What is a knee sprain? A sprain is an injury to a ligament that develops due to overstretching. A ligament is a band of tough tissue that holds bones together and supports joint motion. Four ligaments are important in maintaining the stability and function of the knee joint, the anterior and posterior cruciate ligaments and the medial and lateral collateral ligaments. Knee and ankle sprains are among the most common types of sprains that occur in the body. A knee sprain typically occurs as a result of trauma, and sports injuries are one of the most common causes of knee sprain. The ligament may tear, either partially or completely, when it is excessively stretched. Symptoms may vary in severity and include pain and swelling along with an inability or decreased ability to move the knee joint. Some people report that they hear a popping noise at the time of the injury. A thorough clinical evaluation will be able to accurately diagnose knee sprain and identify if any other knee injuries are present. Many cases of knee sprain can be treated at home with rest, ice packs, and over-the-counter pain relief medications. More-serious cases may require treatment with a brace or other device to help you keep the knee immobilized. If a ligament is completely torn, surgery may be needed to repair the knee joint. A knee sprain is rarely a serious injury that would require emergency care. However, it may be associated with more-serious injuries. Seek immediate medical care (call 911) for serious symptoms, such as paralysis, loss of sensation, absent pulses in the feet, the inability to move the knee joint, severe bleeding, or uncontrollable pain. Seek prompt medical care if you have any pain, soreness, or other problems with your knee, or if you are being treated for a knee injury and symptoms recur or are persistent.