The knee is designed for its own protection. It is completely surrounded by a joint capsule that is flexible enough to allow movement but strong enough to hold the joint together. The capsule is lined with synovial tissue, which secretes synovial fluid to lubricate the joint. Wear-resistant cartilage covering the ends of the thighbone (femur) and shinbone (tibia) helps reduce friction during movement. Pads of cartilage (menisci) act as cushions between the two bones and help distribute body weight in the joint. Fluid-filled sacs (bursae) provide cushioning between structures such as the tibia and the tendon attached to the kneecap (patellar tendon). Five ligaments along the sides and the back of the knee reinforce the joint capsule, adding stability. The kneecap (patella) protects the front of the joint.