Acute Septic Arthritis Caused by Finegoldia magna After ACL Reconstruction Diagnosed by PCR

Case: A 35-year-old immunocompetent woman presented 4 weeks after anterior cruciate ligament reconstruction with acute knee pain, swelling, erythema, restricted motion, and markedly elevated inflammatory markers. Synovial fluid Gram stain was negative, and preliminary cultures showed no growth. Rapid multiplex polymerase chain reaction (PCR) identified Finegoldia magna within hours. The patient underwent prompt arthroscopic debridement with graft retention and received 6 weeks of pathogen-directed antibiotics. Intraoperative anaerobic cultures subsequently confirmed F. magna . At the 1-year follow-up, she had full recovery without recurrence. Conclusion: This case highlights the diagnostic value of rapid multiplex PCR in culture-negative postoperative infections, enabling early targeted therapy and graft preservation.