A Cost-Utility Analysis Comparing the Cost-Effectiveness of Simultaneous and Staged Bilateral Total Knee Arthroplasty

Odum, Susan M. PhDc; Troyer, Jennifer L. PhD; Kelly, Michael P. MD; Dedini, Russell D. MD; Bozic, Kevin J. MD, MBA

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.L.00373
Scientific Articles
Abstract

Background: The safety and efficacy of simultaneous or staged bilateral total knee arthroplasty have long been debated among orthopaedic surgeons. Advocates for simultaneous bilateral total knee arthroplasty posit that the benefits of decreased costs and recovery time, with no difference in functional outcomes, outweigh the economic costs of potential complications. The purpose of the study was to conduct a cost-utility analysis comparing simultaneous bilateral total knee arthroplasty with staged bilateral total knee arthroplasty.

Methods: A Markov model was designed to compare the cost-effectiveness of simultaneous bilateral total knee arthroplasty with that of staged bilateral total knee arthroplasty. Nationwide Inpatient Sample data sets from 2004 to 2007 were used to identify 24,574 simultaneous and 382,496 unilateral procedures. On the basis of the codes of the International Classification of Diseases, Ninth Revision, Clinical Modification, perioperative complications were categorized as minor, major, and mortality, and respective probability values were calculated. Nationwide Inpatient Sample data were used to determine hospital costs conditional on procedure type and complications. Rehabilitation costs, anesthesia costs, and heath utilities were estimated from the literature. To minimize selection bias, propensity score matching was used to match the groups on comorbid conditions, socioeconomic variables, and hospital characteristics.

Results: Using the matched sample, all complication rates were higher for the staged group. The estimated mean cost (in 2012 U.S. dollars) was $43,401 for simultaneous bilateral total knee arthroplasty compared with $72,233 for staged bilateral total knee arthroplasty. The quality-adjusted life years gained were 9.31 for simultaneous bilateral total knee arthroplasty and 9.29 for staged bilateral total knee arthroplasty. On the basis of these matched results, simultaneous bilateral total knee arthroplasty dominated staged bilateral total knee arthroplasty with lower costs and better outcomes.

Conclusions: On the basis of this analysis, simultaneous bilateral total knee arthroplasty is more cost-effective than staged bilateral total knee arthroplasty, with lower costs and better outcomes for the average patient. These data can inform shared medical decision-making when bilateral total knee arthroplasty is indicated.

Level of Evidence: Economic and decision analysis, Level II. See Instructions for Authors for a complete description of levels of evidence.

Author Information

1OrthoCarolina Research Institute Inc., 2001 Vail Avenue, Suite 300, Charlotte, NC 28207. E-mail address: Susan.Odum@orthocarolina.com

2Belk College of Business, University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, NC 28223-0001

3Washington University in St. Louis, Campus Box 8233, One Brookings Drive, St. Louis, MO 63130

4Department of Orthopaedic Surgery (R.D.D.) and Philip R. Lee Institute for Health Policy Studies (K.J.B.), University of California San Francisco, 500 Parnassus Avenue, MU 320W, San Francisco, CA 94158

Article Outline

Osteoarthritis of the knee is a common degenerative condition affecting almost five million people in the United States1. When conservative treatments fail, total knee arthroplasty is a highly efficacious procedure for reducing pain and improving quality of life and function in patients with advanced knee osteoarthritis. Kurtz et al.2 estimated that between 1997 and 2003 the U.S. economic burden of total knee arthroplasty ranged from approximately $3 billion to $6 billion.

When end-stage osteoarthritis is present in both knees, bilateral total knee arthroplasty may be warranted3. Patients, with appropriate guidance from their surgeons, may choose a simultaneous or a staged bilateral total knee arthroplasty on the basis of their preference and health status. Simultaneous bilateral total knee arthroplasty involves one surgical event under one anesthesia. Staged bilateral total knee arthroplasty is performed as two separate procedures under two separate anesthetics.

