A knee sprain means that you have injured one of the ligaments around the knee joint. Saying you have a "knee sprain" is not a terribly useful diagnosis for two reasons:
First: it does not tell you which ligament is injured. This is important because different ligaments are treated very differently. For example, ACL (anterior cruciate ligament) injuries often need surgical reconstruction. On the other hand MCL (medial collateral ligament) injuries seldom require surgery.
Second: it does not tell you how badly the ligament is injured. The reason this is important is that minor injuries usually require minor treatment. More severe injuries may require more substantial treatment, rehabilitation, and possibly surgery.
Also Known As: Knee ligament tear, knee ligament injury