A sprain is a joint injury that causes a stretch or tear in a ligament, a strong band of tissue connecting one bone to another. The lateral collateral ligament is located on the outer side of the knee.
Sprains vary from minor tears in a few fibers of ligament to complete tears of entire ligaments.
Complete tears make the joint very loose and unstable. The lateral collateral ligament can be injured by a twisting motion or from a blow to the inner side of the knee. Your knee is swollen and tender. You have the feeling of your knee giving way. You hear or feel a pop or snap at the time of injury.