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Background: Medial meniscus posterior root tear can result in medial meniscus extrusion. However, the severity of medial meniscus extrusion is different in each root tear patient. The purpose of this study was to identify the factors that contribute to the severity of medial meniscus extrusion with medial meniscus posterior root tear, such as duration of disease, the degree of arthritis—chondral wear, subchondral edema, osteophyte size, and Kellgren–Lawrence (K/L) grade—and mechanical alignment for appropriate treatment method. Methods: From January 2009 to August 2014, we retrospectively analyzed magnetic resonance imaging (MRI) and simple x-ray of 99 patients with medial meniscus posterior root tear. The duration of the disease was identified through retrospective chart review. The severity of medial meniscus extrusion, the presence of subchondral edema, the degree of chondral wear, and the size of the osteophyte were measured on MRI. K/L grade was confirmed on simple x-ray, and the mechanical axis was measured on whole extremity radiographs. Statistical analysis was performed by using bivariate correlation analysis and one-way analysis of variance. Results: The mean medial meniscus extrusion was 4.61 mm, and the mean duration of the disease was 15.52 months. The mean degree of chondral wear was 25.8%, and 63 out of 99 cases showed subchondral edema. The average alignment was 4.30 degrees, and the average size of the osteophyte was 1.48 mm. There were 40 cases (40.4%) with K/L grade I, 48 cases (48.5%) with grade II, 11 cases (11.1%) with grade III, and no cases with grade IV. In the group mean analysis between the K/L grade and the severity of medial meniscus extrusion, the average medial meniscus extrusions were 3.97 mm in grade I, 4.93 mm in grade II, and 5.59 mm in grade III. There was a statistical significance between the size of the osteophyte and the severity of medial meniscus extrusion (P = 0.000), K/L grade, and the severity of medial meniscus extrusion (P = 0.001). Conclusions: The severity of medial meniscus extrusion with medial meniscus posterior horn root tear is associated with the size of the osteophyte and K/L grade. [ABSTRACT FROM AUTHOR]