The knee joint is an incredibly strong, intricate and complex part of human anatomy. It is the largest joint in the human body. Its role is to provide strength, support, and flexibility while standing, walking, and bending down.

The knee is a hinge joint, meaning it allows the leg to extend and bend in one direction. The knee is considered a mobile, or pivotal hinge joint as it also allows minimal medial and lateral rotation.

The knee is a hinge joint made up of bone, cartilage, muscles, and ligaments.

To allow the necessary movements and provide a great deal of strength and structure, there are many structures in the knee joint that need to function well:

  • Knee bones. The bones must maintain their strength and smooth surface in order to move easily against each other. Development of boney growths, called osteophytes or bone spurs, may impede this function and cause pain.
  • Knee cartilage. The cartilage must be smooth and strong to allow the bones to move against each other, acting as the hinge in the joint, without too much friction.
  • Knee muscles. The muscles - which include the quadriceps muscles in front of the knee and the hamstrings on the back of the knee - must be both flexible to allow normal range of motion and strong to adequately support the knee joint.
  • Knee ligaments and tendons. The multiple ligaments and tendons around the knee must be strong to bind the knee joint together and encapsulate it in a tough but flexible structure.

Problems occur when any of these parts of the knee joint start to degenerate or are in some way damaged or irritated.

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Structure of the Knee Joint

The human knee joint is constructed and functions as follows:

  • The knee is located where the femur (thigh bone) meets with the tibia (shin bone).
  • The kneecap, called the patella, sits along a groove at the bottom end of the thigh bone.
  • The knee joint has two places where bones join and move against one another:
    • At the junction where the femur meets the tibia
    • At the junction where the femur meets the patella.
  • Where the bones meet they are covered with articular cartilage, an extremely slippery, strong, flexible material that allows the bones to glide over each other as the knee bends and straightens. The articular cartilage also acts as a shock absorber, cushioning bones against impacting each other (e.g. during jumping).
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  • In addition to the articular cartilage, two "C" shaped pieces of cartilage fit between the femur and tibia, each called a meniscus, which provide additional shock absorption.
  • The knee joint has many stabilizing structures, including two major tendons that connect bone to muscle and four major ligaments that connect bones to bones. These tendons and ligaments essentially encase the movable parts of the joint, providing support for the integrity of the joint while allowing for full range of motion.
  • There are also bursae around the knee joint. A bursa is a little fluid sac that helps the muscles and tendons slide freely as the knee moves.
  • The entire knee joint is encapsulated in the synovial membrane, which produces synovial fluid. This viscous fluid lubricates and circulates nutrients to the joint.
    • When the knee is at rest, the synovial fluid is contained in the cartilage, much like water in a sponge.
    • When the knee bends or bears weight the synovial fluid is squeezed out. Therefore, joint use is necessary to keep joints lubricated and healthy.

Since the knee joint bears most of the weight of the body, it is particularly prone to both acute injury, such as a sports injury, and degeneration over time, such as with osteoarthritis.

See Possible Causes of Severe Knee Pain

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