🎈Baker’s Cyst: Causes & Treatment Options . Baker’s cyst are basically a fluid-filled sac that forms in the hollow space behind your knee joint. Baker’s cysts typically develop when your knee produces too much synovial fluid (the lubricating fluid in your joints) secondary to underlying issues including: • Arthritis (esp osteoarthritis) • Meniscus tears • Rheumatoid arthritis • Cartilage injuries . This excess fluid builds up and creates a bulge that can range from the size of a marble to a gol