Knee Plica Syndrome (Synovial Plicae)

What a plica is. Synovial plicae are normal embryological folds of synovium. They are present in a large proportion of normal knees and are usually asymptomatic, incidental findings. Four are recognised, and the medial (mediopatellar) plica is the one that most often becomes symptomatic.

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Where they come from. In the developing knee, mesenchymal tissue between the femur and tibia is resorbed to create the joint cavity, but in stages. Three separate synovial compartments form first, a medial, a lateral and a suprapatellar cavity, divided by synovial septa. The septa normally break down around the third to fourth month of gestation, leaving a single cavity.

A remnant of a septum. 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.

Why finding one means little. A plica is normal tissue in an unusual amount, not a lesion. Incomplete resorption is common and highly variable, and reported frequencies differ widely between arthroscopic, cadaveric and MRI series. That variability is itself the argument against treating a plica as a diagnosis.

From fold to band. What turns a normal fold into a symptomatic one is acquired change: repetitive flexion and extension, a direct blow, or a haemarthrosis. These produce inflammation, fibrosis and loss of elasticity, so the thickened fold stops gliding and starts to bowstring.

The snap. 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, and why stairs and squatting provoke them.

The chondral footprint. Over time the plica can abrade the medial femoral condyle and the adjacent patellar cartilage. 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.

The arthroscopic Sakakibara classification grades the medial plica by how much it covers the anterior medial femoral condyle, a measure that tracks its symptomatic and chondral-damage potential. It runs from a cord, through a shelf, to the fenestrated or reduplicated ("double") variant.

History. Anteromedial knee pain, often in a young and frequently female patient, with clicking or snapping and a sensation of pseudo-locking. The pain is worse on stairs, squatting and prolonged sitting.

Examination. The classic finding is a palpable, tender, cord-like band medial to the patella. These manoeuvres localise it and reproduce the snap:

• Rolling the band. Palpate the band medial to the patella and roll it under the fingers; tenderness localised to it is suggestive.
• Mediopatellar plica (MPP) test. With the knee in extension, push the patella medially (and inferomedially) with the thumb to compress the plica between the patella and the medial femoral condyle. Reproduction of the patient's pain is positive, and the pain typically eases as the knee flexes and the plica moves off the condyle.
• Hughston's flexion-extension ("stutter") test. While palpating the medial band, flex and extend the knee through roughly mid-range. A palpable, sometimes painful snap or "stutter" as the plica rolls over the medial femoral condyle supports the diagnosis.

Because plicae are so often incidental, these tests are paired with a full knee examination for the mimics set out below.

MRI. Diagnosis is clinical, supported by MRI, which can show a thickened medial plica, best seen when there is a joint effusion.

Arthroscopy. The diagnosis is frequently confirmed, and treated, at arthroscopy, which directly visualises a pathological plica and any chondral footprint.

A diagnosis of exclusion. Plica syndrome is partly a diagnosis of exclusion. Before the pain is attributed to a plica, the other causes of anterior and anteromedial knee pain must be excluded:

• Patellofemoral pain
• Meniscal pathology, including meniscal tears
• Chondral lesions

Conservative first. Many symptomatic plicae settle with non-operative treatment:

• Activity modification
• NSAIDs
• Physiotherapy: quadriceps/VMO and flexibility work
• Sometimes a corticosteroid injection

When to operate. Arthroscopic resection is for the genuinely symptomatic plica whose symptoms are recalcitrant to an adequate trial of conservative treatment. It gives excellent results: in young patients, knee scores and pain improve significantly, and the outcome is not adversely affected by trauma, high-impact sport, plica type or a low-grade (ICRS grade I) cartilage lesion of the medial femoral condyle. A reduplicated or atypical medial plica can also respond to resection, with fat-pad debridement if the fat pad is impinging.

Resect it, do not divide it. 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 back to the synovial wall, taken far enough proximally and distally that no thickened band remains to re-tether.

At arthroscopy. The practical points:

• 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, 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.
• Secure haemostasis. The plica is vascular at its base, and a postoperative haemarthrosis both hurts and promotes the fibrosis you were trying to remove.

Complications. Counsel for recurrence from incomplete excision, persistent pain when the plica was never the real source, haemarthrosis, and the general arthroscopic risks including stiffness and infection. Recurrence is a failure of technique, prevented by complete excision. Persistent pain is a failure of selection, which is why the diagnosis has to be earned before the arthroscope is picked up.