A locked knee is a condition where, on bending the knee, it becomes fixed in position, often at a 45 degree angle, and the person is unable to straighten the leg or bend it any farther. It is usually possible to maneuver the leg back into a straightened position using the hands. Often, the condition is caused by a tear in a piece of cartilage, called a meniscus, which lies inside the joint. When the knee joint moves, the fragment of torn meniscus becomes caught and prevents the movement from continuing normally. When a locked knee is caused by a meniscus tear, the problem is usually treated surgically, and the torn section of cartilage is removed so it no longer interferes with joint mobility.
Other causes of a locked knee can include a ligament injury, loose tissue fragments other than cartilage becoming lodged inside the joint, and certain fractures. Sometimes a swollen, injured knee may appear to be locked, but the lack of movement is because the muscles have gone into spasm. A torn meniscus is probably the most common knee disorder that results in a locked knee. While a ligament injury can cause locking, it is more typically associated with the knee giving way, a condition sometimes referred to as having a "trick knee."
Inside each knee there are two menisci, known as the medial meniscus and the lateral meniscus. Each one is what is called a semilunar cartilage, which is a crescent-shaped piece of strong, flexible tissue. The shinbone and the thighbone meet at the knee and the semilunar cartilages are positioned between the ends of these bones in such a way that they help distribute the weight-bearing forces acting on the joint. If a twisting movement is made while the joint is bearing weight, one of the cartilages may become trapped, leading to a tear. The detached portion of cartilage may then wedge inside the joint, causing it to seize up and resulting in a locked knee.
When knee joints become locked, treatment most often involves surgery, as the damage is likely to be too severe to improve with more conservative measures, such as resting the joint. A type of keyhole surgery may be carried out, known as arthroscopy. Here, a special elongated surgical instrument, similar to a telescope, is introduced into the knee, allowing the surgeon to view the interior of the joint while surgery is performed using fine tools. The technique has the advantage that only small cuts in the skin are required in order to operate. Patients generally recover more quickly, experience less pain and have a lower risk of infection than those who undergo traditional surgery.