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.

tear of the posterior root of the medial meniscus meniscal extrusion Arthritis can develop very quickly as the joint cartilage of femur and tibia break down.
Tear of the posterior root of the medial meniscus. The meniscus becomes incompetent at absorbing vertical forces. The vertical pressure pushes it over the edge of the tibial plateau, leaving the bones in contact with one another.  
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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

Medial Meniscus Extrusion on Knee MRI: Is Extent Associated with Severity of Degeneration or Type of Tear? Costa CR1, Morrison WB and Carrino JA. American Journal of Roentgenology. Volume 183, Issue 1.

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.


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