What is juvenile idiopathic arthritis (JIA)?

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JIA can occur in young people, up to age 16. Discover the differences in arthritis between young people and adults, how common arthritis is in young people, and how it can affect the joints.

Key points

  • Arthritis is inflammation in the joints, the places where the bones come together.
  • JIA affects about 10,000 children and teens in Canada.
  • Symptoms of joint inflammation include redness, swelling, warmth, stiffness, and pain.

The word arthritis comes from the Greek words "arthron" meaning joint, and "itis," meaning inflammation.

A joint is where two bones are joined or come together. Inflammation occurs when the body reacts to infection, irritation or other injury. Symptoms of inflammation include redness, warmth, swelling and pain. Arthritis means you have inflammation in your joints.

How the knee joint moves
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.

An animation of a knee joint bending.

Many people think that arthritis is something only older people get. Children and teenagers get a kind of arthritis called juvenile idiopathic arthritis (JIA). “Juvenile” means young (younger than 16 years old) and "idiopathic" means the cause is unknown. JIA is also sometimes called juvenile rheumatoid arthritis (JRA).

Here are some differences between arthritis in young people and arthritis in adults:

  • Osteoarthritis, a common form of arthritis in adults, is not common in young people with JIA.
  • It may take longer for young people with JIA to be diagnosed because many people (even some doctors) don’t realize young people can get arthritis.
  • JIA in young people may affect their growth​. If the JIA 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 that joint and make that part of the body grow faster. Injury can also occur to the growth plates , which may cause growth to be slower in some children or teens.
  • Children and teenagers with JIA may also get inflammation in their eyes.
  • Medications​ used to treat JIA 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?

JIA affects about 10,000 (one in 1,000) young people in Canada. Four times as many girls as boys are diagnosed with JIA.

How does JIA affect the joints?

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

Normal vs. arthritic joint

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 JIA, 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 warmth, stiffness, swelling, and pain. If left untreated, joint inflammation can damage the cartilage and underlying bone.

Last updated: January 31st 2017