Forecasting US Total Knee Arthroplasty Volumes to 2040: The Shift to Ambulatory Surgery Centers Has Just Begun.

INTRODUCTION: Total knee arthroplasty (TKA) is among the most common and cost-effective orthopaedic procedures in the United States. Policy changes, including removal from Medicare's inpatient-only (IPO) list, and advances in surgical and perioperative care have accelerated its shift to outpatient and same-day settings. The aim of this study was to model national TKA volume trends and forecast growth through 2040. METHODS: We conducted a retrospective analysis using data from the National Inpatient Sample and Nationwide Ambulatory Surgery Sample from 2016 to 2022. Procedures were stratified by setting: inpatient with overnight stay, inpatient same-day discharge, and outpatient in ambulatory surgical centers (ASCs). Survey-weighted regression models were used to evaluate trends, and future volumes were projected using negative binomial and autoregressive integrated moving average modeling. Regional cost trends and payer composition were also analyzed. RESULTS: Between 2016 and 2022, inpatient TKA volumes with length of stay ≥1 day declined by 85.0%, while same-day discharge volumes decreased by 46.7%. Outpatient TKA performed in ASCs rose from 104,971 cases in 2018 to 539,175 in 2022, representing a 413.6% increase after Medicare's removal of TKA from the IPO list. Forecasting models project that by 2040, annual ASC TKA volumes will exceed 2.1 million cases, while inpatient volumes will continue to decline substantially. Regionally, the South is expected to lead in ASC volume, reaching 881,000 cases annually by 2040. In 2022, ASC charges were highest in the West ($74,123) and lowest in the Midwest ($50,200). Medicare's share of ASC TKA cases increased from 42.5% in 2016 to 59.2% in 2022, reflecting a payer shift toward outpatient care. CONCLUSION: TKA has undergone a dramatic shift toward outpatient delivery, with ASC procedures poised to dominate the national landscape by 2040. These findings highlight the need for policy and infrastructure planning to support the continued growth of outpatient arthroplasty.