Ligaments are elastic bands of tissue that connect bones to each other and provide stability and strength to the joint. The major knee ligaments are the anterior and posterior cruciate ligaments (ACL & PCL), the medial and lateral collateral ligament (MCL & LCL) and the posterolateral corner. Any direct knee injury or strong muscle contraction or even a sudden change of direction while running can injure and provoke isolated or combined knee ligament injury. Symptoms of a knee sprain vary depending on the specific ligament that is torn. The ACL is the most common ligament to be injured followed by the MCL. Arthroscopic or open procedure may be necessary to deal with knee instability.