[Differential diagnosis of chronic synovitis]

This review presents an algorithm for the standardised histopathological diagnostics of synovial biopsies and synovectomy specimens. In general, changes of the synovium can be inflammatory or non-inflammatory. To the latter group belong certain benign tumors such as the diffuse variant of the tenosynovial giant cell tumor, lipoma or synovial chondromatosis, additionally the rare group of storage diseases should be kept in mind. Inflammatory diseases can be discriminated into crystal-induced arthropathies such as gout and pseudogout, into granulomatous diseases such as tuberculosis, sarcoidosis and foreign-body inoculation, and into the large group of non-granulomatous synovitis. This group is by far the most common, and it often causes difficulties in assigning the histopathological findings to a concrete diagnosis. Therefore, the synovitis-score should be applied as a diagnostic device in these cases, leading to the diagnosis of a low-grade synovitis (which is associated with degenerative arthropathies) or of a high-grade synovitis (associated with rheumatic diseases), the sensitivity and specificity being 60.5% and 95.5%, respectively.