The common causes of knee immobility in 8-year-old boys are: patella fracture, knee synovitis, meniscus injury and so on. 1. Patella fracture: if the child is playing basketball, ice hockey, soccer and other sports, the knee is accidentally hit or fall, it is easy to have a patella fracture. The patella can play a role in assisting joint flexion and stabilizing the knee joint. So when the patella is fractured, it can lead to the child’s knee is not sharp, and pain, there is a sense of bone rubbing. 2. Knee synovitis: If the child is cold or traumatized, it may cause congestion and edema of the synovial membrane of the knee joint, resulting in aseptic inflammation, which in turn causes pain, swelling, and restriction of activities of the knee joint, leading to knee discomfort. 3. Meniscus injury: mostly caused by knee sprains. When the injury is serious, it can lead to localized blood seepage, causing knee swelling, activity limitation, joint popping, and so on, and thus can also cause knee discomfort. When used as a twisting or rotating motion, it can make the pain worse. In conclusion, when the child’s knee is not moving well, it is recommended to improve the X-ray, MRI and other imaging examinations in time to clarify the cause of the disease and treat the symptoms in time.