Rheumatoid arthritis (RA) causes an inflammatory reaction in the joints, leading to the formation of abnormal tissue called rheumatoid pannus. This article discusses how rheumatoid pannus forms and how it contributes to joint pain and RA damage.
In medicine, a pannus is any abnormal tissue that:
- It contains blood vessels.
- Covers the normal structure of the body.
In the case of rheumatoid arthritis, the pannus tissue invades the space between the bones of the joint, covering the bones and their protective layer of articular cartilage.
How is rheumatoid pannus formed?
Rheumatoid arthritis causes the immune system to attack the thin synovial membrane or synovium that surrounds most joints. The following is a description of how the synovial membrane thickens and changes, this process is called rheumatoid pannus formation.
- Before the onset of RA, the smooth synovial membrane has a thickness of only a few cells and produces synovial fluid, which lubricates and nourishes the joint.
- RA can cause white blood cells to attack a healthy synovial membrane.
- White blood cells secrete cytokines (proteins), which cause the blood vessels of the synovial membrane to multiply. This change is called hypervascularization.
- Increased blood flow leads to excessive tissue growth. Synovial cells multiply at an unusually high speed, which leads to a thickening of the synovial tissue.
- The synovial membrane develops microscopic projections called villi on its surface. This makes the fabric coarse and uneven.
- Thickening tissue requires space, and it invades a small space between the bones of the joint. This invasion causes the pannus to cover the surface of the bones and their articular cartilage.
The development of pannus can lead to joint damage and symptoms such as inflammation, pain, and swelling.
The role of rheumatoid pannus in joint pain and damage.
Pannus contributes to joint pain and damage in several ways, including:
- Excessive fluid production. Unlike healthy tissues of synovial fluid, which produces a small amount of nutrient joint fluid, the inflamed pannus produces excess fluid, which contributes to joint pain and swelling. In addition, this fluid contains damaging proteins that destroy joint tissues.
- Cartilage destruction. Pannus releases microscopic structures called lysosomes that contain and secrete enzymes (proteins). These enzymes, called matrix metalloproteinases, destroy cartilage.
- Bone destruction. Pannus contains high concentrations of cells called osteoclasts, which secrete acids and proteins that damage bone. These acids and proteins are part of the normal cycle of destruction and replacement of bone cells in the body, but in RA this process can break out of control, and bone cell loss can occur in concentrated areas at too high a rate to be replaced.
Over time, the destruction of cartilage, bone, and other tissues can cause pain, loss of joint mobility, and even permanent deformation.