Total knee arthroplasty in the outpatient vs inpatient settings: impact of site of care on early postoperative economic and clinical outcomes

BACKGROUND: The incidence of total knee arthroplasty (TKA) surgery performed in the outpatient setting has increased as a result of improved perioperative recovery protocols, bundled payments, and challenges brought by the coronavirus disease of 2019 (COVID-19) pandemic on health systems. This study evaluates early postoperative clinical and economic outcomes of patients treated in the inpatient vs outpatient setting using the Attune Knee System (AKS). METHODS: Patients with an elective, primary TKA implanted with the AKS, from Q4 2015 to Q1 2021, were identified within the Premier Healthcare Database. The index was defined as the admission date for inpatient cases and the service day for outpatient procedures. Inpatient and outpatient cases were matched on patient characteristics. Outcomes included 90-day all-cause readmissions, 90-day knee reoperations, and index- and 90-day costs of care. Generalized linear models were used to evaluate outcomes (Reoperation: binomial distribution; costs: Gamma distribution with log link). RESULTS: Before matching, 39,337 inpatient and 9,365 outpatient cases were identified, with greater comorbidities in the inpatient cohort. The outpatient cohort had a lower average Elixhauser Index (EI) compared to the inpatient cohort (1.94 (standard deviation (SD): 1.46) vs 2.17 (SD: 1.53), p < 0.001), and the rates for each individual comorbidities were also slightly lower in the outpatient compared to the inpatient cohorts. Post-match, 9,060 patients were retained in each cohort [mean age: ~ 67, EI = 1.9 (SD: 1.5), 40% male]. Post-match comorbidity rates were similar between inpatient and outpatient cohorts (outpatient EI: 1.94 (SD: 1.44)–inpatient EI: 1.96 (SD: 1.45), p = 0.3516): in both, 54.1% of patients had an EI between 1 and 2, and 5.1% had an EI ≥ 5. No differences were observed in 3-month reoperation rates (0.6% in outpatient, 0.7% in inpatient cohort). Index and post-index 90-day costs were lower in the outpatient vs inpatient cases [(savings for index-only costs: $2,295 (95% CI: $1,977–$2,614); 90 days post-index knee-related care only: $2,540 (95% CI: $2,205–$2,876); 90 days post-index all-cause care: $2,679 (95% CI: $2,322–$3,036)]. CONCLUSIONS: Compared to matched inpatient cases, outpatient TKA cases treated with AKS showed similar 90-day outcomes, at lower cost. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13018-023-03750-4.