A medial plica is a remnant of an embryonic partition in the knee, and runs along the inner aspect of the kneecap.
As the knee bends the 'shelf' of tissue may also give a sudden snap over the rounded ends of the femur bone.
The patient may experience a sharp pain when the knee is in a certain position and the plica is being nipped.
Recurrent nipping traumatises and thickens the plica. When a medial plica becomes abnormally thickened, it can be felt with the finger as a string-like object to the inner aspect of the kneecap.
The inset in the illustration on the right is a photograph taken during arthroscopy of a medial plica. The camera ( arthroscope ) is in the suprapatellar pouch , looking down on the top of the patella. You can see the thickened plica becoming nipped between the patella (above) and the femur (below) - as demonstrated in the illustration.
A pathological (abnormal and symptomatic) plica is something that is often missed by surgeons, but in the right patient symptomatic relief can be immediate after its removal.