Periprosthetic joint infection (PJI) is a serious complication after joint arthroplasty, with polymicrobial PJIs representing a distinct subset associated with worse outcomes. This study aims to characterize the risk factors, microbiological profiles, and clinical outcomes of polymicrobial PJIs in a single tertiary care center. A retrospective analysis was conducted on 499 patients diagnosed with PJI between 2010 and 2023. Polymicrobial infection was defined by isolation of ≥2 distinct pathogens from intraoperative samples. Demographic, microbiological, and clinical data were analyzed. Treatment success was defined as infection eradication without recurrence or chronic suppressive therapy. Polymicrobial PJIs accounted for 18.2% of cases. Patients with polymicrobial infections had higher rates of obesity, insulin-dependent diabetes, and higher Charlson comorbidity scores. Coagulase-negative staphylococci and gram-negative bacilli were more frequently isolated in polymicrobial infections, while S. aureus predominated in monomicrobial cases. Treatment success rates were significantly lower in polymicrobial infections, both in acute (61.5% vs. 94.5%, p = 0.003) and chronic settings (51.3% vs. 75.3%, p = 0.02). Polymicrobial PJIs are associated with distinct microbiological patterns, increased comorbidity burden, and significantly worse clinical outcomes. Recognition of specific risk factors and pathogen profiles is essential to optimize management strategies for this complex condition.

Infections following spinal surgery can result in potentially devastating complications. An accurate microbiological diagnosis is crucial for proper treatment. Sonication is a diagnostic method that can be beneficial in patients with acute or low-grade infections. This study aimed to assess the sensitivity and effectiveness of sonication as a method for diagnosing spinal implant infections in cases of both suspected and unsuspected infections during spinal surgical revision. We conducted a retrospective observational study that included all patients who underwent revision spinal surgery between March 2011 and October 2022. We collected the implants and surrounding tissues from these patients for microbiological analysis. The implant sonication was performed according to a previously published protocol. Patients were categorised into those undergoing surgical revision for suspected spinal implant infection (SSII) and those for non-suspected spinal implant infection (NSSII). We collected comprehensive patient data, including demographics, risk factors, Charlson Comorbidity Index (CCI), surgical details, microbiological findings, antibiotic regimens, and clinical outcomes. Sensitivity and specificity analyses were conducted on both sonicated and non-sonicated samples. A total of 158 patients met the inclusion criteria; 51 of them were diagnosed with infection during surgery revision. Patients with SSII had higher CCIs than those with NSSII. The sensitivity was significantly higher in sonicated samples (68.6%; 95% CI: 55.9–81.4%) than in non-sonicated samples (42%; 95% CI: 28.3–55.7%). The specificities were similar, with sonicated samples at 93.5% (95% CI: 88.8–98.1%) and non-sonicated samples at 99.05% (95% CI: 97.2–100.9%). Combining both methods resulted in sensitivity and specificity rates of 76% (95% CI: 64.2–87.8%) and 93.3% (95% CI: 88.2–98.1%), respectively. Methicillin-susceptible Staphylococcus aureus (MSSA) was common in SSII, whereas Cutibacterium acnes and coagulase-negative Staphylococcus (CNS) were predominant in NSSII. This study supports the routine use of implant sonication as a valuable supplementary method for peri-implant tissue cultures, especially for identifying low-grade spinal implant infections.