In the photo here, you can see a model of the bones and ligaments of the knee. This is the right knee, in the bent position, looking from the front. The kneecap has been removed so you can see the structures inside. When a person stands up the two “knuckles” of the upper bone (the femur) roll into position so that the surfaces you see here are resting on the lower bone (the tibia). There are 4 main ligaments that attach the bones together:
The Lateral Collateral Ligament (LCL) runs from top to bottom along the outside of the knee joint
The Medial Collateral Ligament (MCL) runs from top to bottom along the inside of the knee joint
The Anterior Cruciate Ligament (ACL) runs diagonally inside the knee
The Posterior Cruciate Ligament (PCL) runs diagonally in the opposite direction behind ACL to form an “X”
Two other structures, the lateral meniscus and medial meniscus, form the cushion between the femur and the tibia. So, all these structures, combined with the surrounding muscles, tendons, bursa and capsule all help to allow the knee to bend and twist, but prevent it from moving too much. Excessive twisting or trauma in a particular direction can damage the structures that are trying to hold it back. The ACL can get torn or ruptured with sudden, traumatic twisting. Blunt force from either side can damage the collateral ligaments. The meniscus can get pinched, the bursa and capsule can become inflamed.