Rheumatoid nodules are found in approximately 25% of patients with definite or classic. They often form at inflamed joints (knuckles, elbows etc) and vary in size from 2 mm to 5 cm, They are usually painless.
It's not currently known why they form.
Although the exact process is unknown, there are a few hypotheses for the generation of rheumatoid nodules. It has been observed that rheumatoid nodules frequently form over extensor sites and areas vulnerable to trauma. The trauma causes inflammatory particles to build up and leads to a secondary inflammatory response which ultimately causes fibrin release and necrosis. Another hypothesis suggests that the inflammation of blood vessels activates complement components, which leads to the deposit of rheumatoid factors and fibrin. Source Wikipedia