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Plica

A plica is a ribbon-like fold of joint lining (synovium) which may be present in any knee as an embryological remnant.

The suprapatellar plica (in 87% of knees) stretches from the top of the kneecap (patella) to the back wall of the suprapatellar pouch, or the big space which exists behind the patella and goes up to about a handsbreadth above the knee joint. A thickened suprapatellar plica can often be felt as a tender cord just above and towards the side of the patella.

The next most common plica is the infrapatellar plica (in 86% of knees). This one is quite different to the others, being very soft and filmy and reaching from the base of the joint in the middle, up and over the cruciate ligaments and attaching to the roof of the notch of the thigh bone (femur). It does not ever seem to do any harm, but its importance is that it may mimic in structure the cruciate ligament and trick the unwary surgeon.

Then on the inner side, running along the side of the patella is the mediopatellar plica (in 72% of knees). This is often called the medial shelf.

Finally, there may be a lateral patellar plica (1.3% of knees).

Plicae exist as normal structures. When they are traumatised, especially the medial plica and if the trauma is recurrent, they may thicked and become problematic, leading to pain, catching, giving way and often being palpable as a tender thickened band under the skin.