Sprain (knee joint): Any injury to one of six different ligaments which stabilise the knee joint. Those ligaments include: medial and lateral collaterals, medial and lateral meniscus and the anterior and posterior cruciate ligaments. Knee sprains are characterised by knee pain, swelling and tenderness with range of motion. Severe sprains may result in a knee joint effusion (blood inside the joint). Completely torn ligaments may require surgical repair to reestablish knee joint stability.