Discover how your knee works    

At first, the knee appears to be a simple, uncomplicated part of the body. The truth, however, is that the knee is actually a complex mechanism with many different parts that need to work together perfectly to provide the kind of mobility that most people take for granted every day.

It is often said that the knee is the largest joint in the body. In essence, this is true, but the knee itself is actually 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 (a smaller bone in the lower leg). Due to the knee’s complexity and the fact that it 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 articulates, or comes in contact with the tibia. These are called the femoral condyles. 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 

The muscles in the front of the thigh are called the quadriceps. There are four of them, which is where their name comes from—"quadriceps" means four- headed in Latin. 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. When ligaments in the knee are torn or ruptured, tendons are sometimes used as grafts to replace the damaged ligament.

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, while also providing a softer, smoother surface to protect the articular cartilage from wear.

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.