ssi after primary joint arthroplasty 1199
Classification ofMedical Acts associated with the corresponding specific code for prosthetic material (implant code).5,20 This hospital stay database is linked to patient data owing to the encrypted anonymized number. We extracted the 32,678 patientswho underwent primary knee or hip arthroplasty during the selection period (Online Appendix). Patients younger than 18 were excluded (8 patients). Follow-up started at the first hospital stay when the procedure was performed (admission day) and continued until the end of 2012. Patients were not recalled for this study but followed up through their consecutive hospital stays and discharges, regardless of the location of admission in France, either inside or outside the region. The minimumtarget time of follow-up was 12months (inclusion up to December 2011 and follow-up until December 2012).
Primary Outcome: PJI
PJI screening was based on a hospital discharge algorithm previously validated in an investigation study (more than 1,000 medical charts checked): the positive predictive value was 87% and the negative predictive value was 98%.20 The criteria used for identifying and classifying surgical site infections were developed by different experts for PJI: orthopedic surgeons, medical doctors specialized in infectious diseases, and data in the medical information system (Appendix). This definition was based on the diagnosis and procedure codes used in the hospital database summary, their position in the summary, and the presence of specific codes, according to the interna- tional admitted clinical definition for deep PJI.13,21 Patient infection rates were calculated from the number of
PJI with respect to the total number of patients who under- went hip or knee arthroplasty during the study period. In our analysis of risk factors for infection and mortality, patients were included only once, regardless of the number of arthro- plasties undergone. The case fatality rate was calculated using the number of stays with in-hospital death as numerator, and all hip or knee arthroplasty patients as denominator.
Covariables of Interest
Variables evaluated as potential confounders were coded patient variables (comorbidities) and hospital variables
table 1. Baseline Characteristics of the Cohort Patients, 2008–2011
Patients undergoing hip or knee arthroplasty
Sex ratio, M/F
Age, median (range), y Follow-up, mean, d
Follow-up, median (range), d Lost to follow-up, N (%)
Hospital sector, N (%) Private Public
≥1 Coded comorbidity N (%) Mortality rate, N (%)
Patients with hip replacement N=21,633
0.71
75 (18–105) 398
169 (0–1,817)
13,368 (61.7) 8,283 (38.3) 11,368 (52.5) 1,429 (6.6)
Patients with knee replacement N=11,045
0.59
72 (19–110) 484
352 (0–1,801)
4,448 (76.6) 2,583 (23.4) 6,305 (57.2) 280 (2.5)
Overall patients N=32,678
0.67
74 (18–110) 427
236 (1–1,217) 10 (0.03)
21,816 (66.8) 10,866 (33.2) 10,785 (33.0) 1,765 (5.4)
(Appendix). Incidence timing and predictive factors of PJI after a primary hip or knee arthroplasty were considered as the statistical unit. Lost to follow-up in this hospital discharge cohort was defined as no further hospital readmission or the moving of the patient toward another region with possible readmission or death.
Statistical Analysis
The main outcome was the PJI incidence describing the occurrence of surgical site infection for different age and sex (incidence density=person-time incidence rate). We used Kaplan Meier estimates to describe PJI occurrence and overall survival. All possible explanatory variables were first tested in a univariate model (Appendix). The criterion for inclusion in the multivariate analysis was P<.2 in the univariate analysis. Joint location, age, and sex were always included. Cox pro- portional hazards models were used to determine the effects of different confounding factors, as well as time period, on the risk of developing PJI. Hazard ratios (HRs) and 95% CIs were calculated. We checked proportionality of hazards and log- rank test by SAS, version 9.1 (SAS Institute).
results Baseline Characteristics
Overall, 32,678 patients underwent hip or knee arthroplasty during the study period (1.8% of the inpatient hospital stays/ year). Table 1 presents the baseline characteristics of cohort patients. The sex ratio (male/female) for replacement was 0.67. Two-thirds of replacements were hip arthroplasty (66.2%). The median (range) age at replacement was 74 (18–110) years. Among men, mean age was significantly lower (69.9 y [95% CI, 69.7–70.1 y]) than among women (73.8 y [73.7–74.0 y]). The mean length of follow-up was 427 days (14.2 months); the median (range) was 236 (1–1,217) days. Ten patients were lost to follow-up. Independent of the reason of hospital admission, two-thirds of the cohort patients were readmitted once (64.2%) and 15% were readmitted 2 times, allowing reliable hospital database follow-up of patients (Figure 1). Arthroplasty surgery
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