N fossa cysts are cystic masses in the posterior aspect of the N fossa. Primary N fossa cysts are most common in children and mostly originate from the bursa in the N fossa. Secondary N fossa cysts are most common in adults, often secondary to osteoarthritis, meniscus lesions and rheumatoid arthritis. The onset of the disease is associated with increased intra-articular pressure, and the cyst is formed when intra-articular fluid spills through the foramen between the joint and the bursa. Patients often unintentionally found N fossa behind the swelling to the clinic, at first no obvious discomfort, the impact on the knee joint function is not great, only the N fossa discomfort or distension, some have lower limb fatigue; when the cysts increase to a certain extent, it can affect the full flexion and extension of the knee joint, or in the knee joint activities after more pain. Color ultrasound is an economical and accurate method, some smaller N fossa cysts are often found when patients undergo magnetic resonance or CT examination for knee joint lesions. Their treatment should in principle be indicated by the patient’s symptoms. For primary caruncle cysts in children, the consensus is that no specific treatment is required, and reports have found that most disappear or atrophy on their own. In adults with secondary caruncle cysts, asymptomatic ones may be left untreated. Symptomatic or large N fossa cysts should be surgically resected, and resection methods include open surgical resection and minimally invasive arthroscopic resection.