No Weekend Effect in Elective Primary Total Knee Arthroplasty: A Nationwide Analysis of 437,121 U.S. Cases

<b>Background</b>: The "weekend effect" describes the concern that patients treated on weekends experience worse outcomes due to differences in staffing, resource availability, and workflow. Evidence for a weekend effect in elective orthopedic surgery is limited and inconsistent, and most prior work does not ensure that surgery itself actually occurs on the weekend. We aimed to evaluate whether weekend admission and surgery are associated with worse in-hospital or 90-day outcomes in a contemporary nationwide cohort of elective primary total knee arthroplasty performed on hospital day 0. <b>Methods</b>: We conducted a retrospective cohort study using the U.S. Nationwide Readmissions Database (NRD) from 2020 to 2022. Adult patients (≥18 years) undergoing elective primary TKA with surgery on hospital day 0 were identified using ICD-10-PCS procedure codes in the primary procedure position. Weekend admissions (Saturday-Sunday) were compared with weekday admissions (Monday-Friday). Baseline demographics, comorbidities, and hospital characteristics were assessed. Outcomes included length of stay, total hospital charges, in-hospital mortality, major postoperative complications, and 90-day all-cause readmissions, time to readmission, readmission length of stay, and procedures during readmission. Continuous variables were compared using <i>t</i>-tests and categorical variables using chi-square or Fisher's exact tests (two-sided α = 0.05). <b>Results</b>: Among 437,121 elective day-0 TKA admissions, 435,822 (99.7%) occurred on weekdays and 1299 (0.3%) on weekends. Baseline characteristics were highly similar between groups. No clinically meaningful differences were observed in in-hospital complications, mortality, or 90-day readmission outcomes. Small statistical differences in blood transfusion, blood-loss anemia, and postoperative pain did not follow a pattern consistent with a weekend effect. <b>Conclusions</b>: In this large contemporary national cohort of elective primary TKA with surgery on hospital day 0, weekend admission and surgery were not associated with worse in-hospital outcomes or higher 90-day readmission rates. Within standardized perioperative pathways, elective TKA appears safe when performed on weekends, without evidence of a weekend effect.