1 Introduction

The landscape of total joint arthroplasty (TJA) has undergone significant transformation in recent years, driven by the COVID-19 pandemic and policy changes that removed TJA from the Medicare "inpatient only" (IPO) list. Value-based care has driven improvements in cost, care, and efficiency. Bundled payment models such as the Bundled Payments for Care Improvement (BPCI) and the Comprehensive Care for Joint Replacement (CCJR) have led to improvement in quality and decreased costs through the episodes of care. Specifically, BPCI has achieved a 20% reduction in 90-day episode costs alongside lower readmission rates, while CCJR has reduced episode costs by up to 35% for certain Diagnosis-Related Groups (DRGs), with additional decreases in skilled nursing facility use, emergency room visits, and readmissions.¹ These results highlight how structured payment initiatives can optimize resource use and elevate patient outcomes in TJA. The need to reduce healthcare costs, improve patient outcomes, and adapt to external challenges has accelerated the transition of total knee arthroplasty (TKA) and total hip arthroplasty (THA) from inpatient to outpatient care settings.

The removal of total knee arthroplasty (TKA) from Medicare's inpatient-only (IPO) list in January 2018, followed by total hip arthroplasty (THA) in January 2020, marked a significant shift in surgical practice. As a result, the percentage of same-day TKA procedures surged from 1.2% in 2016 to 62.4% in 2020, while outpatient THA cases increased from 2.0% to 54.5% over the same period.² Meanwhile, the overall volume of total joint arthroplasty (TJA) procedures has continued to rise, with a 2019 study projecting a 176% increase in THA and a 139% increase in TKA by 2040.³ The transition to same-day TKA has also led to a median cost savings of approximately 30%, primarily due to reduced post-anesthesia care unit (PACU) time, inpatient floor costs, pharmacy expenses, and meal-related costs.⁴ Similarly, following the removal of THA from the IPO list, the volume of outpatient THA procedures increased nearly eightfold—from 2518 in 2019 to 21,424 in 2021—while inpatient THA cases declined by 55% during the same period.⁵

Beyond financial benefits, patient satisfaction has also been positively impacted, with patients undergoing outpatient surgery reporting higher satisfaction with staff communication regarding medication, pain management, facility quality, and overall experience.⁶ Improvements in surgical techniques, anesthesia, pain management, rehabilitation protocols, and perioperative care pathways have made outpatient TKA a viable option for appropriately selected patients.⁷

Crucially, the shift toward outpatient TJA has not compromised patient outcomes. A study by Schloeman et al. (2023), demonstrated that TKA patients experienced significantly lower rates of 30-day and 90-day hospital readmissions, as well as reduced emergency department visits, following the removal of TJA from the IPO list.⁸ However, patient selection remains critical, as successful outpatient procedures are more common among individuals with fewer comorbidities. Research suggests that post-IPO removal, patients undergoing outpatient surgery were significantly less likely to have a sodium level below 137, a BMI over 40, or a history of smoking.⁴

While extensive literature has documented the benefits of outpatient TJA, fewer studies have examined its impact on inpatient populations. This study aims to characterize the changes in TJA inpatient and outpatient volume, assess the evolving health status of inpatient populations, and analyze demographic differences since the removal of TKA from the IPO list. We hypothesize that the progressive shift to outpatient TJA has led to a sicker and more complex inpatient TJA population.

2 Methods

The primary objective was to evaluate changes in the proportion of procedures named as inpatient versus outpatient status, and to assess changes in patient comorbidity burden over time. Secondary objectives include establishing baseline patient demographics and assessing 30-day readmission rates. All data was collected prospectively as part of a quality improvement project and subsequently analyzed retrospectively. The study was submitted to Western Institutional Review Board (WCG) for IRB expedited review, with IRB approval number 367.

This study included all Medicare patients who underwent total joint arthroplasty (TJA) at a high-volume, Level 1 academic medical center serving a major metropolitan area from June 2018 to 2024. The orthopedic surgeons involved in providing care are independent contractors to the facility, and were not employed by the hospital.

