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Anatomy of the Knee

To better understand how knee problems occur, it is important to understand some of the anatomy of the knee joint and how the parts of the knee work together to maintain normal function.

This guide will help you understand

  • What parts make up the knee
  • How the parts of the knee work
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Important Structures

The important parts of the knee include

  • Bones and joints
  • Ligaments and tendons
  • Muscles
  • Nerves
  • Blood vessels

The knee is the meeting place of two important bones in the leg, the femur (the thighbone) and the tibia (the shinbone).

The patella (or kneecap, as it is commonly called) is made of bone and sits in front of the knee.

The knee joint is a synovial joint. Synovial joints are enclosed by a ligament capsule and contain a fluid, called synovial fluid, that lubricates the joint.

The smaller bone of the lower leg, the fibula, never really enters the knee joint. It does have a small joint that connects it to the side of the tibia. This joint normally moves very little.

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Articular cartilage is the material that covers the ends of the bones of any joint. This material is about one-quarter of an inch thick in most large joints. It is white and shiny with a rubbery consistency. Articular cartilage is a slippery substance that allows the surfaces to slide against one another without damage to either surface. The function of articular cartilage is to absorb shock and provide an extremely smooth surface to facilitate motion. We have articular cartilage essentially everywhere that two bony surfaces move against one another, or articulate. In the knee, articular cartilage covers the ends of the femur, the top of the tibia, and the back of the patella.

Ligaments and Tendons

Ligaments are tough bands of tissue that connect the ends of bones together. Two important ligaments are found on either side of the knee joint. They are the medial collateral ligament (MCL) and the lateral collateral ligament (LCL).

Inside the knee joint, two other important ligaments stretch between the femur and the tibia: the anterior cruciate ligament (ACL) in front, and the posterior cruciate ligament (PCL) in back.

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The MCL and LCL prevent the knee from moving too far in the side-to-side direction. The ACL and PCL control the front-to-back motion of the knee joint.

The ACL keeps the tibia from sliding too far forward in relation to the femur. The PCL keeps the tibia from sliding too far backward in relation to the femur. Working together, the two cruciate ligaments control the back-and-forth motion of the knee. The ligaments, all taken together, are the most important structures controlling stability of the knee.

Two special types of disc shaped cartliages called menisci sit between the femur and the tibia.

The two menisci of the knee are important for two reasons: (1) they work like a bushing to spread the force from the weight of the body over a larger area, and (2) they help the ligaments with stability of the knee.

Without the menisci, any weight on the femur will be concentrated to one point on the tibia. But with the menisci, weight is spread out across the tibial surface. Weight distribution by the menisci is important because it protects the articular cartilage on the ends of the bones from excessive forces. Without the menisci, the concentration of force into a small area on the articular cartilage can damage the surface, leading to degeneration over time.

In addition to protecting the articular cartilage, the menisci help the ligaments with stability of the knee. The menisci enhance the stability of the knee and protect the articular cartilage from excessive concentration of force.

Taken all together, the ligaments of the knee are the most important structures that stabilize the joint. Remember, ligaments connect bones to bones. Without strong, tight ligaments to connect the femur to the tibia, the knee joint would be too loose. Unlike other joints in the body, the knee joint lacks a stable bony configuration.

Tendons are similar to ligaments, except that tendons attach muscles to bones. The largest tendon around the knee is the patellar tendon. This tendon connects the patella (kneecap) to the tibia. This tendon covers the patella and continues up the thigh.

The portion of tendon above the patella is called the quadriceps tendon since it attaches to the quadriceps muscles in the front of the thigh. The hamstring muscles on the back of the leg also have tendons that attach in different places around the knee joint. These tendons are sometimes used as tendon grafts to replace torn ligaments in the knee.

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Muscles

The extensor mechanism is the motor that drives the knee joint and allows us to walk. It sits in front of the knee joint and is made up of the patella, the patellar tendon, the quadriceps tendon, and the quadriceps muscles. The four quadriceps muscles in front of the thigh are the muscles that attach to the quadriceps tendon. When these muscles contract, they straighten the knee joint, such as when you get up from a squatting position.

The hamstring muscles are the muscles in the back of the knee and thigh. When these muscles contract, the knee bends.

Nerves

The most important nerve around the knee is the popliteal nerve in the back of the knee. This large nerve travels to the lower leg and foot, supplying sensation and muscle control. The nerve splits just above the knee to form the tibial nerve and the peroneal nerve. The tibial nerve continues down the back of the leg while the peroneal nerve travels around the outside of the knee and down the front of the leg to the foot. Both of these nerves can be damaged by injuries around the knee.

Blood Vessels

The major blood vessels around the knee travel with the popliteal nerve down the back of the leg. The popliteal artery and popliteal vein are the largest blood supply to the leg and foot. If the popliteal artery is damaged beyond repair, it is very likely the leg will not be able to survive. The popliteal artery carries blood to the leg and foot. The popliteal vein carries blood back to the heart.

Summary

The knee has a somewhat unstable design. Yet it must support the body’s full weight when standing, and much more than that during walking or running. So it’s not surprising that knee problems are a fairly common complaint among people of all ages. Understanding the basic parts of the knee can help you better understand what happens when knee problems occur.