The 'plica syndrome' refers to a collection of associated symptoms which are caused by the nipping inside the knee of a fold of joint lining (the plica) between the bone of the femur (thigh bone) and the extensor structures of the knee (kneecap or the muscles above it).
Plicae (the plural - pronounced 'ply-kee') are simply embryonic remnants of the partitions which separated the foetal (in the womb) knee into compartments. They are like fragments of frail curtain hanging from the lining of the knee, but in predictable places because of the original foetal compartments. They are elastic and thin and usually just move around with the knee, like seaweed with the tide.
But sometimes something happens to damage a plica - an injury or a repetitive exercise - and the plica swells, it may bleed, then it thickens up and the elastic properties are lost - and it then may become a mechanical problem within the knee, periodically 'catching' between the other structures (usually at specific angles of extension of the knee) - giving a sudden pain or discomfort, a feeling of temporary sticking in that position and residual discomfort in the front of the knee.
Treatment depends on the stage - a damaged plica may simply get better on its own. Or physiotherapeutic intervention may diminish the inflammation. Some surgeons will inject a palpable plica with an anti-inflammatory, like a steroid. But once it is thickened up, only surgery will really help. A major problem is that the plica can be missed during surgery. Either the surgeon may think it trivial or he/she may fail to look at it from a portal ('keyhole') above the kneecap (the suprapatellar portal) which clearly demonstrates the mechanism of nipping.