Muscle injuries are known as strains, but knee sprains result from ligament injuries. Ligaments support the joints in your body and connect your bones to each other. Formed from fibrous tissue, ligaments stretch much like elastic. Most sprains occur in the ankle, knee or wrist and result from sports activities. Knee sprains can be particularly painful.

A sprained knee often results from a traumatic event, such as twisting your knee into an unnatural position, falling or slowing your movement too quickly. As the ligaments in your knee stretch beyond their limits, ligament fibers begin to fail sequentially. The fiber failure eventually results in the ligament tearing completely. Four major ligaments provide stability to your knee and can be sprained: the medial collateral ligament, on the inside of the knee; the lateral collateral ligament, on the outside of the knee; and the anterior cruciate ligament and posterior cruciate ligament, both deep within the knee joint.

If a sprain has occurred, you may notice a pop, snap or tearing sensation at the time of your injury. This is typically followed by pain, swelling and difficulty bearing weight on your knee. Stiffness and bruising typically develop 24 to 48 hours after the injury. Most common MCL injuries occur from side impact to the outside of the knee, forcing it to bend forcefully inward, such as in a collision. ACL strains are more likely to happen from a non-contact twisting mechanism, such as in an abrupt stop and pivot during a cutting maneuver.

Seek medical attention, especially if your knee is swollen. To determine the stability of your knee and the degree of your injury, a doctor will examine the area for swelling, tenderness and bruising. If your injury is deep inside your knee joint, symptoms may not be readily apparent. It is always a good idea to compare the injured knee to the uninjured one. The examination of your injury may be difficult because your joint may be extremely painful. Some ligamentous injuries require special clinical examination techniques to determine the degree of injury and instability. Varus or valgus stress tests are used to diagnose sprains of the MCL or LCL. During this test the examiner will place a hand on the thigh, bend the knee to 30 degrees of flexion and either force the lower leg inward (varus) or outward (valgus). Pain on the inside of the knee with a valgus stress represents a strain to the MCL and likewise pain on the outer aspect of the knee with a varus stress would represent an LCL strain. An untreated sprain or chronic injury can cause joint instability, and persistent joint instability can hasten knee deterioration.