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.