Patients were initially grouped by the procedure performed, either TKA or THA, and subsequently categorized by billing admission status (inpatient vs. outpatient) as determined by the hospital billing department. The decision to designate a patient as inpatient versus outpatient admission status was made by the anesthesiology team in consultation with the surgical team based on clinical factors such as CCI, ASA, length of stay (LOS), complexity of the procedure, home-care readiness, the patients' comorbidities, and a combination of the patient's health including body mass index (BMI) and pre-existing conditions. Regardless if the patient was qualified for an outpatient procedure, all procedures were performed in the same hospital operating setting, where patient's may receive different discharge instructions based on the status of the patient, including the individuals medical needs, recovery progress, home situation, as well as pre-existing conditions that may require the use or disuse of certain medications. This grouping strategy aimed to minimize the impact of the removal of THA from the inpatient-only (IPO) list on study results. It is acknowledged that this selection process creates an inherent relationship between comorbidity scores and admission status, which must be considered when interpreting our findings.

For each year, TJA volume, the proportion of procedures designated as inpatient versus outpatient admission status, 30-day readmission rates, and infection rates were assessed. Patient demographics, including American Society of Anesthesiologists (ASA) classification, Charlson Comorbidity Index (CCI) scores, age, sex, body mass index (BMI), and race, were collected and analyzed for each cohort. ASA and CCI scores are determined by the anesthesiologist based off the patient's age, presence and the severity of comorbidities such as diabetes or heart disease, smoking status, and BMI.

Comparisons between groups were conducted using a chi-square test of independence for categorical variables and a one-way analysis of variance (ANOVA) for continuous variables. Statistical significance was set at p < 0.05.

The primary analysis of volume, readmission rates and comorbidity scores focused on Medicare patients (n = 14,424). Demographic data was taken and stratified by patient class and compared across sex, age, and race categories, as shown in Table 1. Statistical analyses of demographic characteristics are reported in the results section below(seeTable 2).

3 Results

A total of 19,151 patients were included in the demographic analysis, with 8418 (44.0%) receiving inpatient total joints and 10,733 (56.0%) receiving outpatient total joints. Among inpatients, 60.3% were female and 39.6% were male. In the outpatient cohort, 56.1% were female and 43.8% were male.

The largest age groups for both inpatient and outpatient settings were 60–69 years (34.6% inpatient, 36.7% outpatient) and 70–79 years (33.3% inpatient, 32.9% outpatient). The average age of inpatient cases was 68.08 years, while outpatient cases had a slightly lower average age of 67.05 years. Racial demographics were recorded based on self-reported race and ethnicity. The majority of patients in both groups identified as White (88.9% inpatient vs. 88.3% outpatient). Minority representation was similar across groups, though outpatient cohorts had slightly higher proportions of Asian (1.6% vs. 1.2%), and patients with unknown race data (1.5% vs. 0.8%).

Between June 2018 and 2024, the percentage of patients undergoing TJA as an outpatient procedure increased from 5.91% (n = 69 of 1167 total procedure) in 2018 to 85.57% (n = 1904 of 2225 total procedures) in 2024 (p < 0.0001). The overall volume of Medicare patients undergoing TJA rose from 2401 in 2018 to 2793 in 2023, representing a 16.3% increase in procedural demand.

From June 2018 to 2023, the average American Society of Anesthesiologists (ASA) score across all TJA patients increased significantly, shown in Table 3. Among inpatients, the mean ASA score rose from 2.5 in June–December 2018 to 3.0 in 2023 (p < 0.01). For outpatients, the mean ASA score increased from 2.0 in June–December 2018 to 2.5 in 2023 (p < 0.01). Similarly, the Charlson Comorbidity Index (CCI) score rose significantly for both inpatient and outpatient populations. Among inpatients, the mean CCI score increased from 3.853 in June–December 2018 to 4.86 in 2023 (p < 0.01), while among outpatients, it rose from 3.109 in June–December 2018 to 3.692 in 2023 (p < 0.01). Although these readmission rates are low, rates may be influenced by COVID-19. These changes indicate an overall increase in comorbidity burden, with inpatients consistently exhibiting higher ASA and CCI scores than their outpatient counterparts each year.

The quarterly readmission rates between 2018 and 2024 showed an improvement beginning in Q2 of 2021, which showed a reduction in readmission rates which were significantly lower than national and regional averages, shown in Fig. 1. The institutional readmission rate started at 7.2% in 2018. A decline was observed in Q3 2021 dropping to 2.0-2.2%. After Q3 2021, there appears to be an increase of 4-5% by 2024 but remains lower than baseline rates. Regional and national averages remained stable but fluctuated from 7.4 to 8.2% for regional and 7.8-9.1% for national between 2018 and 2024. The Top 10% were the lowest at 4.6-5.2%, while premier average fluctuated between 7.6 and 9.5%, which were higher than the baseline and regional admission rates..

