Baker's cyst of the knee is a fairly common problem. It was originally described back in 1877--by Dr. Baker, of course! Modern medicine has been able to use ultrasound to view Baker's cysts. Updated surgical methods have allowed doctors to find out what causes this condition.
At first, it was thought that a Baker's cyst was a problem unto itself. But using a special instrument with a tiny TV camera on the end (called an arthroscope), doctors have been able to see inside the joint. An actual connection can be seen between the cyst and the backside of the joint. There is an open channel between the cyst and the joint through which fluid can pass.
What causes this channel to form? In almost all cases, there has been some damage to the joint. The most common cause of a Baker's cyst is either a tear in the knee cartilage (the meniscus) or damage to the bone from trauma or arthritis. Once doctors figured this out, they wanted to know if treating the cyst was necessary. Perhaps just treating the underlying problem would be enough to get rid of the cyst.
It turns out that this approach works in some cases. The cyst does go away when the meniscus is repaired. But cases of bone damage have a different outcome. Mild damage can be successfully treated with surgery, and the cyst disappears. But more extensive damage to the bone or cartilage means that surgery will not eliminate the cyst.
Doctors now know that Baker's cysts are caused by some other damage in the knee joint. This new information has changed the way doctors treat this problem. Instead of removing the cyst, doctors attempt to repair the damage to the bone or joint. When the injury isn't too great, the cyst goes away. Patients with severe joint damage may end up with a cyst that won't go away. And they also tend to have more pain and poor results.
Stefan Rupp, MD, et al. Popliteal Cysts in Adults. Prevalence, Associated Intraarticular Lesions, and Results After Arthroscopic Treatment. In The American Journal of Sports Medicine. January/February 2002. Vol. 30. No. 1. Pp. 112-115.'