Orthopaedic Surgery - Discoid Meniscus

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Basics

A discoid meniscus is an abnormally broad, thickened, disc- or pancake-shaped meniscus rather than the normal semilunar structure.

It most commonly involves the lateral meniscus, while medial discoid menisci are much less common.

The condition may produce popping, snapping, locking, or pain during childhood or early adulthood, although many individuals remain asymptomatic.

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Laterality

Approximately 75% of cases are unilateral, while around 25% are bilateral.

Because bilateral involvement is possible, the contralateral knee may also require evaluation if symptoms develop.

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Watanabe Classification

Discoid lateral menisci are traditionally divided into three types according to the Watanabe classification.

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Type 1: Complete

A complete discoid meniscus covers essentially the entire lateral tibial plateau.

The meniscus is markedly wider than normal and forms a nearly complete disc over the lateral compartment.

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Type 2: Incomplete

An incomplete discoid meniscus is larger and broader than a normal meniscus but does not completely cover the lateral tibial plateau.

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Type 3: Wrisberg Variant

The Wrisberg-type discoid meniscus has a thickened posterior horn and lacks the normal posterior meniscotibial attachment to the tibia.

Because of this absent attachment, the meniscus is excessively mobile and unstable.

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Epidemiology

The reported incidence is approximately 3–5% in the United States, although prevalence is considerably higher in some Asian populations and has been reported at more than 20% in Japan.

The true incidence is probably higher than recognized because many affected individuals never develop symptoms.

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Etiology

A discoid meniscus is considered a developmental or anatomic abnormality of the meniscus.

Genetic or familial influences may also contribute, although the exact mechanism remains uncertain.

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Diagnosis

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Clinical Presentation

Presentation varies considerably depending on the type of discoid meniscus, its stability, and whether a tear is present.

Some patients are entirely asymptomatic and the abnormality is discovered incidentally.

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Snapping Knee Syndrome

The classic presentation is the so-called snapping knee syndrome, characterized by a palpable or audible snap or clunk near the terminal limits of knee flexion or extension.

This is particularly associated with the unstable Wrisberg variant.

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Snapping in Other Variants

Snapping may also occur in an otherwise stable discoid meniscus if it becomes torn or develops secondary instability.

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Pain and Mechanical Symptoms

Symptomatic patients may report pain, clicking, popping, swelling, locking, or intermittent inability to fully move the knee.

Mechanical symptoms are especially suggestive of an associated tear or unstable meniscal tissue.

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Onset

Symptoms commonly begin gradually without a specific injury.

When symptoms appear suddenly after trauma, an acute tear of the discoid meniscus should be suspected.

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Physical Examination

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Effusion

A knee joint effusion may be present, particularly when the discoid meniscus has torn.

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Anterolateral Bulge

With the knee in full flexion, a visible or palpable anterolateral bulge may occasionally be present because of the increased meniscal tissue.

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Joint-Line Tenderness

Tenderness may be present along the lateral joint line.

It is often mild in an intact discoid meniscus but may become more pronounced when a tear is present.

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Mechanical Click

A pop or click may be appreciated during meniscal provocative testing, particularly the McMurray maneuver.

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Loss of Extension

Some patients have a mechanical block to full knee extension.

Others may show apprehension or discomfort as the knee approaches complete extension.

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McMurray Test

The McMurray test is performed by taking the knee into maximal flexion, applying rotation, and progressively extending the knee.

A positive test consists of pain, clicking, or popping along the lateral joint line.

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Imaging

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Plain Radiographs

Radiographs are not diagnostic in every case, but AP, lateral, tunnel, and skyline views may reveal secondary skeletal features associated with a discoid lateral meniscus.

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Radiographic Findings

Possible findings include widening of the lateral joint space, lateral joint-line lipping, cupping of the lateral tibial plateau, flattening or squaring of the lateral femoral condyle, hypoplasia of the tibial eminence, and elevation of the fibular head.

These findings may suggest the diagnosis but are not always present.

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MRI

MRI is the preferred imaging study for confirming the morphology of the meniscus and identifying associated tears or instability.

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Sagittal MRI Findings

A discoid meniscus is suggested when continuity between the anterior and posterior horns is visible on three or more consecutive 5-mm sagittal images.

This reflects the abnormal width of the meniscus.

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Coronal MRI Findings

On coronal imaging, the meniscus may demonstrate a broad block-like or bow-tie appearance, with increased meniscal width across the lateral compartment.

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Pathological Findings

The affected meniscus is disc-shaped rather than semilunar.

Histologically, discoid menisci demonstrate reduced collagen content and abnormal orientation of collagen fibers, which may contribute to their increased susceptibility to tearing.

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Stable Variant

A stable discoid meniscus retains normal peripheral and tibial ligamentous attachments.

