The knee is a joint that allows for the motion of your leg, by bending and extending. It is essential for sitting, walking and running. The bones that make up the knee are the femur, or thighbone, the tibia, or shinbone, and the patella, or kneecap. Ligaments, which are bands of tough fibers, hold these bones together. Inside the knee, two ligaments hold the thighbone to the shinbone.
The first is the anterior cruciate ligament or ACL. This is especially important in running and other athletic activities. Its function is to keep the shinbone from sliding forward while you are moving. The second is the posterior cruciate ligament or PCL. It keeps the thighbone from sliding forward during knee motion.
Two types of cartilage are present in the knee joint. The first type of cartilage is articular cartilage. This cartilage covers the ends of the bones and provides a smooth gliding surface. The second type of cartilage is the meniscal cartilage. This consists of two C-shaped pieces of cartilage that sit between the thighbone and shinbone.
There is one meniscus on the inside and one on the outside of the knee joint. Each meniscus acts as a shock absorber between the shinbone and thighbone. To lubricate the knee, a lining of tissue inside the joint, called the synovium, produces a lubricating fluid.
The knee joint is surrounded by a joint capsule, that attaches to each side of the joint and helps keep the lubricating fluid inside. There are also muscles, ligaments, and tendons outside of the knee joint that function to stabilize the knee and allow active movement.