Discoid Meniscus Workup

Updated: Sep 27, 2021
  • Author: Ralph DiLibero, MD; Chief Editor: Thomas M DeBerardino, MD  more...
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Workup

Radiography

The widened and thickened discoid meniscus may be demonstrated on routine radiography of the knee. Radiography may reveal any combination of widening of the lateral joint clear space and cupping (see the images below). Cupping is a reversal of the normally flat-to-convex bony shape of the lateral tibial plateau into a more concave shape.

Radiograph of 8-year-old child with bilateral disc Radiograph of 8-year-old child with bilateral discoid menisci, diagnosis confirmed by MRI. Patient is only symptomatic on left side. Patient underwent arthroscopy and partial meniscectomy and is now asymptomatic. Image courtesy of Dennis P Grogan, MD.
Knee radiograph of 17-year-old athlete with a disc Knee radiograph of 17-year-old athlete with a discoid lateral meniscus. Lateral joint space is widened, and tibial plateau has flattened appearance. Image courtesy of Robert D Bronstein, MD.
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Magnetic Resonance Imaging

The positive predictive value (PPV) of magnetic resonance imaging (MRI) for demonstrating a discoid meniscus tear is approximately 57%, whereas the PPV for predicting a discoid meniscus is approximately 92%. [12]  (See the images below.) The PPV is determined by creating a fraction in which the sum of the number of true- and false-positive results is the denominator and the number of true-positive results the numerator, then multiplying that fraction by 100%.

MRI scan of typical discoid meniscus. Image courte MRI scan of typical discoid meniscus. Image courtesy of William Morrison, MD.
MRI scan of typical discoid meniscus. Image courte MRI scan of typical discoid meniscus. Image courtesy of William Morrison, MD.
Coronal MRI scan demonstrating complete discoid me Coronal MRI scan demonstrating complete discoid meniscus (arrow). Image courtesy of Robert D Bronstein, MD.

Larger meniscal size may be one of the main risk factors predisposing to discoid meniscus tears. [22]

MRI is the modality of choice for evaluating a discoid meniscus before surgery. [7, 8]  The most common diagnostic finding is that of a "bow-tie" sign, a viewing of the anterior and the posterior meniscal sections connected together on more than two sagittal MRI serial images.

A discoid lateral meniscus commonly occurs bilaterally; in patients who are symptomatic, an intrameniscal signal is also commonly found.

A study by Hashimoto et al found that two-position MRI effectively visualized the snapping phenomenon and that meniscal shift seen with this modality was an important predictor of overall peripheral rim instability (PRI), anterior PRI, and posterior PRI in no-shift-type complete discoid lateral mensicus. [23]

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Procedures

When the McMurray test is performed on a patient with a discoid meniscus, a loud click or snap is both felt and heard. For more information on the McMurray test, please see Meniscal Tears on MRI.

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