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1.64% and 4.02% (Fig. 1). However, we detected a significant increasing trend in deep SSI rate (P<.001 for trend). The proportion of deep SA-SSIs increased annually by 9% (95% confidence interval [CI], 1.03–1.14). The trends were similar for both HPRO and KPRO, but the increase in the proportion of deep SA-SSIs was greater for HPRO (12% vs 5%). In the multivariable analysis, independent factors significantly


associated with increased risk of deep SA-SSIs included male gender, hip revision, operation during summer, ASA classifica- tion>1 , and duration of operation>120 minutes (Table 1). The effect of age differed between genders. The risk decreased with age among males, but no such effect was found among females. The risk factors were the same for HPRO and KPRO, except during the summer months (adjusted odds ratio [OR], 1.40; 95% CI, 1.05–1.87). Surgeries after 11 a.m. (adjusted OR, 1.39; 95% CI, 1.08–1.79) were associated with increased the risk in the HPRO group only, and the duration of operation (adjusted OR, 1.77; 95% CI, 1.39–2.26) was associated with increased the risk in the KPRO group only.


Discussion


Our study based on 17-year surveillance data of 157,082 ortho- pedic procedures showed that despite the decreasing trend of overall SSI rate, there were significant increases in the deep SSI rate and in deep infections caused by SA following HPRO and KPRO. Our overall annual SSI rate decreased from 4.0% to 1.6%,


and the trend was significant after adjustment by various patient, operation, and hospital-related factors. The similar decreasing trends of crude overall SSI rates in orthopedic sur- gery have also been observed in other national surveillance systems4,5 and European surveillance.6 In the ECDC 2016 annual report, the SSI rate after HPRO varied in different countries between 0.1% and 4.0% and after KPRO this rate varied between 0.1% and 1.4%. Deep SSI rates are rarely reported separately; the European figures are available in the ECDC Surveillance Atlas.7 We found a significant increase in deep SSI rates and deep SSIs


caused by SA. Unlike us, our Swiss colleagues showed a decreasing incidence of SSIs due to SA during 2004–2014 despite the increase in invasive MSSA infections, which might have been related to their national prevention activities.8 They did not


Jana Prattingerová et al


differentiate deep and organ-space infections, and they included all types of surgery. Staphylococcus aureus was responsible for 39% of microbiologically confirmed deep SSIs. This finding is in line with that of a Spanish study in which SA accounted for 36% of deep SSIs, and 22% were MRSA.9 We detected only 25 infec- tions (0.7%) caused by MRSA. The risk factors for SA-SSI we identified are mainly well


known: male gender, high ASA score, long duration of opera- tion, revision arthroplasties.3 However, the risk of deep SA-SSI was higher among young males than older, and in HPROs performed after 11 a.m. Notably, emergency operation was not a risk factor, neither were partial HPROs, all of which are pre- sumably fracture related. This finding suggests that all emer- gency operations might not be correctly coded and/or that there may be some patient-related risk factors or gaps in pre- or perioperative practices in certain patient groups, such as young men. Among factors not frequently studied, we found season- ality: summermonthswerearisk factor fordeep SA-SSI in HPRO. Similar findings have been reported by Kane et al,10 who speculated that the optimal condition during hot summer months might lead to increased bacterial population in the environment and on human skin. Our study has several limitations. First, we did not have


complete data on the use of perioperative antimicrobial prophy- laxis, and we do not know whether nasal decolonization and/or preoperative bathing was in use or when it was implemented. Neither did we have data on body mass index, smoking, alcohol abuse, or underlying conditions (eg, fractures or diabetes melli- tus). Second, the study period was long, and it is very likely that other changes occurred, for example in diagnostics of joint infections and postdischarge surveillance, which influenced our results. Realizing these possible biases, the proportion of culture confirmed SSIs and SSIs detected by postdischarge surveillance were included in our model. In summary, our study demonstrates that the increased rates


of deep SSIs could be hidden behind the overall SSI rates. HPRO and KPRO are rather heterogeneous procedure groups in terms of operation types (ie, elective or emergency, total or partial, and primary or revision), which should be considered when giving feedback to surgeons and ranking hospitals by SSI rates. These results suggest that pre- and perioperative practices of certain patient groups need further evaluation and that some groups may benefit from these efforts, such as those colonized with SA.


Fig. 1. Overall, deep and deep Staphylococcus aureus (SA) rate of surgical site infections (SSIs) following hip and knee arthroplasties, 19 Finnish hospitals during 1999–2016.


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