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Abstract
Background Many factors are driving total knee arthroplasty to be performed more commonly as an outpatient (<24 hour discharge) procedure. Nonetheless, the safety of total knee replacements performed in the outpatient setting is not well established when compared with inpatient setting. The purpose of this study is to compare the postoperative outcomes of outpatient and inpatient total knee arthroplasties.
Methods The 2015 and 2016 American College of Surgeons National Surgical Quality Improvement Program data sets were queried to extract patients who underwent primary, elective, unilateral total knee arthroplasty. The primary outcome was serious adverse events defined by a composite outcome including: return to operating room, wound-related infection, thromboembolic event, renal failure, myocardial infarction, cardiac arrest requiring cardiopulmonary resuscitation, cerebrovascular accident, use of ventilator >48 hours, unplanned intubation, sepsis/septic shock, and death. Propensity matched analysis was used to adjust for potential confounding covariates.
Results 1099 patients undergoing outpatient total knee arthroplasty (1% of total cases) were successfully matched to 1099 patients undergoing inpatient surgeries. The composite rate of serious adverse events was greater in outpatient procedures compared with inpatient procedures (3.18% vs 1.36%, p=0.005). In contrast, failure to rescue and readmission rates were not different between groups.
Conclusions Outpatient total knee arthroplasty is associated with a higher composite risk of serious adverse events than inpatient procedures. Anesthesiologists and surgeons should inform patients and discuss this information when obtaining consent for surgery and planning for discharge timing.
- ambulatory care
- postoperative complications
- outcomes
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Introduction
With the demand for total knee arthroplasty (TKA) expected to increase exponentially by 2050, healthcare systems are exploring strategies to meet this demand from both a safe and cost-effective approach.1–3 This has resulted in the increasing trend of performing TKA on an outpatient basis in selected patients.4 5 The shift from inpatient to outpatient TKA has significant potential cost savings not only for healthcare systems, but also to government payers (eg, Medicare).6
Total knee arthroplasty can result in significant morbidity (eg, infection, bleeding, deep venous thrombosis) to patients.7 8 In addition, patients in the ambulatory setting cannot rely on hospital support (eg, nurses, intravenous medications) to manage their postoperative recovery.9 Previous studies evaluating the safety of outpatient TKA are scarce and limited to single surgeon case series, analysis of a single subset of private insurance data with no covariate adjustments and smaller samples of outpatient TKAs.4 10 11 Currently, the safety of TKA performed in the outpatient setting is not well established when compared with inpatient TKA.
The purpose of this study is to compare the postoperative outcomes of outpatient and inpatient TKA using the most up-to-date sample of outpatient TKA in the American College of Surgeons National Surgical Quality Improvement Program (ACS-NSQIP) database. We hypothesized that there would be observed differences in postoperative complications between the outpatient and inpatient TKA cohorts. In addition, we sought to compare readmission rates of outpatient and inpatient TKA procedures.
Methods
The Institutional Review Board determined that the study qualified for exemption under 45 CFR 46.101(b). The exemption was granted because the study involved a retrospective review of existing data recorded in such a manner that subjects cannot be identified, directly or through identifiers linked to the subjects. Clinical information of the subjects was obtained for the years 2015 and 2016 from the ACS-NSQIP database. The study is reported following the STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) guidelines for reporting observational studies.12
The ACS-NSQIP database is a national prospective database that compiles voluntarily reported data from over 680 institutions in the USA. Over 1 million cases were submitted as part of the 2016 update to the NSQIP database. Data is collected on over 300 variables that include preoperative risk factors, intraoperative variables and postoperative outcomes including complications up to 30 days after surgical procedures.13 Data collection has been previously described in detail.14 In brief, data are collected in 8-day cycles, with the first 40 procedures in the cycle included in the data set. The most commonly performed procedures are capped at five within each cycle to increase procedure heterogeneity. Trained clinical nurses assigned at each site collect data for 30 days postoperatively using isolated telephone interviews and operative and clinical notes. Inter-rater reliability audits of selected participating sites help ensure the collected data are of the highest quality possible. The combined results of inter-rater reliability audits completed to date revealed an overall inter-rater disagreement rate of approximately 1.8% for all assessed program variables.15
De-identified patient information is freely available to all institutional members who comply with the ACS-NSQIP Data Use Agreement. The Data Use Agreement implements the protections afforded by the Health Insurance Portability and Accountability Act of 1996 and the ACS-NSQIP Hospital Participation Agreement. The ACS-NSQIP and the hospitals participating in this program are the sources of the data used in this study; however, these entities have not verified and are not responsible for the statistical validity of the data analysis or the conclusions derived by the authors.
The 2015 and 2016 NSQIP Participant Use Data Files were queried to extract all patients scheduled. Patients who underwent primary, elective, unilateral TKA were identified using the Current Procedural Terminology (CPT) code 27 447. Cases involving trauma, fracture, neoplasms, infectious diseases, or patients under 18 were excluded; patients who had fatal complications on the day of the surgery (<24 hours) were excluded. Patients who qualified for the study under these criteria were then separated to an outpatient TKA cohort, defined as length of stay (LOS) of 0 days (less than 24 hours), and an inpatient TKA cohort, defined as LOS≥1 day(s). The NSQIP database does not distinguish whether the procedure is performed in a free-standing surgical facility, hospital-based ambulatory surgical center or a hospital.
