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.
Meniscal extrusion and knee arthritis
Extrusion of the meniscus may be a result of a tear of the meniscus near the root or an avulsion (pulling away) of the root from the underlying tibia bone. As the meniscus slips over the edge of the tibia, it becomes totally incompetent as a shock absorber, and the cartilage covering the bones may rapidly become stressed and damaged, resulting in arthritis. Arthritis can develop very quickly as the joint cartilage of femur and tibia break down.
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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.
Medial Meniscus Root Tear in the Middle Aged Patient: A Case Based Review. Carreau JH, Sitton SE and Bollier M. Iowa Orthop J. 2017; 37: 123–132..