The ACL is often ruptured during a pivoting or twisting movement, usually while playing sport. It can also be caused by a blow to the outside of the knee when the knee is bent. Sometimes, it is caused by hyper-extending (over straightening) the knee, such as when jumping up and landing awkwardly.
The knee joint is made up of the femur (thigh bone) on the tibia (shin bone) with the patella (knee cap) gliding over the end of the femur. Usually, the patella sits in the trochlear groove, which is a groove in the end of the femur. This forms the patellofemoral joint. Dislocation of the patella occurs when the patella moves out of the patellofemoral groove.
Most joints are lined by a layer of cartilage. In the knee and the hip this cartilage is yaline articular cartilage which is a very smooth and strong type of cartilage. The cartilage is lubricated by a thin layer of fluid (synovial fluid) and this produces a joint surface that has minimal friction and is able to bear a very heavy load for many years.