It prevents the femur from moving backwards onto the tibia, and the posterior cruciate ligament (PCL) prevents the femur from sliding forwards. There are two collateral ligaments, medial and lateral, that also help to provide support. The meniscus (lateral and medial) is tissue that sits between the femur and the tibia, providing ease of movement between the two bones. There is also articular cartilage that sits behind the patella. The knee is surrounded by bursae, fluid-filled sacs that help to cushion the knee joint.