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.
In a normal joint the end of the bone is covered by a layer of cartilage which is very strong and very smooth. The joint is lubricated by synovial fluid and the combination of cartilage and the lubricating fluid allows the joint to move with minimal friction which allows smooth pain free motion over a long period of time.
Anterior Knee Pain
Anterior knee pain is pain that is felt in the front of the knee, usually behind the patella (knee cap). It is commonly seen in people in their teens or early twenties and tends to affect females more than males.
Most commonly, the pain originates in the patellofemoral joint, the joint made up of the patella and the groove in the end of the femur (thigh) bone that the patella sits in.
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.