The most common cysts in the knee joint are meniscal cysts and popliteal cysts, both of which are benign cysts. In the case of meniscal cysts or popliteal cysts, they are generally not cancerous. Because they are benign swellings, they are caused by the fluid in the joint cavity running outside the joint, so they usually do not cause cancer. Meniscal cysts or popliteal cysts are formed mainly because the meniscus is damaged or torn, and the joint fluid seeps out along the fissure and forms cysts outside the joint. Therefore, as long as the meniscal tears are sewn up so that the joint fluid cannot leak out, there is usually no further cyst problem. The most fundamental problem with meniscal cysts and popliteal cysts is to have arthroscopic surgery to sew up the meniscal tears inside.