A plica is a band of thick, fibrotic tissue that extends from the synovial capsule of a joint. Plica can be present in multiple joints, but this article will review plica in the knee, the joint most commonly affected by plica tissue. As a result of overuse or injury, plica can become inflamed or irritated due to friction across the patella or the medial femoral condyle. When the plica becomes inflamed or irritated, it can cause plica syndrome, which is anterior knee pain due to the plica.
Healthcare professionals do not universally agree upon the embryological development of the knee joint. One common theory is the menisci, cruciate ligaments, and a joint cavity all develop when the fetus is 8 weeks old. Over the next 2 weeks, the septa of the synovium are resorbed, and larger cavitation develops which ultimately becomes the knee joint. If this larger cavitation fails to join together fully, mesenchymal tissue may develop synovial folds. These synovial folds are plicae.
The literature varies widely regarding the estimated prevalence of plica syndrome. Most report a 10% prevalence of plica syndrome based on arthroscopic studies. It is estimated that plica syndrome is underdiagnosed because the symptoms are similar to other etiologies of knee pain. A study in Japan looked at 3889 knee joints during arthroscopy and found the incidence of medial plica to be 79.9%. This did not specify that the plica was the etiology of symptoms in all these patients and thus this was the incidence of plica, not plica syndrome. Other literature reviews show autopsy results that found plica are present in approximately 50% of individuals. Thus, the prevalence of plica on arthroscopy does not correlate with the prevalence of clinical plica syndrome.
Plicae are normal structures in the knee joint that come together in utero. Plicae typically involute when the fetus is around 12 weeks old, but autopsy results indicate plicae are present in 50% of individuals.
For the approximately 50% of people who have a plica, there are four different normal plicae. These are suprapatellar, infrapatellar, medial, and lateral. It is not uncommon for a patient to have more than one plica.
The suprapatellar plica is found between the knee joint and the suprapatellar bursa. The infrapatellar plica is found between the intercondylar notch and the synovium around the infrapatellar fat pad. The medial plica is found between the infrapatellar fat pad and the medial aspect of the knee joint. Medial plicae are the most common type of plicae and are the most common to be symptomatic. The lateral plica is the rarest of the plica. It is found between the infrapatellar region and the lateral patellar facet.
It is important to note that not all plicae cause pain. Normal plicae can become painful as a result of undergoing inflammation. This inflammation can then lead to a tight, fibrotic plica that can manifest in symptoms during flexion of the knee when it leads to impingement between the patella and femur.
History and Physical
Patients with plica syndrome will experience pain on the anterior aspect of the knee associated with clicking or popping. The anterior knee pain is a hallmark of plica syndrome. Pain can be brought on by rising from a chair, squatting, stairs, or other activities that load the patellofemoral joint. The history may include a twisting injury or blunt trauma, or there could be no history of injury or trauma. There are secondary causes of plica that should be considered when obtaining a history. These include hemarthrosis secondary to hemophilia, intra-articular lesions, loose foreign bodies, and rheumatoid arthritis.
On physical exam, a taut band may be palpable under the skin which may be tender to palpation. If this is the case, the contralateral knee should be examined to see if there is a plica causing tenderness to palpation on that knee as well. Other findings on physical exam may include an effusion, tight hamstrings, and tight quadriceps.
Two physical exam techniques can suggest plica syndrome. These tests are the Stutter test and the Hughston test. When both tests are positive, the diagnosis of plica syndrome should be suspected. If only 1 of the 2 is positive, plica syndrome is less suspicious, but should still be considered. The Stutter test is performed by having the patient sit upright with the legs dangling off the edge of the exam table at a 90-degree angle. The examiner places his or her index and middle fingers on the center of the patella of the affected leg. The patient then extends the affected leg while the examiner feels for a stutter of the patella. Feeling a stutter is a positive test. In the Hughston test, the patient lies supine with the knee extended. The examiner stands on the side of the affected knee with one hand around the plantar aspect of the patient's heel and the other palm covering the patient's patella. The examiner then pushes the patella medially and internally rotates the tibia while taking the patient's knee through flexion and extension. A positive Hughston's test is when the patient experiences pain or the practitioner appreciates popping during the range of motion.
Anteroposterior (AP), lateral, and skyline radiographs should be obtained when plica syndrome is suspected, although they will often be normal even if plica syndrome is the diagnosis.
The utility of MRI is controversial in the workup of plica syndrome. Plicae are only occasionally apparent on MRI. It is easier to see plica on MRI when an effusion is present. If plicae are apparent, they will appear with low-signal intensity. Symptomatic plicae can sometimes be differentiated from normal plicae on MRI because symptomatic plicae could appear thick and may have synovitis. This is opposed to normal plicae which may appear thin. As a result, MRI can be useful as part of pre-operative planning and is important in evaluating other potential causes of knee pain.
