The most commonly injured knee ligament is the anterior cruciate ligament, also referred to as the ACL. The ACL is one of four primary ligaments in your knee, and it connects your femur (thigh bone) to your tibia (shin bone). A torn ACL injury will not only be very painful when you put any weight on your leg, but will cause your knee to swell up and leave you unstable. ACL tears most often occur while playing sports, due to sudden stops or changes in direction. And because many of the people who suffer ACL injuries have active lifestyles, one pressing question many of them ask is: how long will I be out of commission?