Temporal Trends in Surgical Site Infections After Hip and Knee Arthroplasty in California, 2015 to 2024.

BACKGROUND: Surgical site infections (SSI) following primary total hip arthroplasty (THA) and total knee arthroplasty (TKA) remain clinically consequential and economically burdensome adverse events. Despite California's decade-long mandatory SSI surveillance system and public reporting of Standardized Infection Ratios (SIR), it remained unclear whether hospital arthroplasty volume, policy implementation, or the COVID-19 pandemic were associated with population-level SSI risk. We examined temporal trends in SSI rates after primary THA and TKA in California, and whether hospital volume, policy epoch, or the COVID-19 pandemic was associated with SSI risk. METHODS: We conducted a retrospective, facility-level longitudinal cohort study using state-mandated healthcare-associated infection surveillance data for hip and knee arthroplasty procedures performed in California from 2015 through 2024. The analytic unit was the hospital-year-procedure stratum, and the final panel comprised 5,880 hospital-year-procedure observations. Hospitals were categorized into low (less than 25 procedures/year), medium (25 to 100), or high (greater than 100) baseline annual volume. Generalized linear models with a log link and log-procedure offset estimated adjusted incidence rate ratios (IRR) for SSI, with covariates for year, policy epoch, procedure type, and volume category. RESULTS: Crude SSI rates were consistently higher for hip than knee arthroplasty (range, 0.5 to 0.7% versus 0.3 to 0.4% annually). In adjusted models, knee arthroplasty had an IRR of 0.62 (95% confidence interval [CI], 0.57 to 0.68) versus hip. Hospital volume demonstrated no association with SSI risk; medium-volume (IRR 1.07; 95% CI, 0.78 to 1.47) and high-volume (IRR 1.07; 95% CI, 0.79 to 1.44) hospitals did not differ from low-volume hospitals. Epoch indicators and linear year terms were non-significant across all models. CONCLUSION: In a statewide analysis, SSI rates following primary THA and TKA remained low and stable across hospital settings, policy periods, and the COVID-19 era. Hospital-level procedure volume was not associated with SSI incidence, and no temporal shifts corresponded to public reporting implementation or pandemic disruption.