Knee Plica Syndrome (Synovial Plicae)

Calling a plica an "embryological remnant" is only useful if you can say a remnant of what.

In the developing knee, mesenchymal tissue between the femur and tibia is resorbed to create the joint cavity, but it does so in stages: three separate synovial compartments form first - a medial, a lateral and a suprapatellar cavity - divided by synovial septa. Those septa normally break down around the third to fourth month of gestation, leaving a single cavity. A plica is the part of a septum that never fully resorbed, which is why each named plica sits exactly where its septum used to be: the suprapatellar plica along the old division between the suprapatellar pouch and the joint, the medial plica on the medial wall, and the infrapatellar plica running from the intercondylar notch to the fat pad.

Two consequences follow. First, a plica is normal tissue in an unusual amount, not a lesion - which is why finding one means very little on its own. Second, incomplete resorption is common and highly variable: reported frequencies differ widely between arthroscopic, cadaveric and MRI series, and that variability is itself the argument against treating a plica as a diagnosis. What turns a normal fold into a symptomatic one is acquired change - repetitive flexion and extension, a direct blow, or a haemarthrosis - producing inflammation, fibrosis and loss of elasticity, so the fold stops gliding and starts to bowstring.

Why the snap happens where it does

The medial plica runs obliquely from the medial wall to the fat pad, and during flexion the medial femoral condyle advances beneath it. Through the first part of flexion the plica sits harmlessly on the condyle; as the knee passes roughly 30 to 60 degrees, a taut, fibrotic plica is drawn tightly across the condyle and then snaps over its anterior margin - which is why the pain and the palpable click occur in mid-flexion rather than at either extreme, why the symptoms are provoked by stairs and squatting, and why compressing the plica against the condyle in extension reproduces the pain while further flexion relieves it. The repeated passage of a hard fibrotic band across the same strip of cartilage is what produces the chondral footprint on the anterior medial femoral condyle.

Resect it - do not simply divide it

The word matters. Dividing a plica, or releasing it at one end, leaves the fold in place and it commonly scars back down and becomes symptomatic again; the operation is a complete excision of the plica back to the synovial wall, taken far enough proximally and distally that no thickened band remains to re-tether. The practical points at arthroscopy:

• Look before you resect. Confirm the plica is genuinely pathological - thickened, fibrotic, avascular or frayed - and look for the corroborating chondral footprint on the anterior medial femoral condyle. A thin, soft, mobile fold in a knee whose pain is coming from somewhere else should be left alone.
• Resect completely, taking the whole band rather than dividing it, and check through a full arc of flexion and extension that nothing remains to catch.
• Address what accompanies it - an impinging hypertrophied infrapatellar fat pad, and any chondral lesion.
• Haemostasis matters, since the plica is vascular at its base and a postoperative haemarthrosis both hurts and promotes the fibrosis you were trying to remove.

The complications to counsel for are recurrence from incomplete excision, persistent pain when the plica was never the real source, haemarthrosis, and the general arthroscopic risks including stiffness and infection. The first two are both failures of selection rather than of technique, which is why the diagnosis has to be earned before the arthroscope is picked up.