Concentration and Frequency of Intraoperative Irrigation as a Strategy Against Implant-Associated Infection: Insights from a Preclinical Rat Model

Peri-implant infections remain a major challenge in orthopaedic surgery. Although the use of intraoperative irrigation to prevent surgical site infections is a standard practice across multiple surgical subspecialties, the choice of irrigation solution, the timing of application, and the frequency of irrigation are largely left to surgeon discretion. In current orthopaedic practice, intraoperative irrigation typically consists of either normal saline solution or antiseptic solutions such as diluted povidone-iodine (PVI) and chlorhexidine (CHG). Despite their ubiquitous use, there is no consensus regarding the optimal irrigation agent or concentration, reflecting a lack of concrete evidence from high-quality comparative trials and reliance on heterogeneous observational data1. In the absence of such studies, preclinical animal models provide an important platform to evaluate the efficacy of various intraoperative irrigation solutions, enabling the assessment of key parameters such as timing, duration, and frequency, which are difficult to standardize in clinical studies2. The present study by Ima and colleagues used an animal model of extra-articular implant-associated infection in which rats were inoculated with methicillin-susceptible Staphylococcus aureus. Their study provides unique insights into how irrigation solution concentration and exposure frequency may influence early bacterial burden and peri-implant tissue response. The authors compared the outcomes of 2 clinically relevant PVI concentrations (0.13% and 0.35%) and a normal saline solution control administered at 30- or 60-minute intervals during a 1-hour operative period. Their results demonstrated that both PVI concentration and irrigation interval were directly related to outcomes, with a higher PVI concentration and a shorter irrigation interval associated with lower recoverable bacterial counts, reduced macroscopic infection scores, and increased peri-implant bone mineral density. Two previously published preclinical models, one by Zhu et al.3 and the other by Penn-Barwell et al.4, offer useful comparisons. Zhu et al. utilized an intra-articular rat model in which a Kirschner wire, inoculated with S. aureus or Escherichia coli, was inserted into the femoral condyle and allowed to incubate for 1 hour before treatment with various irrigation solutions3. Irrigation with 0.3% PVI did not significantly reduce the joint bacterial burden compared with normal saline solution at postoperative day 7, in contrast with the findings of Ima et al. The differences in their findings may be explained by differences in methodology. In the intra-articular environment, the effectiveness of intraoperative irrigation may have been influenced by the 1-hour incubation prior to irrigation, the use of a single lavage, and the concentration of the PVI3. Penn-Barwell et al. evaluated CHG irrigation with use of a contaminated rat open-fracture model4. A segmental femoral defect was inoculated with S. aureus and left untreated for 6 hours prior to irrigation and debridement. Multiple CHG concentrations (0.5%, 0.05%, and 0.005%) were compared with normal saline solution, with no significant reductions in bacterial burden observed. The differences in outcomes may again be attributed to differences in methodology. Although Penn-Barwell et al. evaluated CHG rather than PVI, and therefore no direct comparison can be made, irrigation was performed after a 6-hour incubation period and consisted of a single lavage. Delayed irrigation, as demonstrated in the 2 comparative studies, may permit early biofilm development, which has been shown to substantially reduce bacterial susceptibility to antiseptic irrigation solutions5. Early and more frequent intraoperative irrigation may therefore represent a strategy for limiting bacterial adherence and reducing the postoperative bacterial burden. Despite its unique insights, the study by Ima et al. has several important limitations. Bacterial outcomes were assessed primarily using colony-forming unit (CFU) assays and macroscopic infection scores, without histologic confirmation using Gram staining or sophisticated techniques (e.g., the use of bioluminescent bacteria with longitudinal in vivo imaging2) to definitively verify bacterial presence or eradication. As the authors allude to, CFU assays carry inherent margins of error, making it difficult to draw firm conclusions regarding complete bacterial clearance. Additionally, the study did not directly evaluate cytotoxicity to surrounding tissues, an important consideration when assessing the safety of the PVI concentrations used. Future studies could build on these findings by applying the same preclinical model to alternative antiseptics, such as CHG; by incorporating histologic or molecular methods to definitively confirm bacterial eradication; and by evaluating longer-term outcomes to capture delayed infection and biofilm formation. Nevertheless, the present study remains unique in its systematic evaluation of multiple irrigation cycles during a single operative period, providing a framework for leveraging repeated, timed lavage in clinical practice to maintain antiseptic activity throughout surgery. Translating these insights to human procedures will require careful balancing of antimicrobial efficacy, tissue safety, and operative feasibility.