Meniscal extrusion is when the meniscus becomes loose from its moorings and slips out over the edge of the top of the tibia.

This is a common complication of a root tear of the posterior horn of the meniscus.

meniscal extrusion    
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An explanation of what it means to have a 'floating' knee meniscus on MRI imaging.

An interpretation of a 2015 published medical article on meniscal root tears.

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CLINICAL PUBLICATIONS

The MRI findings of meniscal root tear of the medial meniscus: emphasis on coronal, sagittal and axial images. Choi SH, Bae S, Ji SK, Chang MJ. Knee Surg Sports Traumatol Arthrosc. 2012 Oct;20(10):2098-2103. Epub 2011 Nov 24.