Anatomy of the Knee
To understand the knee joint, we first need to understand the bones of the leg that form the joint. Three bones make up the knee joint – the femur, the tibia and the patella. The femur (thigh bone) is the largest bone in the body and extends from the hip to the knee where it ends in structures known as condyles that are covered in cartilage. The condyles of the femur form the upper part of the knee joint that articulates (bends and glides) against the top of the tibia (shin bone) in a region termed the tibial plateau that is also covered in cartilage. The patella (knee cap), the backside of which is covered in cartilage, sits on the front of the knee and articulates against the femoral condyles.
Between each of the femoral condyles and the tibial plateau sits a crescent shaped soft tissue structure called the meniscus. The meniscus is made from fibrocartilage and acts as a shock absorbing, stabilizing pad between the femur and the tibia. Both meniscus and cartilage tissues lack blood vessels and nerve endings that make repair of these tissues difficult once they are damaged.
Ligaments are another key component of the knee that hold the joint in a stable and properly aligned position as it articulates through a normal range of motion.
What is Cartilage
Cartilage is a tissue that can provide structure, act as a shock absorber, and provide a smooth, friction-free surface that allows our joints to work and our bones to painlessly move against each other. Cartilage is made up of special cells, called chondrocytes, that lay down a tissue made of a specialized protein and collagen fibers around themselves. This tissue, called cartilage, is incredibly important to the human body.
There are three kinds of cartilage:
Elastic cartilage makes up flexible structures of the body, such as the nose and the ear.
Fibrocartilage is considered inflexible and tough. It acts more like a shock absorber. Examples may be found between the vertebrae of the spine and the menisci of the knee.
Hyaline cartilage is found in the articular cartilage that covers the ends of the bones within a joint. It provides a smooth surface to reduce friction when a bone moves over another bone within the joint. Articular cartilage covers the ends of all bones found in joints (knee, ankle, hip, shoulder, elbow, wrist and finger) and allows them to slide freely in the joint.
For the sake of clarity, the knee cap is not shown.