There are 2 menisci in the knee; one in the inside of the joint (medial) and one on the outside (lateral). They are important structures, as they bear load, shielding the joint surfaces for stress and making the joint more congruent.
During the knee’s movement, they glide backwards and forward with the femur (thigh bone) with respect to the tibia (shin bone) as the knee flexes and extends. During this movement, the edge of the meniscus may get caught between the two bones, tearing it. This both defunctions the torn portion of the meniscus and results in a loose flap which may get caught in the joint causing pain. If the flap is big enough, the joint will lock.
Meniscus was previously thought to be largely irreparable, although with current techniques and specific surgical training our ability to repair meniscal tears has greatly increased.
If symptoms persist, patients are best treated with an arthroscopy and either a partial meniscectomy or repair, depending on the tear pattern and the age of the patient.