What is the cause of baker’s cyst?

A Baker’s cyst (named for the 19th century surgeon, William Morrant Baker) is a collection of fluid behind the knee. Baker’s cysts develop due to an accumulation of fluid in the knee joint that travels behind the knee and becomes trapped there. The reason fluid accumulates in the knee in the first place is usually trauma, torn cartilage, or some type of arthritis (such as rheumatoid arthritis or osteoarthritis).