The synovial folds of the knee are synovial compartments that exist during embryonic life but begin to degenerate in late fetal life and are referred to as residual tissue in the developing knee joint. It is divided into suprapatellar, infrapatellar, and medial patellar folds by location. Synovial folds in the knee are often caused by strenuous exercise, trauma, inflammation, etc., causing knee pain and a series of dysfunctions, called synovial fold syndrome in the knee. There are various views such as congenital variation, fibrin adhesions from injurious inflammation, and reactive repair of the synovial membrane. Among the various views, the theory of congenital variation is generally accepted. This theory suggests that in early embryonic life, the knee is divided into 3 chambers: medial, lateral, and suprapatellar, separated by a septum of sparse elastic fibrous tissue. At 3 months of embryonic age, the septum degenerates and the 3 chambers begin to fuse. If septum degeneration is incomplete, synovial folds are formed. Second, the classification of synovial folds customarily divided into four types of synovial folds: suprapatellar folds, infrapatellar folds, medial patellar folds and lateral patellar folds. The most common ones are infrapatellar and suprapatellar folds. The medial patellar crease is less common than the first two, but it is more likely to cause clinical symptoms and is therefore the focus of clinical research. The lateral patellar crease is less common. The main etiology of synovial crease syndrome (1) direct trauma, blunt impact on the crease. (2) Indirect injury, excessive exercise, forced repeated flexion and extension of the knee joint, twisting, the crease is subjected to strain and repeated extrusion and friction of the patellofemoral joint surface. (3) Chronic inflammation caused by other pathologies within the knee that involve the crease. These factors make the synovial crease inflammatory changes and congestion and edema, and over time it becomes hyperplastic, hypertrophic, fibrotic, and loses its original elasticity. When the knee joint flexes and extends, the fibrotic folds cannot be deformed and elongated, resulting in mechanical irritation of the femoral condyles, which can lead to secondary inflammation of the synovial membrane at the edge of the femoral condyles in mild cases, or erosion of the patella and femoral condylar cartilage in severe cases. In the early stage of the disease, the symptoms mostly come from the inflamed crease itself; in the late stage, the symptoms are mostly caused by fibrosis and damage to the articular cartilage caused by the strain on the crease. (4) Synovial folds are thick, wide, fibrotic, hard and inelastic, and most people believe that such folds are more susceptible to pathological changes, leading to clinical symptoms.