What is Juvenile Idiopathic Arthritis?

The word arthritis comes from the Greek words "arthron" meaning "joint," and "itis", meaning inflammation. Thus, arthritis is inflammation in the joints, the places where the bones come together.

Many people think that arthritis is something only old people get. Children and teenagers get a type of arthritis called juvenile idiopathic arthritis (JIA). "Juvenile" means young (16 years of age or younger) and "idiopathic" means the cause is not known. JIA is also sometimes called juvenile rheumatoid arthritis (JRA).

How the Knee Joint Moves
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The main movements of the knee joint are flexion (bending) and extension (straightening). There is also rotation and sliding of the femur on the tibia. The movement is controlled by the muscles and ligaments in and around the knee.

Here are some differences between arthritis in children/teenagers and arthritis in adults:

  • One of the most common forms of arthritis in adults, called osteoarthritis, is not common in children or teenagers with JIA.
  • Rheumatoid arthritis (RA) is another common type of arthritis in adults that does not occur often in children or teenagers.
  • There are some types of arthritis in children and teenagers that do not occur in adults.
  • It may take longer for children and teenagers with JIA to be diagnosed because many people (even doctors) do not realize that children and teenagers can get arthritis!
  • JIA in children and teenagers may affect their growth. If the arthritis affects many joints or other body systems, they may grow more slowly overall. On the other hand, if only one joint such as the knee is affected, the inflammation can cause more blood flow to the joint and make that part of the body grow faster.
  • Children and teenagers with JIA may also get inflammation in their eyes.
  • Medications used to treat arthritis are generally the same in all age groups. However, doses need to be adjusted for children and teenagers based on their weight.

How common is JIA in children and teenagers?

  • JIA affects about 10,000 (one in 1000) children and teenagers in Canada .
  • Four times as many girls as boys get JIA.

How does arthritis affect the joints?

Look at the illustration below of a healthy joint and a joint with arthritis. See if you can pick out the differences between them.

Can you list the six parts that make up the joints?

Normal vs. Arthritic Joint
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In a healthy joint, a special sac called the articular capsule surrounds the space between two bones. The inner lining of this sac is called the synovial lining. The synovial lining makes a fluid that keeps the joint slippery. This fluid is similar to motor oil that keeps the parts of a car engine working properly. The ends of the bones are covered with a smooth substance called cartilage. The cartilage allows the bone ends to glide easily across each other.

In arthritis, changes in the immune system cause the lining of the sac to become inflamed and thickened. Extra fluid is produced, which contains inflammatory cells. These inflammatory cells make and release substances that can cause the symptoms of joint inflammation. These symptoms include redness, swelling, warmth, stiffness, and pain. If this is left untreated, joint inflammation can damage the cartilage and underlying bone.