Anatomy of Juvenile Idiopathic Arthritis
Previously known as Juvenile Rheumatoid Arthritis (JRA)
For juvenile idiopathic arthritis (JIA), previously known as juvenile rheumatoid arthritis, the most important part of your anatomy to understand is the joints. JIA affects the various joints, and the affected joints won't be the same from patient to patient. Which joints become inflamed or swollen depends mostly on the type of JIA your child has.
- Polyarticular tends to affect the smaller joints (eg, fingers, wrists), although it can affect load-bearing joints, such as the knees.
- Pauciarticular tends to affect the larger joints (eg, knees, elbows, ankles)
- Systemic affects joints large and small
The spine's joints can be affected by JIA. There's a particular subtype of pauciarticular juvenile idiopathic arthritis that affects primarily the spine: juvenile ankylosing spondylitis.
To learn more about ankylosing spondylitis, please visit our Ankylosing Spondylitis Condition Center.
In polyarticular JIA, the spine's joints can also be affected. Usually, arthritic symptoms and signs are found in the neck (cervical spine). It's not as common for this type of JIA to go into the upper back (thoracic spine) or low back (lumbar spine).
This article will talk about the joints in your spine that can be affected by JIA. It will describe the components of the joints.
Cervical Spine Joints
To reiterate, polyarticular JIA can affect the neck (cervical spine).
At the top of your spine—at the place where your spine and skull come together—you have the C1 and C2 vertebrae. The 'C' stands for cervical, and doctors number each of the vertebrae to make it easier to talk about where there are problems in the spine.
C1 and C2 also get other names, though: C1 is the atlas vertebra. In Greek mythology, Atlas was doomed to carry the weight of the world on his shoulders. The atlas vertebra, as the first vertebra in your spine, is what your head rests on (you can think of your head as an entire world, if you'd like).
C2 is the axis vertebra. It has a special bony projection that serves at the pivot point for C1, so since it helps the atlas rotate, it's called the axis. The special bony projection is the odontoid process, also known as the dens. It's labeled as the dens in the drawing below.
The atlantoaxial joint can be affected by JIA, although researchers have found that only 2% of children with JIA show effects of it at the atlantoaxial joint when the disease first develops.
The facet joints in the cervical spine can also be affected by JIA. The facet joints are also called the zygapophyseal joints. As all joints in your body do, the facet joints help you move and are very key in your flexibility.
Although this description is of facet joints, the details apply to many other joints in the body. It's good to keep that in mind when thinking about how JIA can affect other joints, such as the fingers, wrists, and knees.
The facet joints have cartilage around them. The cartilage is there to protect bones as you move, and it makes it easier for your bones to move. The cartilage around joints is called hyaline cartilage, which is the most common type of cartilage. It is made of collagen fibers; collagen is a type of protein.
Surrounding the joint is a very, very thin lining called the synovium membrane. The whole joint—cartilage and all—is covered by a thin membrane called the synovium. The synovium produces synovial fluid, which is what truly helps your joints move well. The synovial fluid lubricates the cartilage and bone.
In juvenile idiopathic arthritis, white blood cells, which are supposed to help our bodies fight infection, can turn against the synovium and cause inflammation. Because of that inflammation, the body releases certain chemicals, and those chemicals thicken the synovium. Swollen joints are caused by the thickened synovium.
Thoracic and Lumbar Spines
It's rare for juvenile rheumatoid arthritis to go into the thoracic (upper or mid-back) or lumbar (low back) spines). It is possible, though. Those spinal regions also have facet joints, just like the cervical spine, so those joints can be affected by JIA.