Vikas Shahi, MD, sports medicine physician at HSS, explained Baker's cysts, including what causes them and treatment options, with Runner's World.

A Baker's cyst is an overflow of fluid from the knee joint that collects at the back of the knee. Typically, the fluid flows from the knee joint to the backside through a one-way valve system, shared Dr. Shahi. So a Baker’s cyst is technically not a true cyst because it’s not actually enclosed in all spaces, he explained.

Baker’s cysts themselves are not problematic, but they’re often caused by underlying issues with the knee joint, Dr. Shahi said. The most common cause he sees in his clinic is osteoarthritis, which causes fluid buildup in the knee that can eventually spill over to the backside and form the cyst. Another reason: Inflammatory arthritis, which can trigger the knee to produce excess fluid, Dr. Shahi added.

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