Outcomes variables and analysis
Preoperative demographic variables such as age, gender, body mass index, functional status, American Society of Anesthesiologists (ASA) classification, smoking status, preoperative hematocrit, hypertension, diabetes, congestive heart failure, bleeding disorder, and chronic obstructive sleep apnea were compared between the two cohorts. Intraoperative variables including surgical duration and anesthesia type (eg, general, spinal, and intravenous sedation) were also compared between the cohorts. Postoperative events were grouped into minor adverse events (MAE), serious adverse events (SAE), and any adverse events (AE) following the same classification of prior studies on patients undergoing TKA.10 16 17 The primary outcome was SAEs defined as a composite that includes: (1) a return to operating room, (2) wound-related infection, (3) thromboembolic event (eg, deep venous thrombosis (DVT) and/or pulmonary embolism), (4) renal failure, (5) myocardial infarction, (6) cardiac arrest requiring cardiopulmonary resuscitation, (7) stroke or cerebrovascular accident, (8) on ventilator >48 hours, (9) unplanned intubation, (10) sepsis/septic shock, and (11) death. MAEs included: (1) blood transfusion, (2) pneumonia, (3) wound dehiscence, (4) urinary tract infection, and (5) renal insufficiency.
We also compared the rates of readmissions and failure to rescue between the groups. Failure to rescue is defined as a death following a postoperative complication.
Statistical analysis
Due to the observational (non-randomized) nature of this data, propensity score matching was used to minimize the effects of confounding when assessing differences in patient demographics between inpatient and outpatient surgeries.18 The propensity score is the probability of treatment group conditional on observed baseline demographics. In this study, the probability for undergoing an outpatient procedure (propensity score) was calculated for each patient based on age, gender, body mass index, ASA, functional status before surgery, smoking status, preoperative hematocrit, hypertension, diabetes, congestive heart failure, bleeding disorder, chronic obstructive pulmonary disease, surgical duration, and anesthesia type. Each outpatient was one-to-one matched without replacement to an inpatient with the nearest propensity score. A caliper of 0.10 was used. All outpatients could be matched with an inpatient case.
After propensity score matching, differences in outcome rates of the matched cohorts were assessed using McNemar’s test for matched data. The rates of AEs between inpatient and outpatient cohorts were compared at 30 days postoperative (events that occurred at any time in the 30-day postoperative period). To adjust for multiple endpoint testing, an adjusted p value was calculated to correct for false discovery rate. All statistical analyzes were conducted with the use of SAS software V.9.4 (SAS Institute Inc, Cary, North Carolina, USA).
Results
A total of 108,314 patients undergoing unilateral TKA were included in the NSQIP database for 2015 to 2016 and 5495 patients were excluded (figure 1). A total of 102,819 patients qualified for the study. Of these, 1099 (1.07%) underwent outpatient TKA and 101,720 had a LOS≥1 day qualifying for the inpatient cohort. Patients in the outpatient group were younger (64.9 vs 66.6, p<0.001), more likely to be male (p<0.001) and a higher proportion were classified as ASA Physical Status II (p<0.001). There were no significant differences in body mass index, preoperative functional status and smoking between the two cohorts. Covariates were well balanced between the propensity matched cohorts, absolute standard mean difference <0.12 for all covariates (table 1).
Study flow diagram. NSQIP, National Surgical Quality Improvement Program; TKA,total knee arthroplasty.
Demographics of patients undergoing total knee arthroplasty after propensity matching
The composite rate of SAEs was greater in outpatient procedures compared with inpatient procedures (3.18% vs 1.36%, p=0.005). MAEs did not differ between study groups. Specific rates for each AE are presented on table 2. In contrast, patients in the outpatient setting had no increased failure to rescue when compared with patients in the inpatient setting, 0 out 57 (0%) and 2 out 41 (4.8%), p=0.17. Similarly, readmission rates did not significantly differ between the groups, OR of 1.41 (95% CI, 0.82 to 2.42). The relative risks for postoperative complications comparing outpatient to inpatient TKAs are presented on figure 2.
Matched comparisons and relative risk of adverse event rates that occurred any time after surgery in outpatient versus inpatient total knee arthroplasty
Forest plots comparing the relative risk of postoperative complications between outpatient total knee arthroplasty and inpatient total knee arthroplasty. Diamonds represent the point estimate for relative risk; line represents 95% CIs. AE, adverse event; MAE, minoradverse event; SAE, seriousadverse event.
Discussion
The most important finding of the current investigation was the greater composite risk of SAEs on patients undergoing total knee arthroplasty in the outpatient setting when compared with the inpatient setting. The significantly higher composite score was driven primarily by a higher number of thromboembolic events and returns to the operating room, but neither of these events were individually significantly higher. Taken together, our findings suggest that the inpatient setting seems to be an option associated with less SAEs for patients undergoing total knee replacements.
