How the Knee Works

The knee is comprised of four separate joints—one that joins the patella (the kneecap) and the femur (the thighbone), two joining the tibia (the shinbone) to the femur, and one connecting the tibia and the fibula.

With all of it’s complexity and the fact that the knee is a weight-bearing joint, it is more likely to be injured than any other joint in the body. The injury can occur in any of the knee’s different components.

  • Bones

    The femur has two rounded knobs on the end where it comes in contact with the tibia. The surface of the tibia on which they rest is the tibial plateau. This is divided into two halves: the lateral tibial plateau (the outer side), which is the half furthest from the other knee, and the medial tibial plateau (the inner side).

    The smaller joint connecting the fibula to the side of the tibia is a static joint, meaning that unlike the other joints in the knee, it moves very little.

  • Muscles

    There are four muscles in the front of the thigh called the quadriceps. To straighten the knee, you contract these muscles. The muscles in the back of the thigh are the hamstrings. When these contract, the knee bends.

  • Ligaments

    Ligaments are the strong bands of tissue that keep the ends of bones connected. There are four major ligaments in the knee. The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) are on the sides of the knee and prevent the knee from moving too much in a side-to-side direction.

    The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are in the front and back, respectively. These regulate the front-to-back bending of the knee.

  • Tendons

    Tendons are like ligaments, but connect muscles to bones. The quadriceps and hamstring muscles each have tendons that connect to the bone beneath. The largest tendon in the knee is the patellar tendon, which connects the patella to the tibia.

  • Articular Cartilage

    Articular cartilage is a tough, rubbery, shiny material that covers the ends of bones. About a quarter inch thick, it serves to absorb shock while providing a smooth surface to facilitate motion. When healthy, this system provides a mechanism that has almost no friction, so the joint can bend freely.

  • Meniscus

    The menisci are rubbery, crescent-shaped sections of fibrocartilage around the bone to fill the space between the round femur and the flat tibia. They primarily serve to preserve the knee by absorbing shock and spreading stress around the joint.

  • Bursae and synovial fluid

    The bursae are fluid-filled sacs that act as a gliding surface to reduce friction between the bones, tendons and muscles. They are filled with synovial fluid, a thick liquid that acts as a lubricant inside the joint.