A meniscal injury refers to damage to one of the two C-shaped cartilages which separate the bones in the knee joint. These cartilages act as shock absorbers within the knee to assist in cushioning forces.
Like the discs in the spine, the knee joint has a lateral and medial meniscus to protect and absorb pressure coming through the joint during jumping, landing or quick changes of direction. Long time meniscal tears are a potential risk factor for osteoarthritis in the knee (Khan et al., 2016).
Some actions can, especially when highly repetitive, however, can cause an overstretch of the meniscal tissue and therefore cause damage: a tear in the cartilage. Most commonly those tears are caused by sports typical movements, for example quick changes of directions: the knee stays grounded, whilst the body moves into a different direction (twisting in tennis to reach for a shot without moving the feet).
Management of the acute injury would be to follow RICE principles acutely and gentle strengthening and range of motion exercises. If the injury is unresolved arthroscopic (keyhole surgery) may be required to repair the meniscus.
Khan H.I. et al., 2016, Natural history and clinical significance of meniscal tears over 8 years in a midlife cohort, BMC Musculoskeletal Disorders, 5 (17), p: 4