Atypical Periprosthetic Joint Infection Post-Total Knee Arthroplasty: A Case of Actinomyces europaeus

Periprosthetic joint infection (PJI) is a significant complication following total knee arthroplasty (TKA), representing a substantial challenge due to the difficulty in diagnosis and management. The main causes are predominantly common bacteria, but rare pathogens such as <i>Actinomyces europaeus</i> can complicate diagnosis and treatment. We report a unique case of a 75-year-old Caucasian patient with a history of multiple comorbidities including obesity, arterial hypertension, total thyroidectomy, rheumatoid arthritis, and prior venous thrombosis. The patient presented with pain, functional impairment, and signs of inflammation in the left knee seven months post-TKA. An active fistula was also noted. Initial management with broad-spectrum antibiotics did not halt the progression of infection, prompting further diagnostic evaluation. <i>Actinomyces europaeus</i> was identified as the causative agent through cultures and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). A two-stage surgical intervention was necessitated, involving removal of the infected prosthesis followed by reimplantation with an antibiotic-impregnated spacer. This case highlights the importance of considering rare organisms like <i>Actinomyces europaeus</i> in atypical PJI. It underscores the necessity of advanced diagnostic tools and susceptibility testing in managing infections effectively. Timely intervention, tailored antimicrobial therapy, and appropriate surgical strategies are crucial for successful outcomes in infections involving uncommon pathogens.