Joint effusion after total knee arthroplasty (TKA) is considered as a manifestation of certain inflammatory reactions within prosthetic joints. This study investigated causes of joint effusion following TKA and analyzed phenotypic characteristics of synovial fluid leukocytes for each cause. Forty-six TKAs for rheumatoid arthritis (RA) and 49 TKAs for osteoarthritis (OA) displaying joint effusion were investigated. Causes of joint effusion were clinically identified and frequencies of each cause were compared between RA and OA. Synovial fluid cell phenotypes were analyzed using a fluorescence-activated cell sorter. Clinical diagnoses for joint effusion were classified into five different groups: deep infection (DI); increased activity of RA (IRA); particle-induced synovitis (PS); metal sensitivity (MS); and nonspecific synovitis (NS). The most frequent cause of post-TKA effusion was IRA in RA, and NS in OA. Biomaterial-related arthritis such as PS and MS were more frequent with OA than with RA. Analysis of synovial fluid cell phenotypes revealed that the characteristic cells for each diagnosis were CD16+CD14 - neutrophils in IRA and DI, CD14+ macrophages in PS, and CD3+CD45RO+ T cells in MS. Post-TKA joint effusion is clinically caused by five different types of arthritis. Phenotypic characteristics of synovial fluid leukocytes reflect joint pathology and contribute to diagnosis and exclusion of biomaterial-related arthritis.
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