What Is Pannus?

Rheumatoid arthritis (RA) symptoms include swollen joints that may feel spongy when the skin over them is pressed. This spongy feeling is caused by pannus, an abnormal tissue that develops because of excess inflammation. Pannus leads to painful arthritis symptoms as well as the destruction of joint cartilage and bone.

This article discusses the formation of rheumatoid pannus, how it contributes to joint pain and damage, and how it is treated.

How Medical Professionals Describe Pannus

Most of the joints in the body are surrounded by a thin, delicate lining. If the lining of a joint becomes inflamed, it is called pannus.

Pannus can grow out of control, covering the surfaces of a joint’s bones and cartilage. The pannus releases fluids and chemicals that can eat away those tissues.

Pannus is a hallmark sign of rheumatoid arthritis. Doctors will look for indications of pannus during diagnosis, checking for spongy or boggy joints. Early in the disease, pannus is most likely to develop in small joints, such as those in the hands, wrists, and feet. Pannus can also appear in other, larger joints, such as the knees and shoulders.

See What Is Cartilage?

Pannus in other types of arthritis

It is not unusual for pannus to develop in joints affected by other types of inflammatory arthritis, such as psoriatic arthritis and lupus. However, rheumatoid pannus is typically much more severe and more damaging.

Pannus in other parts of the body

The term pannus is not exclusive to joints. Pannus is Latin for cloth or garment. In medicine, pannus is any abnormal tissue that:

• Contains blood vessels (necessary for tissue growth)
• Covers up a normal body structure

For example, doctors may use the term pannus to describe a thin film that grows over the eye’s cornea as a result of chronically severe dry eyes¹Zhang X, M VJ, Qu Y, He X, Ou S, Bu J, Jia C, Wang J, Wu H, Liu Z, Li W. Dry Eye Management: Targeting the Ocular Surface Microenvironment. Int J Mol Sci. 2017 Jun 29;18(7):1398. doi: 10.3390/ijms18071398. PMID: 28661456; PMCID: PMC5535891. or a past eye injury.²Jakobiec FA, Stacy RC, Mendoza PR, Chodosh J. Hyperplastic corneal pannus: an immunohistochemical analysis and review. Surv Ophthalmol. 2014 Jul-Aug;59(4):448-53. doi: 10.1016/j.survophthal.2013.10.005. Epub 2013 Oct 20. PMID: 24309126.

How Pannus Formation Occurs

Joint lining is called synovium. In healthy joints, synovium is thin and delicate. Rheumatoid arthritis causes the immune system to attack the synovium. The synovium then becomes inflamed, forming pannus. This process typically happens gradually, causing symptoms to develop over weeks or months.

Pannus formation occurs in these steps:

1. In a healthy joint, the smooth synovium is just a few cells thick. The synovium produces synovial fluid, which lubricates and nourishes a joint.

   See What Is a Synovial Joint?

2. Rheumatoid arthritis causes white blood cells to attack healthy synovium. This attack is the start of inflammation.
3. The white blood cells release chemicals (cytokines) that prompt the synovium’s blood vessels to multiply (hypervascularization). As blood vessels multiply, blood flow increases.
4. The increased blood flow encourages synovium’s cells to multiply. As more cells are produced, the synovium thickens.
5. Microscopic, finger-like projections grow on the normally smooth surface of the synovium. These projections, called villi, make pannus tissue rough and uneven.
6. The thickening synovium requires space and invades the space in between a joint’s bones. It spreads over the surface of the bones and their protective layer of articular cartilage. It also pushes out at the edges of the joint, making it look swollen.

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

How Rheumatoid Pannus Causes Joint Damage

Pannus contributes to joint pain and destruction. It does this by releasing chemicals that eat away at the joint tissue. Three different chemical reactions may be at work:

1. Pannus produces excess fluid

Healthy synovium tissue produces small amounts of fluid (synovial fluid) that helps nourish and lubricate the joint. In contrast, pannus tissue produces too much fluid, causing the joint to swell. In addition, the fluid contains damaging proteins that injure and destroy joint tissues.

2. Pannus cells destroy cartilage

The pannus cells contain microscopic structures called lysosomes. The lysosomes contain and release enzymes (proteins) called matrix metalloproteinases (MMPs) that eat away at cartilage.

3. Pannus cells destroy bone

Cells called osteoclasts produce chemicals—acids and proteins—that destroy bone cells. In a healthy joint, osteoclasts are part of the body’s normal cycle of bone cell destruction and replacement. In a joint affected by RA, there are too many osteoclasts and bone cells are destroyed faster than they can be replaced.

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

RA symptoms not related to pannus

Not all rheumatoid arthritis symptoms are related to pannus. Symptoms such as fatigue, fever, and weight loss have to do with body-wide (systemic) inflammation.

See Rheumatoid Arthritis (RA) Symptoms

Thankfully, treatment for rheumatoid arthritis can slow down or stop the formation of pannus as well as the body-wide inflammation, which can cause other RA symptoms.