The potential benefits of simultaneous bilateral total knee arthroplasty include decreased hospital stay, single anesthetic event, and decreased rehabilitation time4. It is debated whether these benefits outweigh the potential increased morbidity and mortality associated with simultaneous bilateral total knee arthroplasties. Advocates for simultaneous procedures posit that the benefits of decreased costs and recovery time and equivalent functional outcomes outweigh potential complications3-10. Several investigators have reported increased complication rates following simultaneous bilateral total knee arthroplasty compared with those following staged bilateral total knee arthroplasty, including gastrointestinal complications, deep-vein thrombosis, pulmonary embolism, fat embolism, cardiovascular events, and mortality3-17. However, there is also evidence that suggests that there is no difference in complication rates between simultaneous bilateral total knee arthroplasty and staged bilateral total knee arthroplasty4,7,10,18-24.

To our knowledge, there have been no cost-effectiveness analyses of simultaneous bilateral total knee arthroplasty compared with staged bilateral total knee arthroplasty. With the conflicting data on outcomes and costs of bilateral total knee arthroplasty, it is important to provide quality, comparative effectiveness data to guide decision-makers. The purpose of the study was to evaluate the cost-effectiveness of simultaneous bilateral total knee arthroplasty compared with that of staged bilateral total knee arthroplasty.

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Materials and Methods

A Markov model (TreeAge Software, Williamstown, Massachusetts) was constructed to compare the cost-effectiveness of staged bilateral total knee arthroplasty with that of simultaneous bilateral total knee arthroplasty. Markov models are based on synthesized data that include costs, events, and utility measures or outcomes25. Quality-adjusted life years (QALYs) and costs are the outcomes. Markov models include mutually exclusive health states, in which experimental units transition through Markov cycles25. Transition probabilities, utility values, and estimated costs are assigned to each health state. Possible outcomes are modeled with use of a Markov cycle tree (see Appendix)25. The patient who underwent either simultaneous or staged bilateral total knee arthroplasty may have had one of five outcomes: (1) no perioperative complications, (2) minor perioperative complications, (3) major perioperative complications, (4) bilateral total knee arthroplasty-related mortality, or (5) mortality from other causes.

Complications were based on previously identified International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes26 and were categorized as minor or major on the basis of an expert panel of three adult reconstructive surgeons (K.J.B., R.D.D., and M.P.K.) (see Appendix). Nationwide Inpatient Sample (NIS) data from 2004 to 200727 were used to determine probability values for in-hospital surgical complications, in-hospital mortality, and surgical costs. This data set did not include direct identifiers for linkage so it was not possible to determine true staged cases. Therefore, a sample of unilateral total knee arthroplasties was derived with use of the Current Procedural Technology (CPT) code 27477. The simultaneous bilateral total knee arthroplasty cases were derived with use of the CPT code 27477 with modifiers 62 and 50. With use of the NIS data set, a total of 382,496 unilateral and 24,574 simultaneous bilateral total knee arthroplasties were available for analysis. The Appendix describes how in-hospital probabilities for complications and in-hospital costs for staged bilateral total knee arthroplasty were computed from the unilateral total knee arthroplasty NIS data.

Within the first postoperative year, patients transitioned from surgical hospitalization(s) to one of five post-hospitalization health states previously described for the remainder of year one. Patients began the second year in one of the five post-hospitalization health states. The transition probabilities between health states in a given year were determined by an expert panel of adult reconstruction surgeons (Table I).

NIS data27 were used to determine probability values for in-hospital surgical complications and in-hospital mortality. For simultaneous cases, the complication and mortality rates were based on all cases. The complication rates for the staged procedures were based on the assumption of two surgeries for those undergoing a unilateral procedure. The details regarding the complication rate calculations for staged bilateral total knee arthroplasty are found in the Appendix.

To determine the probability of mortality unrelated to bilateral total knee arthroplasty, NIS data were used to compute the average patient age and sex composition for staged and simultaneous bilateral total knee arthroplasty. The average patient age at the time of surgery was sixty-seven years for the staged group and sixty-five years for the simultaneous group, and the percentage of female patients was 64% for the staged group and 58% for the simultaneous group. This information was used with 2006 Period Life Tables from the Social Security Administration to determine the probability of mortality unrelated to bilateral total knee arthroplasty in each year (Table II).

Health state utility values for the surgery year and subsequent years were derived from the Tufts CEA (Cost-Effectiveness Analysis) Registry and the literature (Table III). For both procedures, the postoperative year-one health state utilities were set at 0.59 for no complications, 0.43 for minor complications, and 0.35 for major complications28-31. In subsequent years, health state utilities were estimated to be 0.75 for no complications, 0.59 for minor complications, and 0.51 for major complications28-31.

