I. Nature: N-fossa cyst refers to bursitis in the N-fossa in general. It is due to the accumulation of fluid in the knee joint and the increased pressure in the cavity during knee flexion, forcing the synovial fluid to move back and forming a cyst in the N fossa. It causes pain and swelling at the back of the knee, and an elastic soft tissue mass can be palpated. Diagnosis: Mostly seen in middle-aged men, often solitary, usually without history of trauma. There is no local pain when the cyst is small, and occasionally there is a tenting sensation when the knee is extended. It is mostly located medially to the N fossa. It can be palpated when extending the knee, with fluctuating sensation and no pressure pain, and is not clearly palpable when flexing the knee. The pain is obvious when it grows gradually. Puncture may extract mucous yellow fluid. x-ray sometimes reveals a spherical soft tissue shadow in the N fossa. MRI can confirm the diagnosis. Differentiation: It should be differentiated from aneurysm (pulsating, blood in puncture fluid), hemangioma (local color change, no change in knee flexion/extension swelling, blood in puncture fluid), and N-fossa tumor (mostly hard swelling, no cystic sensation, no change in knee flexion/extension swelling). Treatment: 1. Small, asymptomatic ones do not need treatment. It can also be treated by puncture and fluid extraction, and local injection of Bonisolone, which is more effective. 2, large, affecting the patient’s life and work, surgery to remove!