Plicas are remnants of tissue from when the knee was developing and are seen as crescenteric bands of tissue within the lining of the knee. Small plicas are normal within the knee but large plicas can get pinched in the knee, normally in the patello-femoral joint. This causes inflammation and pain. The inflammation can lead to a thickening of the plica making it more likely to be pinched. Arthroscopic removal of the plica resolves the symptoms.
Athletes that do repetitive deep flexion and extension movements of the knee, such as cyclists, and people who kneel at work are most likely to suffer from plicas.