Rheumatoid arthritis (RA) causes an inflammatory response in joints that leads to the formation of abnormal tissue called rheumatoid pannus.

Hand RAA rheumatoid pannus is abnormal tissue between the bones of the joint.
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Pannus is Latin for cloth or garment. In medicine, pannus is any abnormal tissue that:

  • Contains blood vessels
  • Covers up a normal body structure

In the case of rheumatoid arthritis, pannus tissue invades the space in between a joint’s bones, covering the bones and their protective layer of articular cartilage.

This article discusses how rheumatoid pannus forms and how it contributes to RA joint pain and damage.

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How Does Pannus Form?

Rheumatoid arthritis causes the immune system to attack the delicate synovial membrane, or synovium, that surrounds most joints. Below is a description of how the synovium thickens and changes, a process called rheumatoid pannus formation.

  1. Before the onset of RA, a smooth synovial membrane is just a few cells thick and produces synovial fluid, which lubricates and nourishes a joint.
  2. Rheumatoid arthritis can causes white blood cells to attack healthy synovium.
  3. The white blood cells release cytokines (proteins) that prompt the synovial membrane’s blood vessels to multiply. This change is referred to as hypervascularization.
  4. The increased blood flow leads to excess tissue growth. The synovial cells reproduce at an abnormally fast rate, causing the synovium to thicken.
  5. The synovial membrane develops microscopic projections called villi on its surface. This makes the tissue rough and uneven.
  6. The thickening tissue requires space, and it invades the small space between the joint’s bones. This invasion causes the pannus to cover the surface the bones and their articular cartilage.

The development of pannus can cause joint damage and symptoms such as inflammation, pain, and swelling.

Rheumatoid Pannus’ Role in Joint Pain and Damage

Pannus contributes to the joint pain and damage in several ways, including:

  • Excess fluid production. Unlike healthy synovial tissue that produces small amounts of nourishing joint fluid, the inflamed pannus produces excess fluid that contributes to joint pain and swelling. In addition, this fluid contains damaging proteins that degrade joint tissue.
  • Cartilage destruction. The pannus releases microscopic structures called lysosomes that contain and release enzymes (proteins). These enzymes, called matrix metalloproteinases (MMPs), degrade cartilage.
  • Bone destruction. Pannus contains high concentrations of cells called osteoclasts that secrete acids and proteins that damage bone. These acids and proteins are part of the body’s normal cycle of bone cell destruction and replacement, but in RA this process can spin out of control, and bone cell loss can occur in concentrated areas at a rate too fast to be replaced.

Over time the destruction of cartilage, bone, and other tissue can cause pain, loss of joint mobility, and even permanent deformity.

Written by Judith Frank, MD
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