D2I2.
congenital

Discoid meniscus

Discoid meniscus is a rare human anatomic that usually affects the meniscus of the knee. Usually a person with this anomaly has no complaints; however, it may present as pain, swelling, or a snapping sound heard from the affected knee. Strong suggestive findings on magnetic resonance includes a thickened meniscal body seen on more than two contiguous sagittal slices.

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Plain-language summary adapted from Wikipedia. Not medical advice.