A knee sprain means that you have injured one of the ligaments in and/or around the knee joint. A sprain is caused by a ligament being stretched too far. The symptoms of a sprain are typically pain, swelling, and bruising of the affected joint. The collateral ligament sprains will affect lateral stability and the cruciate sprains will affect stability in the frontal plane. Many knee sprains can be treated conservatively however if there is significant instability, there may need to be surgical intervention.