A Baker cyst typically is caused by an issue with the knee joint, such as arthritis or a cartilage tear. Both conditions can cause the knee to produce too much synovial fluid. This fluid lubricates joints to reduce friction for pain-free motion. But the fluid can build up and move into a bursa, which is a small, fluid-filled sac that cushions the joint and reduces friction. In the case of a Baker cyst, the fluid collects behind the knee, causing it to swell and form a cyst.

Symptoms

• Swelling behind the knee, and sometimes in the leg.
• Knee pain.
• Stiffness that can make it hard to fully bend the knee.

Causes

A baker cyst can be caused by:

• Inflammation of the knee joint,
  arthritis
  osteoarthritis
• rheumatoid arthritis.
• A knee injury
  cartilage tear.

Complications

This may cause:

• Sharp pain in the knee.
• Swelling in the calf.
• A change in skin color of the calf or a feeling of water running down the calf. The change in color may be a shade of red, purple or brown depending on your skin color.