A knee sprain occurs when you over stretch or tear the ligaments in your knee.  This can occur with pivoting, landing wrong, or by a direct blow the knee.  If you have sprained your knee, it is important to ice your knee to bring down the swelling.  Minor sprains typically heal in a few weeks with Rest, Ice, Compression (sleeve) and Elevation.  For more serious knee sprains, wear a knee brace to help compress and support the knee.  Talk to your physician for a full diagnosis.  You can also prevent knee sprains by wearing a knee brace when you play sports.  This will provide increased stability for the running, jumping, and cutting movements you may be doing.