RESUMO
INTRODUCTION AND IMPORTANCE: Actinomycosis is a chronic granulomatous disease associated with the Actinomyces species. This unusual condition, especially in the musculoskeletal system, has been considered a diagnostic challenge due to its initial non-specific symptoms requiring high clinical suspicion and an adequate diagnostic approach for its identification. CASE PRESENTATION: We present the case of a 39-year-old Hispanic female with right knee pain and associated purulent secretions for the past four years, who demonstrated persistent synovial actinomycosis despite arthrotomy with cleansing and debridement plus a long-term antibiotic regime. CLINICAL DISCUSSION: Actinomyces species remain a rare cause of musculoskeletal disease. Its presentation could include localized swelling, tissue fibrosis, sinus tracts, or an abscess, yet these symptoms are not specific, requiring high clinical suspicion to avoid a potential misdiagnosis. Culture in an anaerobic media and pathologic specimens are vital diagnostic tools. Among the treatment alternatives, antimicrobial therapy and surgery are usually required to manage bone and joint infections. Adequate antibiotic selection is crucial, as suboptimal treatment could promote the development of a persistent infection. CONCLUSION: This case highlights the diagnostic challenge of synovial actinomycosis, a rarely reported condition in native knee joints. High clinical suspicion is critical as early diagnosis, and adequate management is essential to avoid a persistent infectious process.