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.
The meniscus is a wedge shaped semi-circular pad made of fibro-cartilage that acts as the shock absorber of the knee.
The MCL is a ligament on the medial or inner side of the knee, that attaches between the femur (thigh bone) and the tibia (shin bone). It helps to stabilise the knee and prevent excessive side to side movement.
The LCL is a ligament on the lateral or outer side of the knee, that attaches between the femur (thigh bone) and the fibula. It helps to stabilise the knee and prevent excessive side to side movement.
The PCL can be ruptured when the top of the tibia is forced backward. This can happen due to a blow to the knee when bent (the classic examples are the knee cap smashing into the dashboard in a head on car accident, and the bent knee hitting down on to the ground in a rugby scrum). It can also happen when the knee is forced into hyperextension (knee pushed back more than straight). Occasionally it happens with a relatively minor injury (such as missing a step or kerb).
Posterolateral Corner Injury
PLC injury is a tear of one or more of those tendons and ligaments. It may occur with a twisting injury, a hyperextension injury (knee being pushed too far back past straight), a blow to the inside or front of the knee. It may also occur with dislocation of the knee when one or more of the other major ligaments (ACL, MCL or PCL) is also ruptured. There may also be injury to the nearby common peroneal nerve, causing weakness and numbness in the leg and foot.