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Fig. 1 Quarterly readmission rates between 2018 and 2024.

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The 30-day readmission rates for inpatient Medicare TJA patients were analyzed from June-December 2018 through 2024, as shown in Table 4a. The inpatient Medicare case count decreased from 1.098 in June-December 2018 to 322 in 2023 before remaining similar at 321 in 2024. The number of readmissions within 30 days of the index procedure fluctuated from 29 in June-December 2018 to a low of 15 in 2023, and 16 in 2024. Corresponding readmission rates fluctuated, starting at 2.64% in 2018, peaking at 4.98% in 2024, with an overall grand total rate of 2.88% across 6072 cases and 175 readmissions, due to an increase in caseload.

Additionally, 30-day readmission rates for outpatient Medicare TJA patients were analyzed from June-December 2018 through 2024, as shown in Table 4b. The outpatient Medicare case count increased from 69 in June-December 2018 to 2144 in 2023 before decreasing slightly to 1904 in 2024. The number of readmissions within 30 days of the index procedure rose from 1 in 2018 to a peak of 59 in 2023, and 37 in 2024. Corresponding readmission rates fluctuated, starting at 1.45% in 2018, peaking at 2.75% in 2023, and dropping to 1.94% in 2024, with an overall grand total rate of 2.33% across 8352 cases and 195 readmissions. These outpatient-specific rates remained consistently low, aligning with the broader trend of improves outcomes in outpatient settings despite increasing comorbidity burdens.

Between June 2018 and December 2024, a total of five postoperative infections were observed in the inpatient total joint arthroplasty cohort. Annual infection events included one case in 2022 (n = 1), two cases in 2023 (n = 2), and two cases in 2024 (n = 2), shown in Table 5. These findings are presented in the context of an evolving surgical landscape in which a growing proportion of lower-risk arthroplasty procedures were performed in the outpatient setting.

4 Discussion

This single-center retrospective study demonstrates a dramatic increase in the designation of TJA procedures as outpatient admission status among Medicare patients at the study institution, rising from 5.91% in 2018 to 86.94% in 2023, consistent with national trends following the removal of total knee arthroplasty (TKA) and total hip arthroplasty (THA) from the Medicare inpatient-only (IPO) list.⁶ This is conflicting the original statements by CMS, who suggested the transition to outpatient designation would be a small subset of patients and should not significantly cause a shift in TJA cases¹ .⁴ This shift, paralleled by an increase in overall TJA volume, reflects the combined influence of policy changes, the COVID-19 pandemic, and advancements in perioperative care, aligning with projections of rising TJA demand through 2040.⁶

During the same time, we have seen a shift of TJA from the inpatient hospital setting to the outpatient surgical center setting. While actual numbers were not recorded in this study we observed this in the community. This has rendered the effect of concentrating the sickest patients into the inpatient hospital and tertiary care setting. The observed increases in ASA scores (from 2.5 to 3.0 for inpatients and 2.0 to 2.5 for outpatients) and CCI scores (from 3.853 to 4.86 for inpatients and 3.109 to 3.692 for outpatients) may support our hypothesis that the progressive transition to outpatient TJA has concentrated sicker, more complex patients in the inpatient setting.

The observed decline in readmission rates beginning in Q3 2021, coinciding with the accelerated shift toward outpatient TJA following the full removal of THA from the IPO list in 2020 and post-COVID recovery, suggests that the stratification of healthier patients into outpatient settings is associated with improved outcomes for the remaining inpatient cohort, though rates began to rebound modestly by Q4 2022 while still remaining below pre-2021 levels and national/regional benchmarks.⁹,¹⁰,¹¹ Unfortunately, the pandemic had a significant impact on patients’ willingness to present in traditional healthcare setting and seek care. This interpretation is drawn from the temporal correlation observed in our institutional data, which shows that rates fluctuated between 7.1% and 7.7% prior to Q3 2021, then experienced a significant decline to 2.0% to 2.2%. Initially, these rates remained low, even as ASA scores increased from 2.5 to 3.0 and CCI scores rose from 3.853 to 4.86 in the inpatient population.