Our results are clinically important given the current shift of practice towards the performance of total knee replacement in the outpatient setting. Given the current financial incentives and economic pressures to reduce costs, it is expected that the number of outpatient total knee replacement procedures will grow substantially over the following years. The increase of outpatient total knee replacement has the potential to harm significantly more patients in the years to come and the medical costs incurred in treating the increased complications may erode the savings generated by performing such procedures in the outpatient setting.
Another important finding of our study was that thromboembolic events and return to the operating room were the two most common, although not statistically significant, SAEs occurring in the outpatient setting. It is possible that patients in the outpatient setting may have lower adherence to prophylactic strategies to reduce thromboembolic events as previously demonstrated in prior studies.19 20 Nonetheless, it seems that these events were appropriately managed and did not result in an increased rate of readmissions and failure to rescue in the outpatient compared with the inpatient setting.
It was also interesting to note a major selection process for the patients undergoing outpatient TKA. In the original cohort, patients in the outpatient setting were younger and healthier than the ones in the outpatient setting. We used propensity score matching to adjust for the covariate imbalances in our analysis and obtained a well-adjusted cohort. If patients are not appropriately selected through a careful preoperative screening process, one may argue that we can expect even greater composite rates of SAEs in the outpatient TKA group when compared with the inpatient TKA group.
Early studies suggested greater complication rates of outpatient TKA compared with inpatient TKA. Bovonratwet et al examined 642 patients undergoing outpatient total knee replacement from the NSQIP database from 2005 to 2014 and noticed an increased in blood transfusion in outpatient TKA when compared with patients undergoing inpatient surgeries.10 Arshi et al used a different database and found a higher complication rate for DVT, acute kidney infection, and need for later revision but the authors did not adjusted the outcomes for important covariates (eg, ASA class and surgical duration).11 Another publication involving NSQIP database included 365 outpatient total knee replacements and showed an increase in bleeding complications and DVT in the outpatient cohort.21 Our study confirms the previous concerns expressed by others when examining the safety of outpatient knee and hip arthroplasty using a larger sample size.10 11 21 22
It is clear from our study that the selection criteria utilized to triage patients for outpatient TKA is a very important step for the success of any program. Several scoring systems have been advocated to facilitate selection of patients undergoing outpatient TKA. For example, Ziemba-Davis et al developed the outpatient arthroplasty risk assessment score based on patients’ comorbidities and concluded that the score was highly effective for identifying patients who can safely elect to undergo outpatient total joint arthroplasty.23 In contrast, the same authors concluded that the ASA Physical Status classification does not provide sufficient discrimination for safely selecting patients for outpatient arthroplasty.
Some studies comparing outpatient knee arthroplasty to inpatient knee arthroplasty have demonstrated some benefits of the outpatient setting. Ford et al demonstrated using an age and comorbidity matched cohort that patients who had knee arthroplasty in the ambulatory setting had less postoperative complications than the ones in the inpatient setting.24 In addition, Carey et al examining a cohort of non-Medicare patients demonstrated that readmissions and postsurgical complications were lower in ambulatory setting when compared with the inpatient setting.25
Our study can only be interpreted within the context of its limitations. Our study was done before the full implementation of nurse navigators, remote monitoring of patients for outpatient total joint arthroplasty, and the use of more motor sparing regional anesthesia techniques. Although some anesthetic types were coded monitored anesthesia care/intravenous sedation or ‘other’, and almost certainly received regional or neuraxial anesthesia, the anesthesia type did not impact the analysis. In addition, the database does not provide information on whether the procedure is performed in a free-standing surgical facility, hospital-based ambulatory surgical center, or a hospital. Lastly, due to limitations on the database, we cannot assess potential causal reasons to explain our findings. For example, it is possible that patients in the outpatient setting received less physiotherapy and this resulted in less mobility and more thromboembolic events.
In summary, patients undergoing outpatient total knee replacement have a greater composite risk of SAEs when compared with patients undergoing total knee replacement in the inpatient setting. It is the responsibility of the surgical and anesthesia teams to protect patient safety during the current movement towards outpatient TKA. Future studies that incorporate up-to-date clinical practices are warranted to determine the safety of outpatient TKA. Meanwhile, anesthesiologists and surgeons should inform their patients of the potential risks of having TKA in the outpatient setting.
References
Footnotes
Presented at Presented in part at The American Society of Anesthesiologists Annual Meeting, Orlando, Florida, October 2019.
Contributors HTM: This author participated in the study design, conduct of the study, and manuscript preparation. TM: This author participated in the study design, conduct of the study, and manuscript preparation. DC: This author participated in the study design, conduct of the study, and manuscript preparation. PA: This author participated in the study design, conduct of the study, statistical analysis, and manuscript preparation. MCK: This author participated in the study design, conduct of the study, and manuscript preparation. GDO: This author participated in the study design, conduct of the study, statistical analysis, and manuscript preparation.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Ethics approval This study was performed under an exempt status granted by the Institutional Review Board of Rhode Island Hospital (IRB#1531731).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data may be obtained from a third party and are not publicly available, https://www.facs.org/quality-programs/acs-nsqip/participant-use.