N fossa cyst is a common clinical disease, which is prevalent in middle-aged and elderly people. They are mostly located in the N fossa between the N cord tendon and the tibial condyle or collateral ligament, and may also be located deep to the medial-lateral head of the gastrocnemius muscle. In most cases it presents as a distended bursa. Many people will incidentally notice a mass on the posterior aspect of the knee, with or without knee pain. In fact, N fossa cyst itself is not scary, it is not a tumor, but a “water sac” filled with viscous fluid, it is due to the accumulation of fluid in the knee joint, the synovial membrane of the knee joint in the joint capsule behind the back of the hernia and the formation, or joint fluid through the semimembranosus muscle or gastrocnemius muscle head of the bursa between the normal channel and the joint and the formation of the outflow. Most N fossa cysts are clearly connected to the knee joint. According to foreign data, the vast majority of patients with N fossa cysts are associated with intra-articular lesions (98%), like meniscus injuries, synovial crease syndrome, osteoarthrosis, and so on. If intra-articular lesions are not treated, N fossa cysts are prone to recur after surgery. In the past, N fossa cysts were treated by using a posterior knee incision with only open resection of the cyst wall. However, the postoperative recurrence rate was high, so some surgeons closed the channel with suture after removing the cystic cavity, or applied the medial gastrocnemius muscle head-turned-tip for reinforcement. The disadvantages of this treatment are high surgical trauma, long postoperative recovery time, and failure to address intra-articular disease. Instead, N fossa cysts actually appear as a result of intra-articular disease. Due to meniscus injury, osteoarthrosis, etc., the joint fluid leakage increases, and finally the synovial membrane of the joint herniates out of the weak part of the knee joint due to fluid pressure, and the herniated outlet is a unidirectional flap, which is only outward but not inward, and gradually grows in size over time, forming a cyst. Therefore, when treating N fossa cysts, we must focus on the treatment of intra-articular disease, only after the treatment of intra-articular disease, we can treat N fossa cysts fundamentally. Nowadays, we have been able to treat N fossa cysts through arthroscopic methods, only 2-3 small incisions of 6 mm around the knee joint can be made, which can treat both N fossa cysts and the intra-articular diseases that cause N fossa cysts. The damage is minimal, the recovery is fast, and it treats the N fossa cyst at its root. We started to treat N fossa cyst patients with arthroscopy in 2008, and from the statistics of the patients we have treated, 50% of them have meniscus injuries, and the other 50% have thickened synovial folds, free bodies, or osteochondral chondromalacia, etc. After arthroscopic treatment, the patients not only have N fossa cysts, but also have a lot of N fossa cysts, but also have a lot of N fossa cysts. After arthroscopic treatment, not only the N fossa cysts disappeared, but also the joints were painless, and the clinical results were very good.