Knee Anatomy and Physiology

Anatomy of the knee.
Anatomy of the knee. SEBASTIAN KAULITZKI/Getty Images

The knee is the largest joint in the body and is often the site of pain and injury. It is a hinge joint, it bends and straightens (flexes and extends). It also allows a small amount of rotation when the lower leg is flexed.

The knee joint is made up of the lower end of the femur (thigh bone) and the upper end of the tibia (shin bone). The patella (kneecap) slides along a groove on the femur, and covers the front of the joint.

The knee joint is often referred to as having three compartments, which have significance in knee replacement. The patellofemoral compartment is where the kneecap rides over the end of the femur. The medial compartment is on the inside aspect of the knee where the femur and tibia meet, while the lateral compartment is on the outside aspect where the femur and tibia meet.

Knee Meniscus, Cartilage and Capsule

The meniscus and cartilage cushion the spaces between the bones of the knee. They act as shock absorbers during movement. The surfaces of the bones of knee are lined with articular cartilage. This elastic tissue not only keeps bone from rubbing on bone, it provides a smooth gliding surface for the movement of the knee. In osteoarthritis, the cartilage is lost or becomes bumpy, which results in stiffness and pain.

The lateral meniscus and medial meniscus are crescent-shaped disks that separate the femur and the tibia.

They protect the ends of the bones so they don't rub against each other and they absorb shock.

The knee joint is a synovial joint, meaning it is covered by a synovial membrane, which produces synovial fluid, keeping the joint lubricated. It is one layer of the articular capsule, with a fibrous membrane and fatty deposits making up the other layers.

There are over a dozen knee bursae, or fluid-filled sacs all around the joint cavity, under the tendons and ligaments.They are filled with synovial fluid and they act as cushions. There is also a the infra patellar fat pad just below the knee that protects the kneecap.

Muscles, Tendons and Ligaments of the Knee

The quadriceps muscle groups on the front of the upper leg help straighten the leg from a bent position, while the hamstring muscle group, on the back of the upper leg, help to bend the knee.

Several large fibrous bands of tissue, called ligaments, support the knee on both sides. They provide strength and stability to the joint. The four ligaments that connect the femur and tibia are:

  • The medial collateral ligament (MCL) provides stability to the inside of the knee.
  • The lateral collateral ligament (LCL) provides stability to the outside of the knee.
  • The anterior cruciate ligament (ACL), in the center of the knee, limits rotation and the forward movement of the tibia.
  • The posterior cruciate ligament (PCL), also in the center of the knee, limits backward movement of the tibia.

    Tendons are tough cords of tissue that connect muscle to bone. In the knee, the quadriceps tendon connects the quadriceps muscle to the patella and helps provide power to extend the leg. The patellar tendon connects the patella to the tibia.

    The blood supply for the knee comes from the femoral artery and the popliteal artery.

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