A Plica is a fold in the lining of the knee joint (Synovium).

The injury causing this condition can be minor and may be difficult to remember. The injury causes the Plica (fold in the lining of the knee joint) to become swollen and inflamed. Repeated use of the knee can cause serious pain when the swollen Plica rubs on the front of the thigh bone (femur). People who use their knees as part of their exercise regime, such as runners, may notice that they are able to run less every week.