A ligament is a band of fibrous tissue that connects one bone to another at a joint. Ligaments act as a check to reign in and limit motion and helps hold the joint together. In the knee, ligaments are susceptible to injury because the joint has only one main plane of motion and any other motion can cause the ligaments to tear.
There are three generalized ligament injuries that can happen to the knee.
The ACL (Anterior Cruciate Ligament) connects the femur to the tibia in the lower leg. The ACL protects the knee joint by limiting hyperextension from a forceful straightening of the leg and assists with lateral restriction. If the leg extends too far forward, such as an athlete having a collision when the knee is already in an extended position, the ACL can tear. Sports like basketball, soccer, football, or gymnastics where sudden stops or collisions are possible, commonly cause this injury.
The PCL (Posterior Cruciate Ligament) controls motion of the tibia, to keep it from extending too far backward. This sort of injury can be caused by an impact to the tibia or in a car accident where the dashboard forces the lower leg backward during collision.
Collateral Ligaments control the side-to-side motion of the knee joint. Damage can occur when the knee joint is forced to bend laterally. This occurs in a twisting motion when a foot is planted or a forceful blow occurs to the side of the knee.
A serious impact to the knee can cause compounded injuries that include any or all of these conditions.