Medial meniscus tears, also called medial meniscal tears, occur when your meniscus inside your knee is torn partially or completely. You have two menisci in each knee: the medial meniscus and the lateral meniscus.
Medial Meniscus Tears - How They Occur - Mechanism of Injury
Most medial meniscus tears happen in athletic people involved with too much bending and rotation of their knees. Risk of meniscal injury increases when a force is exerted on your outer knee.
You can tear your medial meniscus when your knee is bent and you turn your femur (thigh bone) inwards (internal rotation) while your foot is planted on the floor. Medial meniscus tear can also happen when there is a direct blow to the outer side of your knee while it is bent. Sometimes, other structures of your knee may be damaged as well, such as the anterior cruciate ligament or the medial collateral ligament.
Magee, David. (1997). Orthopedic Physical Assessment (3rd ed). Philadelphia, PA: W.B. Saunders Company.
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS). (2010). Knee Problems. Retrieved from http://www.niams.nih.gov/Health_Info/Knee_Problems/default.asp