Unfortunately, current economic climate pulls academic centers to employ and retain hospital outpatient departments, making the shift to outpatient ASC. Although majority of surgeons are health system employed, this may still support the conclusion that diverting lower-risk cases is associated with more focused resource allocation and risk management in the hospital setting, even as the inpatient population grew more complex.¹²,¹³,¹⁴ This improvement aligns with broader advancements in bundled payment models like CJR, which emphasize optimized perioperative care and resource allocation, potentially mitigating risks in increasingly complex inpatient population characterized by higher ASA and CCI scores.¹⁵,¹⁶,¹⁷,¹⁸

However, the gradual increase to 4-5% by 2024, amid rising episode volumes through 2023, underscores potential challenges such as the concentration of sicker patients in hospital settings, necessitating enhanced risk stratification and monitoring to sustain improved outcomes.¹⁹,²⁰ Compared to stable national (7.8-9.1%) and regional (7.4-8.2%) averages, our rates remained favorable, often approaching top 10th percentile benchmarks (4.6-5.2%), highlighting the efficacy of institutional protocols. These protocols do not vary between patients. The disparity with higher Premier averages (7.6-9.5%) indicates room for further quality initiatives, particularly as TJA demand continues to grow.²¹,²²

The outpatient-specific 30-day readmission rates, ranging from 1.45% to 2.75% with an overall rate of 2.33% further reinforce that outpatient TJA maintains low complication rates even as volumes expand and patient comorbidities increase. This supports the safety of outpatient procedures for selected patients while highlighting the need for cautious monitoring in the inpatient cohort.

The stable expansion of outpatient TJA to 85.57% of cases suggests that these protocols are effectively targeted an increase in the volume of outpatient procedures at our center. This shift reflects a significant transition from inpatient to outpatient status settings over the study period. Conversely, the disproportionate rise in inpatient ASA (3.0) and CCI (4.86) scores in 2024 highlights a growing challenge: as outpatient TJA becomes the norm for healthier patients, inpatient populations may require more intensive resources to manage their elevated risk profiles.

This evolving patient stratification has significant clinical implications. The success of outpatient TJA underscores the efficacy of perioperative advancements, delivering cost savings of approximately 30%.⁴ Higher patient satisfaction was also observed with communication and pain management.⁶ However, the increasing complexity of inpatient TJA patients, evidenced by higher ASA (3.0) and CCI (4.86) scores in 2024, necessitates tailored strategies, such as enhanced preoperative risk assessment and postoperative monitoring, to optimize outcomes. This is especially important given the increase in TJA volume that is projected to surge to 139% for TKA and 176% for THA by 2040, respectively.³ Decreased inpatient reimbursements exacerbates hospital burdens for complex cases.

Limitations of this study include its reliance on a single high-volume academic center, which may not reflect patterns in smaller or rural settings. Additionally, the absence of detailed outcome data (e.g., readmissions, infections) limits our ability to fully characterize the impact of this shift on patient safety. Minority underrepresentation in this cohort is attributable to the demographic characteristics of the surrounding geographic area. Other limitations include the role of IPO temporality in evaluation, primary versus revision procedures, and the complexity of the procedure. No regression analyses were performed to directly link comorbidity scores to outcomes, representing an opportunity for future research. Future research should incorporate multicenter data and longitudinal outcomes to validate these trends and explore the economic and quality-of-life implications of outpatient TJA expansion.

5 Conclusion

The transition to outpatient TJA has redefined the surgical landscape, offering efficiency and satisfaction for healthier patients while being associated with the concentration of higher-risk cases in the inpatient setting. With the higher-risk patients in the inpatient setting, this studies confirms increased disease severity with inpatient TJA patients compared to outpatient intervention. As this trend continues, healthcare systems must balance the benefits of outpatient care with the resource demands of an increasingly complex inpatient population.

Ethical statement

Exemption ID number 367 in June 2024.

Patient-guardian consent

The study did not require consent to be obtained.

Credit author statement

John Dundon: Conceptualization, Methodology, Supervision, Project administration, Writing – Reviewing & editing, visualization, investigation. Nicholas Brown: Data curation, Writing – Original draft preparation, Writing – Review & Editing. Skylar Lewis: Data Curation, Writing – Original draft preparation, Writing – Review & Editing. Noe Trevino: Data curation, Writing – Original draft preparation, Writing – Review & Editing. Joshua Uffer: Data curation, Writing – Original draft preparation, Writing – Review & Editing.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.