This expansion of the synovial membrane, sometimes referred to as "pannus," can lead to bony erosions and cartilage degradation at the site of the cartilage-bone junction in the periarticular bone.It is important to note that the cause of rheumatoid arthritis is unknown. It is speculated that genetic and environmental factors can both contribute to the development of rheumatoid arthritis. Gene loci and antigens, such as HLA class II antigens, have been seen as closely associated with rheumatoid arthritis. Environmental risk factors include smoking, periodontitis, viral infections and gut, mouth and lung microbiomes. Researchers have noted that Prevotella species, which are expanded in the gastrointestinal tract in early RA, and Porphyromonas gingivalis, which is associated with periodontitis, may have a role in pathogenesis. Pathology
Histological examination of nodules shows that they consist of a shell of fibrous tissue surrounding a center of fibrinoid necrosis. Pea-sized nodules have one centre. Larger nodules tend to be multilocular, with many separate shells or with connections between the necrotic centers. Individual necrotic centers may contain a cleft or several centers of necrosis may all open on to a large bursal pocket containing synovial fluid. The boundary between the necrotic center and the outer fibrous shell is made up of the characteristic feature of the nodule, which is known as a cellular palisade. The palisade is a densely packed layer of macrophages and fibroblasts which tend to be arranged radially, like the seeds of a kiwifruit or fig.
A rheumatoid nodule is a lump of tissue, or an area of swelling, that appear on the exterior of the skin usually around the olecranon (tip of the elbow) or the interphalangeal joints (finger knuckles), but can appear in other areas. There are four different types of rheumatoid nodules: subcutaneous rheumatoid nodules, cardiac nodules, pulmonary nodules, and central nervous systems nodules. These nodules occur almost exclusively in association with rheumatoid arthritis. Very rarely do rheumatoid nodules occur as rheumatoid nodulosis (multiple nodules on the hands or other areas) in the absence of rheumatoid arthritis. Rheumatoid nodules can also appear in other areas of the body other than the skin. Less commonly they occur in the lining of the lung or other internal organs. The occurrence of nodules in the lung of miners exposed to silica dust was known as Caplan’s syndrome. Rarely, the nodules occur at diverse sites on body (e.g. upper eyelid, distal region of the soles of the feet, vulva and internally in the gallbladder, lung, heart valves, larynx, and spine).Rheumatoid nodules can vary in size from 2 mm to 5 cm and are usually rather firm to the touch. Quite often they are associated with synovial pockets or bursae. About 5% of people with rheumatoid arthritis have such nodules within two years of disease onset, and the cumulative prevalence is about 20 to 30%. Risk factors of developing rheumatoid nodules include as smoking and trauma to small vessels.In the majority of the time, nodules are not painful or disabling in any way.