MRI is a non-invasive, highly sensitive tool that allows for detailed visualization of synovial plicae and surrounding knee structures, including the menisci, ligaments, and cartilage. Normal synovial plicae appear as thin, low-signal-intensity bands on T1- and T2-weighted images. Pathological changes are seen as thickened bands with signs of inflammation or associated joint effusion. The four types of synovial plicae are classified based on their anatomical location within the knee joint: the suprapatellar plica, infrapatellar plica, lateral  and medial plica.     

1. Suprapatellar plica: a fold of synovial tissue located above the patella, within the suprapatellar pouch of the knee joint; it is typically present in all individuals and in most cases, it does not cause pain or other symptoms;   
2. Infrapatellar plica:  located below the patella, near the insertion of the patellar ligament; it can be more prominent in some individuals, especially in those engaging in activities that involve repeated knee flexion, and may occasionally become symptomatic;   
3. Lateral patellar plica:  the least common type,  is located on the lateral aspect of the knee joint;   
4. Medial plica is the most commonly symptomatic  one and it can also be classified in four  types according to Sakakibara, which helps in understanding the severity and potential impact on knee function:  

•  type A: the simplest and smallest form of medial plica, presenting  as a thin structure that rarely causes symptoms;   
•  type B:  the plica becomes thickened, forming  a shelf-like structure that partially extends into the joint cavity; while typically asymptomatic, it may occasionally cause mild discomfort during physical activity;    
•  type C: characterized by a more prominent  fold that extends significantly into the joint, often coming into contact with other knee structures, which increases  the risk of mechanical irritation and inflammation;    
•  type D:  the most complex form, is characterized by a large plica with a fenestration within its structure, this can lead to impingement between the patella and the femoral condyle, making it more likely to cause persistent symptoms and functional limitations.
  

  Fig 2: Sakakibara classification of medial patellar plicae.