A sprain is a joint injury that causes a stretch or tear in a ligament, a strong band of tissue connecting one bone to the other. The medial collateral ligament is located on the inner 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.
This injury usually occurs when a blow to the outer side of the knee causes stretching or tearing of the medial collateral ligament. It can also be caused by twisting 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.