A series of intra-articular knee lesions, such as instability, popping and pain, caused by repeated injury or stimulation of the synovial folds of the knee joint, is called synovial fold syndrome. The synovial folds of the knee are divided into suprapatellar, infrapatellar, intrapatellar and lateral synovial folds according to their relationship with the patella, and their frequency, size, thickness and clinical significance vary. The infrapatellar crease, also known as the mucosal ligament, is usually asymptomatic but may interfere with arthroscopic surgery; the suprapatellar crease, located above the patella, divides the suprapatellar capsule into two parts and rarely causes symptoms; the lateral patellar crease, which is extremely rare; and the medial patellar crease, which is the most common, with a 10-50% incidence in the knee and can cause knee pain. The medial patellar synovial crease begins just above the patella and sometimes extends proximally with the suprapatellar crease, which can thicken and lose elasticity if there is trauma or chronic inflammation, before clinical symptoms appear. Etiology The medial synovial crease lies flat against the medial femoral condyle during knee flexion, and a normally mobile synovial crease does not produce any symptoms. After blunt impact, local synovial inflammation and edema can occur, followed by fibrosis; repeated entrapment and entrapment of the medial patellar synovial crease can lead to bleeding, synovial inflammation, and fibrosis. Both of these conditions can lead to loss of elasticity of the synovial folds, creating a structure that cannot be flexed, thus causing pain. When the knee is flexed and extended, the loss of elasticity of the synovial crease can cause mechanical irritation to the medial femoral condyle, eventually causing damage to the articular cartilage; in addition, this structure may interfere with the function of the quadriceps muscle, causing chondromalacia of the patella. Clinical manifestations It can occur in patients of any age, but is mostly seen in adolescents. The most common symptom is knee pain, which may be total knee pain, anterior superior knee pain, or anterior medial knee pain, but the medial gap pain and medial joint line of the patellofemoral joint of the knee are predominant and are mostly dull. There is usually no significant quadriceps atrophy or joint cavity effusion. The pain increases after repeated activities, and is worse when jumping, going up and down stairs, or standing up suddenly from a squatting position, or even when unable to stand up after squatting. Some patients have sudden pain during walking, and the pain stops immediately after the activity. In some cases, the pain is simply soreness in the knee joint cavity. Some patients have interlocking (stuck feeling), popping, and knee rubbing sensation when extending and flexing the knee joint, or may even have limited knee extension, or have limited movement after a day of activity, with symptoms improving after a night’s rest. Physical examination The medial patellofemoral space may be painful with pressure and palpable sclerotic bands, and pain may be induced by pushing the patella inward when the knee is flexed at 30 degrees. Due to the medial synovial membrane embedded between the patella and femur, patellar compression test and patellar grinding test may be positive. Some signs of meniscal injury, such as McSweeney’s sign and Apply’s grind test, may be positive, but the pain is often located above the joint line, between the crease and fat pad, rather than at the joint line. MRI can detect synovial folds with significant thickening of the medial patellofemoral joint space and can provide insight into femoral and patellar cartilage damage and identify the presence of meniscal injury. Arthroscopy is an important tool in the diagnosis of this condition. It allows direct observation of the location, extent, and course of the synovial folds, as well as the presence of pathological changes such as congestion, edema, fibroplasia, hypertrophy, and rupture, and allows observation of the movement of the synovial folds during knee extension. Other intra-articular lesions may also be found. Diagnosis The diagnosis can be made by combining history, physical examination, and ancillary tests. Treatment The disease should be treated conservatively, including lifestyle changes to reduce repetitive knee extension activities and avoid prolonged knee flexion. Other treatments include oral NSAID medications, isometric contraction exercises for the quadriceps and N cord muscles, physical therapy, and local closed injection therapy. If the symptoms are short-lived, they can mostly be cured with conservative treatment, but may recur after exercise. If conservative treatment is not effective, synovial crease resection can be performed under arthroscopy. After the surgery, the patient can gradually return to sports after 2-3 months by applying pressure bandages and performing quadriceps exercises immediately.