Pannus is defined as thickening synovial tissue (also referred to as synovium or the lining of the joint) that covers articular cartilage, the type of cartilage found in the joints. As synovium thickens and proliferates, the joint actually fills with synovium and the abnormal synovium migrates across the articular cartilage, eventually producing erosions. As pannus progresses, it can invade the bone and bone marrow, and destroy surrounding structures like the joint capsule and tendons. (See this illustrated.)
Little is known about the behavior of osteoarthritis (OA) pannus compared to rheumatoid arthritis (RA) pannus. Researchers did a study for the purpose of comparing OA with RA pannus in vitro (in the lab environment). Results published in Clinical and Experimental Rheumatology found similarities between OA and RA pannus -- but why OA pannus invades the cartilage surface yet does not cause marginal erosions typically seen in RA was not explained.
Sources:
Rheumatoid Arthritis: Survey in MRI Features in Musculskeletal System. J HK Coll Radiol 2002;5:63-68
http://www.hkcr.org/publ/Journal/vol5no1/full/Vol.%205%20No.%201_63-68.pdf
Osteoarthritis and rheumatoid arthritis pannus have similar qualitative metabolic characteristics and pro-inflammatory cytokine response. Furuzawa-Carballeda J. et al. Clin Exp Rheumatol. 2008 Jul-Aug;26(4):554-60. http://www.ncbi.nlm.nih.gov/pubmed/18799084
