Knee Anatomy

What Makes Up Your Knee?

Healthy Knee illustration

Basic Knee Anatomy

The knee is a complex joint made up of bones, muscles, tendons, ligaments, and cartilage. It’s often described as a hinge joint, similar to the hinge on a door. The knee not only bends back and forth but also has a complex rotational component that occurs with bending and straightening.

Each knee has two crescent-shaped cushion pads made up of cartilage, called meniscus. The lateral meniscus lies at the outer side of the knee and the medial meniscus lies at the inner side of the knee. The strong but flexible meniscus acts as shock absorber between the femur (upper-thigh bone) and the tibia (shin bone), especially during weight-bearing activities such as walking or climbing stairs. The meniscus also stabilizes and evenly distributes your body weight across the knee joint.

The ACL is one of a pair of crucial ligaments that forms a cross in the center of the knee joint. Both the ACL and the PCL function to stabilize the knee from front to back. The ACL limits the forward movement of the tibia, and the PCL limits the backward movement of this bone. Additional ligaments provide stability to other movements at the joint, including angulation (left/right motion) and rotation. Without such stability, you may feel that your knee is simply giving out from under you.

How A Healthy Knee Moves
Knee Condyles

X-ray showing the three compartments of a healthy knee.

The three bones that make up the knee joint are the femur (upper-thigh bone), the tibia (shin bone), and the patella (kneecap). The cartilage, or meniscus, is a protective cushioning that keeps the bones from rubbing against one another. In a healthy knee, a thin, smooth tissue liner called the synovial membrane releases a fluid that lubricates the knee, reducing friction as the bones move.

The knee can be divided into three compartments:

  • The medial compartment (inside of the knee)
  • The lateral compartment (outside of the knee)
  • The patello-femoral compartment (below the knee cap)

All the components of the knee — bones, cartilage, synovial membrane, ligaments, tendons and muscles — must work together properly for the knee to move smoothly.