Causes / typical injury mechanism: Plicae are thick bands of fibrotic tissue that extend from the synovial capsule of a joint. They are normal structures that develop in utero but are not present in all people. They are most common in the knee. Plicae can become inflamed due to chronic friction caused during knee joint range of motion (ROM) or during acute trauma to the anterior knee. This can cause a focal synovitis as a result of mechanical irritation from this thickened band of tissue.

Classic history and presentation: Plica syndrome most often presents as atraumatic anterior or anterior-medial knee pain associated with clicking and popping, especially during knee flexion. It usually has an insidious onset, but it may also present acutely after trauma or any activity that loads the patellofemoral joint.

Prevalence: It is believed that plica syndrome is underdiagnosed due to its similar presentation to many other pathologies. Arthroscopic studies have shown a prevalence of 10%. However, autopsy reports have shown plicae to be present in approximately 50% of people, suggesting that the presence of plicae themselves do not always cause pain.

• Age – First to third decade of life.
• Sex / gender – Equal.

Risk factors: Risk factors include activities that repetitively load the patellofemoral joint and conditions that cause intraarticular pathology in the knee joint, such as hemophilia causing hemarthrosis, rheumatoid arthritis, and trauma causing fractures or meniscal / ligamentous damage.

Pathophysiology: As the cavitation that ultimately becomes the knee joint forms during embryological development, mesenchymal tissue folds form if the cavitation does not fuse together perfectly. These folds become synovial plica and are often present bilaterally.

Grade / classification system: There are 4 different plicae that can be present in the knee –

• Suprapatellar plica: found between the knee joint and suprapatellar bursa.
• Infrapatellar plica (ligamentum mucosum): found between the intercondylar notch and synovium surrounding the infrapatellar fat pad.
• Medial plica: most common type and most common to be symptomatic; found between the medial aspect of knee joint and the infrapatellar fat pad.
• Lateral plica: rarest; found between the infrapatellar region and the lateral patellar facet.