The knee is the lower extremity joint connecting the femur, patella, and the tibia. Since in humans the knee supports nearly the entire weight of the body, it is the joint most vulnerable both to acute injury and the development of osteoarthritis.

Human anatomy

Upon birth, a baby will not have a conventional knee cap, but a growth formed of cartilage. In human females this turns to a normal bone knee cap by the age of 3, in males the age of 5.

The knee is a complex, compound, condyloid variety of a synovial joint which hovers. It actually comprises two separate joints.

• The femoro-patellar joint consists of the patella, or "kneecap", a so-called sesamoid bone which sits within the tendon of the anterior thigh muscle (m. quadriceps femoris), and the patellar groove on the front of the femur through which it slides.

• The femoro-tibial joint links the femur, or thigh bone, with the tibia, the main bone of the (lower) leg. The joint is bathed in a viscous (synovial) fluid which is contained inside the "synovial" membrane, or joint capsule.

The recess behind the knee is called the popliteal fossa. It can also be called a "knee pit."

Menisci

These are cartilaginous elements within the knee joint which serve to protect the ends of the bones from rubbing on each other and to effectively deepen the tibial sockets into which the femur attaches. They also play a role in shock absorption. There are two menisci in each knee, the medial meniscus and the lateral meniscus. Either or both may be cracked, or torn, when the knee is forcefully rotated and/or bent.

Synovial Fluid

Synovial fluid is a thick, stringy fluid found in the cavities of synovial joints. With its egg-like consistency (synovial comes from Latin for "egg"), synovial fluid reduces friction between the articular cartilage and other tissues in joints to lubricate and cushion them during movementhttp://en.wikipedia.org/wiki/Synovial_fluid.

Movements

The knee permits the following movements: flexion, extension, as well as slight medial and lateral rotation. Also, the knee has special locking and unlocking mechanisms, related to movement by the femoral condyles on the tibial plateau. The ligaments and menisci, along with the muscles which traverse the joint, prevent movement beyond the knee's intended range of motion. It is also classified as a hinge joint.

The range of movement is as follows: Flexion is permitted up to 120º when the hip is extended, 140º when the hip is flexed and 160º when the knee is flexed passively. Medial rotation is limited to 10º and lateral rotation to 30º .

Blood supply

The femoral artery and the popliteal artery help form the arterial network surrounding the knee joint (articular rete). There are 6 main branches: