Microbiological profile of prosthetic joint infections in orthopedic oncology: a comparison with conventional joint arthroplasty

Background: Periprosthetic joint infections (PJIs) are a devastating complication following oncologic endoprosthetic reconstruction (EPR). Despite significant efforts to characterize the microbiologic profile of PJI in traditional joint arthroplasty, data are lacking in orthopedic oncology. Our study analyzed the causative microorganisms and time to positivity (TTP) of PJI in oncologic EPR and conventional joint arthroplasty (C-TJA). Methods: We retrospectively compared sample cultures for lower-extremity oncologic EPR and C-TJA patients diagnosed with PJI between 2000 and 2022. All positive microorganisms were assessed, along with clinical and culture method data. Comparisons utilized the Mann-Whitney $U$  test. Results: We included 70 oncologic EPR and 153 C-TJA patients diagnosed with PJIs. Staphylococcus epidermidis (16.8 % vs. 10.6 %, $p=0.01$ ), Enterococcus spp. (12.6 % vs. 4 %, $p<0.001$ ), and Peptostreptococcus spp. (5.3 % vs. 1.3 %, $p<0.001$ ) were common and more frequently isolated in oncologic EPR than C-TJA PJI. Conversely, Staphylococcus aureus predominated in samples from C-TJA patients (31.7 % vs. 15.1 %, $p<0.001$ ). Differences in endoprosthetic microorganism prevalence were observed between primary versus metastatic bone disease and bone versus soft tissue sarcoma. TTP was highly variable among microorganisms and was significantly faster ( $p<0.05$ ) for bone and soft tissue vs. synovial fluid (3 d vs. 4 d) and for broth and solid media vs. broth only (2.5 d vs. 4.5 d). Conclusion: The microorganism profile in oncologic EPR PJI was distinct from C-TJA PJI. The oncologic EPR population highlighted variability in the prevalence of Gram-negative rods and slower TTP for broth-only cultures. Further investigation of the mechanisms behind these differences will allow care teams to provide prompt, individualized, and targeted antimicrobial therapy.