Severe rheumatoid ahritis causes a proliferative synovitis with extensive damage to the synovium of the joint. The synol membrane becomes markedly thickened (pannus formation), with edematous villous projections that extend into the joint space. The intense inflammatory infiltrate that is present is typically composed of plasma cells, lymphocytes, and macrophages. Ref: Bednar M.S., Light T.R. (2006). Chapter 10. Hand Surgery. In H.B. Skinner (Ed), CURRENT Diagnosis & Treatment in Ohopedics, 4e.