The knee joint, which appears like a simple hinge-joint, is one of the most complex joints. It is held together by four ligaments. A ligament is not only designed to hold bones together, it also provides the joint with stability and mobility to move.
There are two ligaments on both sides of your knee known as Collateral ligaments and two ligaments deep inside your knee. These latter cross over each other (hence the name cruciate) and are called Anterior Cruciate ligament (ACL) and Posterior Cruciate ligament (PCL).
The ACL prevents the femur moving forward and rotating abnormally on the tibia. The ACL is required for normal function of the knee. One of the main functions of the ACL is to provide stability during rotational movements such as turning, twisting and sidestepping.
An ACL tear is most often a sports-related injury. Most often ACL tears occur when pivoting or landing from a jump.