N-fossa pain, a very common phenomenon after knee surgery, is often a difficult problem to solve and causes a lot of pain to patients. The problem of N-fossa pain is summarized in the following six common situations, and an analysis and management of each situation is provided. Most of these situations summarized below are judged by experience and logical analysis, and they lack effective data support, so there is still room for improvement in terms of scientificity. Please give your valuable opinions and exchange with each other. 1. Foreign body During the surgical operation, bleeding, and some microscopic magnitude of debris will inevitably occur, and the bleeding and debris will be deposited near the posterior joint capsule due to the factor of gravity, causing local inflammatory reaction and subsequently triggering N fossa pain. The location of this pain is usually not clearly defined in one spot, but rather a widespread area. Moreover, the pain can be relatively constant and does not change depending on the flexion and extension activities of the joint. The nature of the pain is often vague. The solution is mainly through rest, physical therapy, or some medication prescribed by the doctor to reduce inflammation, and the pain will be gradually relieved when the inflammation disappears. 2.Points For ACL reconstruction, the positioning of the upper and lower bony tracts is very critical. If the points are out of alignment, the ligament stabilization effect will be poor in the light cases, and in the heavy cases, the ligament may not be equally spaced and affect the joint function. In the case of the femoral tract, it is usually located at the posterior edge of the medial femoral epicondyle near the cartilage, and because it is sometimes very close to the posterior joint capsule, it may irritate the surrounding tissues and cause pain in the N fossa. For this type of pain, there is no particularly effective way to treat it during rehabilitation. 3. Entrapment In the knee joint, due to the long-term restricted movement, some normal spaces can grow other tissues, such as synovial membrane or scar, causing occupancy. The posterior joint capsule part of the occupancy, during the knee flexion, will be squeezed, jammed, and subsequently cause pain. This pain is often sharp and becomes more pronounced as the angle of flexion increases, and disappears immediately upon straightening the knee joint. In this type of condition, prevention is more significant and requires achieving the required joint angle within the prescribed time to avoid the development of an occupancy. For those who have already developed a dominant position, it is necessary to frequently do some posterior joint capsule stretching, such as passive knee extension, so that the posterior tissue elasticity gradually increases and increases the posterior lateral space. 4. Abnormal trajectory We all know that the normal knee flexion process, between the tibia and the femur is a complex movement of rolling plus sliding, however, for the long-term restricted movement of the knee, the soft tissue elasticity of the joint is not good, so in the process of knee flexion, it is very easy to change into a simple rolling mode, and this directly caused by the posterior pressure of the joint suddenly increased, and then caused pain. This pain, although also increasing with the angle, is not sharp, but rather a dull pain, unlike the previous condition. The location of the pain is usually the entire N-fossa portion and is not deviated. The solution to this type of pain is twofold. One is to increase the posterior glide of the tibia through arthrodesis techniques (mainly the posterior tibial glide technique), which coincides with the theory underlying arthrodesis, the “law of convexity and concavity”. The second is to encourage the patient to increase the strength of the posterior femoral muscles, which can also increase the posterior tibial glide. It is important to note that extra caution should be exercised in patients with posterior cruciate ligaments. 5.Tendon extraction For many ligament reconstruction procedures, it is necessary to take the N cord muscle tendon to reconstruct the ligament. So this also inevitably results in pain at the N fossa. Firstly, the pain is mainly soreness and swelling, similar to the pain of muscle strain. Second, the location of the pain is the medial N fossa, sometimes radiating to the mid-thigh. This type of pain usually does not last long, but it should not be taken lightly because many patients feel most comfortable in a slightly flexed state of the knee, but over time, scar healing will form at the tendon extraction site, seriously affecting the normal muscle elasticity and causing limited knee extension at a later stage, so in the early postoperative period, patients should be educated to tolerate this pain as much as possible and practice passive knee extension. 6. Micromovement In the lateral side of the knee joint against the bottom, there is actually an inconspicuous joint – the upper tibiofibular joint. During normal knee flexion, the upper tibiofibular joint moves slightly to match the movement of the knee joint. However, in knees that have been limited in motion for a long time, the upper tibiofibular joint will gradually lose its micro-movement. Eventually, the upper tibiofibular joint does not compromise during flexion, and pain ensues. This type of pain is usually concentrated near the fibular tuberosity, and the pain is also sharp and sometimes popping. The treatment is to do a corresponding release of the upper tibiofibular joint and to do an oblique anterior-posterior sliding of the fibular tuberosity. The above six situations are a little bit of experience that I have summed up in the last few years. I hope that it will be helpful to patients and to rehabilitation practitioners, and I also hope that there will be more people and sum up more ideas.