IS A DISCOID MENISCUS?

• Predominantly affects the lateral meniscus.

• Frequently bilateral.

• Characterized by:

  • Abnormal meniscal shape

  • Increased thickness

  • Possible instability due to abnormal attachments

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INCIDENCE

• True incidence remains unknown.

• Estimated prevalence ranges from 3 to 15 percent.

• Bilateral involvement is common.

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MORPHOLOGY

• Meniscus is:

  • Thicker than normal

  • Wider than normal

• Covers a larger portion of the lateral tibial plateau.

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MENISCAL INSTABILITY

• Instability results from inadequate attachment to the joint capsule or tibia.

• Instability may occur:

  • Anteriorly

  • Posteriorly (Wrisberg type)

  • In the midportion

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STRUCTURAL CHARACTERISTICS

• Reduced vascularity compared to a normal meniscus.

• Fewer collagen fibers with disorganized orientation.

• Increased susceptibility to rupture, especially horizontal tears.

• Mucoid degeneration is frequently present.

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CLASSIFICATION

Watanabe Classification (1969)

Based on arthroscopic assessment of morphology and stability:

• Type 1: Stable, complete discoid meniscus

• Type 2: Stable, incomplete discoid meniscus

• Type 3: Unstable discoid meniscus (Wrisberg type)

  • Posterior attachment is only via the Wrisberg ligament

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INCREASED RISK OF MENISCAL RUPTURE

• Risk is increased up to 30 times compared to a normal meniscus.

• Contributing factors include:

  • Increased thickness and width

  • Inferior meniscal tissue quality

  • Abnormal collagen orientation

  • Reduced vascularity

  • Mucoid degeneration

  • Mechanical instability

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CLINICAL SYMPTOMS

• Mechanical symptoms such as:

  • Popping

  • Snapping

  • Locking

• Pain and swelling may be present.

• Many patients remain asymptomatic unless rupture or instability occurs.

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INSTABILITY AND DISLOCATION

• An unstable discoid meniscus may dislocate intra-articularly during knee motion.

• Displacement may occur:

  • Toward the intercondylar notch

  • Toward the peripheral compartment

• May result in protrusion and joint locking.

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AGE AT PRESENTATION

• Children younger than 10 years:

  • Typically present with instability

  • Often bilateral

  • Usually not trauma related

• Adolescents:

  • Symptoms often follow sports activities

• Adults:

  • Symptoms typically follow minor trauma

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CLINICAL EXAMINATION

• May reveal:

  • Lateral joint line resistance

  • Palpable or audible clunk near full knee extension

  • Snapping knee phenomenon

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RADIOGRAPHIC EVALUATION

Plain Radiography

• Used as an initial investigation and for differential diagnosis.

• Indirect signs may include:

  • Block-shaped lateral femoral condyle

  • Hypoplastic lateral tibial spine

  • Increased concavity of the lateral tibial plateau

  • Increased lateral joint space exceeding 11 millimeters

Magnetic Resonance Imaging

• Confirms diagnosis.

• Classic findings include the bow tie sign.

• Allows assessment of associated meniscal tears and cartilage injury.

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TREATMENT PRINCIPLES

• No evidence supports prophylactic surgery in asymptomatic patients.

• Initial rehabilitation is recommended for mild symptoms.

• Surgical intervention is indicated for:

  • Persistent symptoms

  • Mechanical locking

• A locked knee requires urgent surgical management.

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SURGICAL MANAGEMENT OF SYMPTOMATIC DISCOID MENISCUS

Saucerization (Meniscal Sculpturing)

• Removal of the central portion of the meniscus.

• Preservation of a stable peripheral meniscal rim.

• Repair of peripheral meniscal tears.

• Central tears are generally not repairable.

• Unstable meniscus should be stabilized.

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INTRAOPERATIVE STEPS

• Diagnostic arthroscopy to assess:

  • Meniscal width (complete or incomplete)

  • Meniscal height (normal or abnormal)

  • Peripheral stability

• Saucerization to restore near-normal meniscal contour.

• Suture repair of repairable peripheral tears.

• Assessment and stabilization of peripheral attachments if instability is present.

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SURGICAL CAUTION

• Avoid excessive meniscal resection, especially in young children.

• Preserve meniscal root attachments at all times.

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POSTOPERATIVE MANAGEMENT

• Saucerization without repair:

  • Weight bearing as tolerated

• Saucerization with repair:

  • Partial weight bearing for 6 weeks

  • Use of crutches and optional orthosis

• Structured physiotherapy program

• Return to sports:

  • Individualized decision

  • Not earlier than 4 months postoperatively

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PROGNOSIS

• Total meniscectomy is associated with early onset osteoarthritis.

• Saucerization with meniscal preservation provides:

  • Good clinical outcomes

  • Sustained results up to 10 years

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SECONDARY COMPLICATIONS

• Increased risk of articular cartilage damage.

• Higher incidence of lateral femoral condyle osteochondritis dissecans.