Studies on surgical site infections after orthopaedic surgery : focusing on air-born transmission

<p dir="ltr">Paper I presents a retrospective cohort study based on data from the Swedish Hip Arthroplasty Register (SHAR) and the Swedish Cause of Death Register. The aim was to investigate the risk of mortality following revision surgery for prosthetic joint infection (PJI) after total hip arthroplasty (THA), compared with revisions for non-infectious causes, and more specifically to assess whether the infection itself contributed to the increased risk of death. We included all patients who underwent revision THA between 1998 and 2017. Revision due to PJI was defined as the exposure, and mortality as the outcome. Patients undergoing revision for non-infectious reasons served as the control group. The outcome was analyzed using a Cox proportional hazards model. A total of 4,943 PJI revisions and 12,529 non-infected revisions were included in the final analysis. The incidence rate ratio (IRR) was 1.19 (95% CI 1.13-1.25), the crude hazard ratio (HR) was 1.19 (95% CI 1.13-1.25), and the adjusted HR was 1.05 (95% CI 0.99-1.12). Mediation analysis identified age and comorbidity status as the strongest predictors of mortality. We concluded that the increased mortality risk following revision for PJI is primarily attributable to patient age and comorbidities rather than the infection itself.</p><p dir="ltr">Paper II presents a register-based retrospective cohort study aimed at investigating the association between air quality in the operating theatre (OT), measured as colony-forming units per cubic meter (CFU/m3), and the risk of surgical site infection (SSI) following orthopaedic surgery. The primary outcome was SSI within 12 weeks postoperatively, defined using a proxy variable termed the post-surgery infection marker (PSIM), a novel composite indicator based on antibiotic prescription data and diagnostic codes. The secondary outcomes were antibiotic use within 30 and 90 days respectively after surgery, and mortality within 2 years after surgery. Associations were analyzed using logistic regression models adjusted for sex, age, comorbidity score, surgery duration, and hospital regarding the primary outcome and first two secondary outcomes. Mortality was analysed using a Cox proportional hazards model. A total of 429 CFU measurements collected between 2013 and 2019 from 101 unique OTs across 30 hospitals were included. Patients who had undergone orthopaedic surgery in these OTs within three months prior to the CFU measurement were identified through the Swedish Perioperative Register (SPOR). After excluding patients with pre-existing infections or contamination at the index surgery, 4,603 procedures were included in the final analysis. No significant association was observed between CFU levels and SSI risk (adjusted OR per CFU unit: 1.00; 95% CI, 0.99-1.01; p = 0.81). The strongest predictors of SSI were male sex (OR: 1.34; 95% CI, 1.08-1.65) and comorbidity burden (OR per Elixhauser unit: 1.32; 95% CI, 1.22-1.43). CFU count showed no significant association with antibiotic use within 30 days (adjusted OR, 0.99; 95% CI, 0.97-1.01) or within 90 days (adjusted OR, 1.00; 95% CI, 0.98-1.01). For mortality, the adjusted hazard ratios (HR) was 1.00 (95% CI, 0.99-1.01; p = 0.90) per unit increase in CFU count. Surgery duration showed a modest but statistically significant association with SSI risk. We concluded that these findings challenge the assumption that lower CFU levels reduce the risk of SSI in modern OTs, suggesting that preventive efforts may be better focused on optimizing preoperative patient health and minimizing surgical duration.</p><p dir="ltr">Paper III represents the largest study in this thesis. It describes a multicenter, cluster-randomized, crossover, placebo-controlled, double-blinded trial designed to assess whether a new type of air purifier using plasma technology could reduce the risk of SSIs following orthopaedic surgery. We installed three air purifiers in each of 34 OTs at seven major hospitals across Sweden. These purifiers were programmed to switch ON and OFF at random intervals. Each set of three purifiers operated in synchronization, making each OT a defined cluster with crossover between intervention and placebo periods. All patients undergoing orthopaedic surgery in these OTs between 2017 and 2021 were identified using data from SPOR and included in the study. Patients who underwent surgery while the purifiers were ON formed the intervention group, while those operated on when the purifiers were OFF formed the control group. The primary outcome was SSI within 12 weeks postoperatively, defined using the same proxy variable (PSIM) as in Paper II. Secondary outcomes were also identical to those in Paper II: antibiotic use within 30 and 90 days postoperatively, and mortality within two years. Logistic regression was used to evaluate the primary and antibiotic-related outcomes, while mortality was analysed using a Cox proportional hazards model. After excluding patients with pre-existing infections or contamination at the index surgery, 40,547 procedures were included in the final analysis. Of these, 19,869 were in the intervention group and 20,678 in the control group. The PSIM rate was 9.2% in the intervention group and 9.4% in the control group, yielding an OR of 0.98 (95% CI, 0.91-1.05) for the intervention. This finding was consistent across various subgroups defined by diagnosis, hospital, and ventilation type. Likewise, no association was observed between air purification status and the secondary outcomes. We concluded that in modern OTs equipped with standard, midrange airflow ventilation systems, the addition of wall-mounted plasma air purifiers did not reduce the PSIM rate after orthopaedic surgery. Furthermore, no impact was observed on antibiotic use or mortality.</p><h3>List of scientific papers</h3><p dir="ltr">I. <b>Persson A,</b> Sköldenberg O, Mohaddes M, Eisler T, Gordon M. Increased mortality after total hip prosthetic joint infection is mainly caused by the comorbidities rather than the infection itself. Acta Orthopaedica 2023 Sep 26:94:484-489. <a href="https://doi.org/10.2340/17453674.2023.18619" rel="noreferrer" target="_blank">https://doi.org/10.2340/17453674.2023.18619</a></p><p dir="ltr">II. <b>Persson A,</b> Sköldenberg O, Gordon M. The correlation between bacterial load in the operating theatre and surgical site infections following orthopaedic surgery: a register-based cohort study [Manuscript]</p><p dir="ltr">III. <b>Persson A,</b> Atroshi I, Tyszkiewicz T, Hailer NP, Lazarinis S, Eisler T, Brismar H, Mukka S, Kernell P-J, Mohaddes M, Sköldenberg O, Gordon M. Effect of Plasma Air Purifiers on Infection Rates in Orthopaedic Surgery. NEJM Evidence 2025 2025 Apr;4(4):EVIDoa2400289. Epub 2025 Mar 25. <a href="https://doi.org/10.1056/EVIDoa2400289" rel="noreferrer" target="_blank">https://doi.org/10.1056/EVIDoa2400289</a><br><br></p>