There are two large ligaments inside your knee each about the size of your little finger that cross deep inside the joint. They are called the ACL (anterior cruciate ligament) and PCL (posterior cruciate ligament) and go from the femur (thigh bone) to the tibia (shin bone). They serve to stabilize the knee and allow it to glide through a smooth range of motion as you bend and straighten the leg. The ACL is the ligament in the front and the one most commonly injured.
ACL is one of the important ligaments present in the knee joint. It is present within the joint in the anterior (front) aspect of knee joint. It prevents the leg bone (tibia) from moving forwards.
Without the ACL the knee is less stable. Without its stabilizing influence, the knee can buckle suddenly as it is used and this leads to meniscal injury, cartilage damage and eventually to arthritis. Usually there is no problem for "straight-ahead" activities such as walking or jogging. However, it can be a big problem for athletic, daily, recreational and work-related activities involving twisting, pivoting, jumping, climbing stairs or suddenly changing direction. Examples of these activities include most sports (especially basketball, football, volleyball, etc.) and many jobs (such as carpentry, warehouse, refinery, etc.).