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Unstable Variant

The unstable variant lacks normal posterolateral meniscotibial attachments, producing excessive mobility of the posterior meniscus.

This is characteristic of the Wrisberg type.

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Differential Diagnosis

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Acute Meniscal Tear

A normal-shaped meniscus with an acute tear can produce similar pain, clicking, locking, and joint-line tenderness.

MRI helps distinguish the underlying anatomy.

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Meniscal Cyst

A meniscal cyst may cause joint-line swelling and pain and is often associated with a meniscal tear.

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Osteochondritis Dissecans

Osteochondritis dissecans may cause knee pain, swelling, catching, or locking and should be considered in younger patients with mechanical symptoms.

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Osteochondral Fracture

An osteochondral fracture can produce acute pain and mechanical blockage, especially after trauma.

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Physeal Fracture

In skeletally immature patients, physeal injury may mimic internal derangement of the knee.

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Tibial Eminence Fracture

A fracture of the tibial eminence can produce pain, swelling, and extension loss and may resemble an ACL-associated injury.

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Anterior Cruciate Ligament Tear

ACL injury may produce swelling, instability, and mechanical symptoms and should be considered particularly after acute trauma.

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Treatment

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General Principles

Many discoid menisci are asymptomatic and require no treatment.

Observation is appropriate when the patient has no pain, locking, swelling, or functional impairment.

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Symptomatic Disease

Patients with persistent pain, locking, swelling, popping, or instability may require surgical treatment.

The operative strategy depends on whether the meniscus is torn and whether it is mechanically stable.

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Nonoperative Treatment

Nonoperative management has a limited role in a persistently symptomatic discoid meniscus.

A short period of immobilization or activity modification may be attempted in patients with acute-onset symptoms, particularly when the diagnosis remains uncertain.

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Return to Activity

Patients may gradually resume normal activities once symptoms have resolved and full motion and function have returned.

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Surgery

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General Surgical Principle

Modern treatment emphasizes preservation of as much functional meniscal tissue as possible.

Total meniscectomy should be avoided whenever feasible because removal of the entire meniscus substantially increases the risk of later osteoarthritis.

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Stable Discoid Meniscus

For a symptomatic stable discoid meniscus, the preferred procedure is arthroscopic partial meniscectomy with saucerization.

Excess central meniscal tissue is removed to recreate a more normal semilunar shape while preserving a stable peripheral rim.

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Meniscal Repair

If a meniscal tear or unstable peripheral attachment is present, repair may be performed at the same time as saucerization.

Preservation of the peripheral meniscus helps maintain normal load transmission across the knee.

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Importance of Meniscal Preservation

Maintaining a functional meniscus allows continued shock absorption, load distribution, and joint stabilization.

This may decrease the risk or delay the development of degenerative osteoarthritis compared with complete meniscectomy.

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Unstable Discoid Meniscus

An unstable meniscus is generally treated with arthroscopic saucerization and stabilization of the remaining peripheral meniscus.

If the posterior attachment is deficient, it may be reattached to create a stable and functional meniscal rim.

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Complete Meniscectomy

Complete meniscectomy is reserved for situations in which the meniscus is not salvageable.

Possible indications include extensive degenerative change, a massive irreparable tear, or severely abnormal meniscal tissue that cannot be reconstructed.

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Follow-Up

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Prognosis

Patients with a stable discoid meniscus generally have good outcomes following appropriately performed saucerization.

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Effect of Age

Younger patients often have better postoperative results than older patients, particularly when the meniscal tissue can be preserved before substantial degenerative change develops.

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Extent of Meniscal Resection

Removal of a larger amount of meniscal tissue is associated with a greater subsequent risk of osteoarthritis.

For this reason, preservation of a stable peripheral rim is preferred whenever technically possible.

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Partial Versus Total Meniscectomy

Partial meniscectomy and saucerization generally produce better clinical and radiographic outcomes than total meniscectomy at both short- and long-term follow-up.

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Complications

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Osteoarthritis

The major long-term complication after excessive meniscal resection is degenerative osteoarthritis of the involved compartment.

This risk is particularly high after complete meniscectomy.

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Recurrent Meniscal Tear

Even after saucerization, the remaining meniscus retains abnormal tissue architecture.

Therefore, the residual discoid meniscus remains more susceptible to future tearing than a normal meniscus.

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Persistent or Recurrent Symptoms

Pain, popping, or locking may recur if residual unstable tissue or a new tear develops.

Repeat arthroscopic evaluation may occasionally be required.

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Patient Monitoring

Patients should be followed until pain, swelling, locking, and mechanical symptoms have resolved and knee motion has returned satisfactorily.

After surgery, follow-up should also assess meniscal stability, range of motion, return to activity, recurrent tearing, and the development of degenerative changes.