A literature review revealed multiple studies which looked at 492 knees and compared the sensitivity and specificity of physical exam compared to ultrasound and MRI. The results were that physical exam had a 90% sensitivity and 89% specificity, ultrasound had 90% sensitivity and 83% specificity, and MRI had 77% sensitivity and 58% specificity. The gold standard for diagnosis is arthroscopy.
Symptoms of plica syndrome are often similar to many other etiologies of knee pain. As a result, the differential diagnosis can be lengthy and may include osteochondritis dissecans, patellofemoral syndrome, patellofemoral subluxation, meniscal disease, osteoarthritis, patellar tendonitis, cruciate ligament pathology, and pigmented villonodular synovitis. These differential diagnoses can be differentiated from plica syndrome as follows:
- Osteochondritis dissecans: Differentiate with radiographs and MRI.
- Patellofemoral syndrome: Patellofemoral knee pain can be difficult to distinguish from plica syndrome as the symptoms overlap significantly. Other causes of patellofemoral pain, such as chondromalacia, may be apparent in history and imaging.
- Patellofemoral subluxation: Differentiate because patients with patellofemoral subluxation will often provide a history consistent with subluxation and may have apprehension with a displacement of the lateral patella.
- Meniscus pathology: Differentiate because meniscus pathology will have tenderness in the joint line, whereas plica pain tends to localize above the joint line. Also, physical exam tests such as Apley, Thessaly, bounce home, and/or McMurray can help distinguish the 2 entities.
- Osteoarthritis: Differentiate with radiographs showing decreased joint space, osteophytes, subchondral sclerosis, subchondral cysts, among others, although this does not rule out also having symptomatic plicae.
- Patellar tendonitis: Differentiate by palpating the patellar tendon on either the proximal or distal attachment.
- Cruciate ligament dysfunction: Differentiate by physical exam techniques suggesting laxity including Lachman, anterior drawer, or posterior drawer would likely be positive in cruciate ligament injury.
- Pigmented Villonodular Synovitis (PVNS): Differentiate via MRI.
Medial plicae are most commonly symptomatic and can be classified by the Sakakibara arthroscopic classification:
- Type A: Elevation in the synovial wall
- Type B: Appear shelf-like, but not covering the anterior surface of the medial femoral condyle
- Type C: Large, shelf-like appearance and covering the anterior surface of the medial femoral condyle
- Type D: Fenestrated plica with a central defect
Once appropriately treated, there is a favorable prognosis for plica syndrome. One study of 969 patients found that at a median follow-up of 27.5 months, 10% had not responded to treatment, whereas 26% improved, and 64% were symptom-free.
One study suggests that the friction between a medial plica and the medial femoral condyle may lead to cartilage damage. Another study which looked at 48 patients who had severe medial compartment osteoarthritis requiring total knee replacement supported this. All 48 of these patients had a medial plica and a cartilaginous lesion on the medial femoral condyle abutting the plica.
If left untreated, medial plica syndrome can cause grade IV Outerbridge chondral lesions, which is when the subchondral bonehead is exposed. These are difficult to treat and preventing grade IV Outerbridge lesions from occurring highlights the importance of prompt diagnosis and treatment.
Plica syndrome can be diagnosed and managed without consultation to a sports medicine physician or orthopedic surgeon as long as the managing physician is practicing within his or her expertise and is confident he or she is making the correct diagnosis. This physician can begin conservative management with home stretching and strengthening and/or formal physical therapy. If the patient does not improve after 3 months of conservative measures, an intraplical corticosteroid injection is a reasonable next step treatment option. If there is no improvement at this point and the patient would consider surgery, the patient should be referred to an orthopedic surgeon.
Deterrence and Patient Education
Patient education should emphasize the prognosis is favorable if this condition is correctly treated promptly. Patients should be encouraged to seek treatment for chronic knee pain to establish a diagnosis and start treatment before their condition progresses to grade IV Outerbridge lesions. At this point, treatment is more difficult.
Pearls and Other Issues
Plica syndrome manifests as anterior knee pain and is diagnosed by history and physical exam. Imaging techniques are most helpful to rule out other conditions on the differential diagnosis although MRI is potentially useful in pre-operative planning. Initial treatment should be conservative management unless cartilage damage is suspected, in which case performing arthroscopy before completing conservative management is reasonable. The prognosis for plica syndrome is favorable when treated promptly before the potential complication of cartilage damage becomes too advanced. One pitfall with plica syndrome may occur if practitioners obtain radiographs and attribute pain to osteoarthritis, when there may also be symptomatic plicae present.
Enhancing Healthcare Team Outcomes
If the initial primary care provider or nurse practitioner is not confident in the diagnosis, referral to a primary care sports medicine physician or an orthopedic surgeon is recommended to establish the diagnosis and initiate treatment. An interprofessional team approach is best for evaluation and treatment. [Level V]
The best outcomes with plica syndrome will occur when other etiologies of anterior knee pain are ruled out, and conservative treatment is initiated early in the disease process. If the patient does not improve with conservative management, they should consult with a surgeon. If an MRI is obtained, it is important to have the MRI read by a radiologist who has experience with plica syndrome.