Medical researchers have been able to show that these cysts form when there has been damage to the knee joint. Usually, this is a tear in the cartilage (meniscus) or damage to the bone from degenerative arthritis.
Medical advances have helped with the identification and treatment of these cysts. When diagnostic imaging such as ultrasonography became available, it was easier for doctors to see these cysts and look for a cause. Then knee surgery was improved by the development of arthroscopy. This method of looking inside the knee joint with a tiny camera gave doctors even more information.
It seems these cysts form when fluid squeezes out of the joint toward the back part of the knee in the area called the popliteal fossa, the indentation felt in the back part of the knee between the two hamstring tendons and the top part of the calf muscle.