Anatomy of the Knee

When patients with knee problems learn a bit about knee anatomy before they visit an orthopadeist, it will be easier for them to communicate with their doctor and understand their situation.


Functional anatomy of the knee:
The knee joint is made up of 3 bones (2 long bones and knee cap) and 2 joints. The main knee joint is the area between 2 long bones (tibia and femur) and it is called the tibiofemoral joint. The second joint is the area between the top long bone (femur) and kneecap (patella), and it is called the patellofemoral joint.
The regions where the bones contact each other are covered with the cartilage tissue, and this cartilage tissue enables the bones to easily slide over each other. Cartilage tissue in the knee joint is usually 2-3 mm thick and it is embedded tightly on the surface of the bones. In a healthy knee joint, the cartilages are very slippery. As a comparison, we might say cartilage slides on cartilage 3 times more easily then 2 pieces of ice sliding over each other. Cartilage tissue is created upon birth, and we live our whole lives with these cartilage tissues. Cartilage tissue unfortunately cannot regenerate or repair itself.
Our patellofemoral joint sometimes carries a lot of load, and sometimes there is noe load on it. As an example, the patellofemoral joint carries no load when we are standing still. When we are walking the load on the patellofemoral joint is a quarter of our body weight; when we are walking up steps, this load is 4 times our body weight and when we are jumping this load becomes as heavy as 8 – 10 times our body weight. The main knee joint (tibiofemoral joint) which consists of 2 long bones almost never rests, and there is almost always some load on this joint depending on the current activity.


Tibia and femur bones which make up the tibiofemoral joint, are actually geometrically incompatible. The lower end of the femur has round surfaces and the upper end of the tibia has straight surfaces. While we never realize it when we are healthy, the tibia and the femur always wear each other out. To decrease this wear and tear, we have “C” shaped tissues in between them called the menusci. The knee joint has 2 menisci; inner (medial) and outer (lateral). The menusci consist of a special type of tissue just like our cartilage tissue. The menusci are flexible so they can bend in harmony with knee joint’s motion.


The shape of the menusci is like a gasket; the outer part of the menusci is thicker, while the central parts are thinner. This unique formation of the menusci enables the round-ended femur and flat-ended tibia to be compatible with each other, enables the load to be equally spread through the tibiofemoral knee joint’s surface, enables it to work as a shock absorber, enables the tibiofemoral knee joint to be stable and provides nutrition to the cartilage tissue. In short, the menusci are vital and valuable tissues.

The knee joint is NOT like a hinge mechanism. There is linear and angular (circular) motion on all 3 axis for a total of 6 types of different motion. The knee joint’s motion has complex dynamics. For example, when we extend our leg, our foot points outside (externally); but when we fold our leg under our body, our our foot points inside (internally). In short, there is significant (approximately 20 degrees) rotation between our extended and fully flexed legs. This complex type of motion is created with the help of our bone shapes, muscles and ligaments. 
Ligaments are tissues that bind bone to bone, and in our knee joint we have 2 pairs (cruciate and collateral) of ligaments. The cruciate ligaments consist of an anterior (ACL) and a posterior (PCL). The collateral ligaments consist of a medial (MCL) and a lateral (LCL) ligament.
Our ligaments provide stability to our knee joint and they are always on the clock. If we damage our ligaments, we compromise the stability of our knees. Our muscles also help provide stability to our knee joint only when they are flexed, but when we are careless and we overload our muscles, our muscles cannot overtake the stability duties of our ligaments. Therefore, our knees give away, we can damage our menusci and cartilage tissues.