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Hannah Wolford et al
Fig. 1. Flow chart depicting general model for obtaining annual arthroplasty and surgical site infection (SSI) counts. Arthroplasty rates were applied to the baseline population, and SSI rates are applied to this resulting population receiving arthroplasties. Mortality rates were not applied to individuals between arthroplasty and SSI branches (by definition 90 days or less). The cycle was repeated annually, with the baseline population as the population aged 1 year from the previous year without an arthroplasty or death. The model was run separately for primary and revision hip and knee arthroplasty (4 separate models).
SSIs. Surgery and SSI burdens were projected annually using 4 separate Poisson models: primary knee, primary hip, revision knee, and revision hip. Arthroplasty rates were applied to the baseline population to create a cohort of individuals who had surgery. The SSI rates were then applied to the resulting cohort. This cycle was repeated annually with the baseline population aging by 1 year, excluding individuals who died or previously had surgery and including individuals who aged into each cohort (Fig. 1). A second cohort of patients who previously had an arthro- plasty were included in a separate model that applied rates of revision arthroplasties and subsequent SSIs in the same manner. Mathematical computations are described more fully in the Appendix. We selected a static model because (1) the infection rates were low, (2) SSIs have different infection etiologies, and (3) previous studies have shown clustering has little effect on overall infection rates.17 Each model was run from January 1, 2020, through December
31, 2030. We used the most recently available data for arthro- plasty and SSI rates, as described in the Model Inputs section. Age and gender cohorts were defined as males and females restricted to 4 age categories: 18–44, 45–64, 65–84, and >85 years old. Confidence intervals were calculated for each annual estimate
by combining the parameter and Poisson variance. The Poisson variance of each projection was equal to the mean of the pro- jection. Parameter variance was estimated using a bootstrapping method. We assigned a normal distribution to each age- and gender-stratified arthroplasty and SSI rate based on their mean and standard deviation. Next, we ran 1,000 trials of the Poisson process, randomly selecting rates from each normal distribution. We estimated our parameter variance as the variance of these trials.
Model Inputs Primary procedures
For our primary arthroplasty models, we created a synthetic population to match the 2015 US Census Bureau population
projections for each birth cohort, stratified by age, sex, race, and Hispanic ethnicity.18 Our population of adults increased from 247,733,509 in 2015 to 268,898,729 in 2030. Primary arthroplasty rates were estimated for years 2012
through 2014 for each age and gender cohort from the National Inpatient Sample (NIS). The number of arthroplasties were identified using International Classification of Disease, Ninth Revision, Clinical Modification (ICD-9-CM) procedure codes located in any position on the discharge record. For hip arthro- plasty, we used codes for primary total hip replacement (81.51) and primary partial hip replacement (81.52); for knee arthro- plasty, we used the code for primary knee replacement (81.54). Hip and knee arthroplasties were standardized by resident population estimates (ie, number of arthroplasties divided by number of residents for each cohort) from the US Census and were then averaged across years 2012 through 2014. These averages were input into the model as time-constant primary procedure rates. We only considered inpatient procedures, as outpatient procedures are rare (ie, 0.75%–6.2% of hip and knee arthroplasties). Some studies have shown no significant difference between adverse events following outpatient versus inpatient surgery; however, outpatient surgical patients tend to have fewer risk factors for surgical complications.19,20 The SSI rates were estimated from National Healthcare Safety
Network (NHSN) data following primary hip and knee arthro- plasties for years 2012 through 2015 stratified by age and gender. Counts of complex SSIs (ie, deep incisional and organ-space SSIs identified within 90 days of the operative procedure) were stan- dardized using NHSN arthroplasty totals, averaged across years 2012 through 2015, and were input into the model as time- constant infection rates.21 For this analysis, to increase precision in the stratified rate
estimates, we used constant SSI rates estimated from pooled 2012–2015 NHSN data for each gender and age strata. Overall, our pooled infection rate (0.56% for hips and 0.33% for knees) did not significantly differ from 2015 estimates (P > .05 for both hips and knees). Thus, we used the overall pooled rates in our modeling.
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