Elbow synovitis related to an intraarticular osteoid osteoma of the humerus, with immunologic and histochemical studies

An 18-year-old boy presented with elbow synovitis. Investigations disclosed an osteoid osteoma of the coronoid fossa confirmed by histology. The synovium appeared hypertrophic with histologic patterns resembling those seen in synovitis in rheumatoid arthritis. Immunohistochemistry showed lymphoid follicles composed of B and T cells. T lymphocytes were mainly of the CD4 phenotype, showing soluble interleukin 2 receptor (IL-2r) in places but were poorly positive for DR antigen. C3, C4, B factor and CH50 activity were decreased and interleukin 1 and soluble IL-2r were increased in synovial fluid. They were normal in peripheral blood except for a slight decrease in C4. These data suggest a local immunologic activation induced by osteoid osteoma, the mechanism of which remains hypothetic. Immunomodulating mediator diffusion from osteoid osteoma itself or as a secondary response to tumoral antigen release could be advocated. Whether such phenomena are specific to the epiphyseal location of osteoid osteoma needs clarification.