NIS data27 were used to compute average hospital costs for simultaneous and staged bilateral total knee arthroplasty for each surgical outcome group, in which doubled unilateral total knee arthroplasty costs were used for staged bilateral total knee arthroplasty. Hospital cost estimates were based on hospital charges, adjusted for the hospital’s all-payer cost-to-charge ratio (or hospital group average all-payer inpatient cost-to-charge ratio, when missing). Additional estimated costs included anesthesia costs, rehabilitation costs32, and physician fees. Costs for patients with no complications post-hospitalization were assumed to be $527 lower than costs for those with major or minor complications33. Cost inputs are shown in Table IV, and details regarding the computation of rehabilitation costs are found in the Appendix.

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Analysis

Given the observational nature of NIS data, it was important to construct a sample of the treatment group (simultaneous bilateral total knee arthroplasty) matched to individuals from the control group (unilateral total knee arthroplasty) with similar characteristics. This matching was done with use of a propensity score, which is the conditional probability of being in the simultaneous bilateral total knee arthroplasty group given a set of characteristics that may be related to group assignment (simultaneous bilateral total knee arthroplasty compared with unilateral total knee arthroplasty). Matching treatment cases to control cases on the propensity score tended to balance the distributions of measured characteristics in each group34. To estimate the probability of being in the simultaneous bilateral total knee arthroplasty group, we used variables that are potentially related to surgery choice, including demographic characteristics (age, sex, race, ethnicity), primary or secondary insurer status, hospital characteristics (size, urban or rural, teaching or non-teaching, region), patient-predicted mortality risk and functional loss, and present comorbidities. Details regarding the matching method are found in the Appendix. The matched sample included 381,002 unilateral cases and 24,020 simultaneous cases.

We considered cumulative costs and utility values for fifteen years following the year of surgery with use of a 3% discount rate. Total costs and QALYs were calculated and the cost-effectiveness was measured by the incremental cost-effectiveness ratio, which is the ratio of the difference in costs between the two bilateral surgical procedures divided by the difference in QALYs. The incremental cost-effectiveness ratio measured the cost per year of life at full health gained from choosing the bilateral surgical approach that provided the greatest number of QALYs.

A Monte Carlo probabilistic sensitivity analysis was performed to evaluate the impact of the model parameter uncertainties on the results. A probabilistic sensitivity analysis repeatedly calculated the outcome of the decision tree over the range of all included variables with use of either the 95% confidence interval (95% CI) or the standard deviation. For this model, the costs of each surgical outcome, the probability of each surgical outcome, the utility value assigned to each surgical outcome, and the health state transition probabilities were all included in the probabilistic sensitivity analysis (Tables V and VI). The cost-effective analysis was conducted on the propensity score matched study sample and the unmatched study sample.

To check the robustness of our results, we conducted a series of two-way sensitivity analyses, in which the inpatient costs, rehabilitation costs, and probabilities of inpatient complications and mortality were set at values that strongly favored staged bilateral total knee arthroplasty.

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Source of Funding

There was no external funding for this study.

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Results

Under this model’s assumptions with use of the unmatched cohort, the probabilities of having a minor or major in-hospital complication was higher for staged bilateral total knee arthroplasties (Table II). The minor complication rate was 8.98% for the staged group compared with 6.84% for the simultaneous group. The major complication rate was 2.36% for the staged group compared with 1.49% for the simultaneous group. The in-hospital mortality rate was 0.20% for the simultaneous procedures compared with 0.18% for staged bilateral total knee arthroplasties.

The total cost (in 2012 U.S. dollars) was $65,297 for staged bilateral total knee arthroplasty compared with $43,245 for simultaneous bilateral total knee arthroplasty. The QALYs gained from the procedures were nearly identical. The model estimated 9.341 QALYs gained following staged bilateral total knee arthroplasty compared with 9.304 QALYs gained following simultaneous bilateral total knee arthroplasty. The incremental cost-effectiveness ratio for the unmatched sample was $590,474 per QALY; this finding means that society would have to be willing to pay at least $590,474 per year of life at full health to make the QALY gains from staged bilateral total knee arthroplasty worth the added costs of staged bilateral total knee arthroplasty. The Monte Carlo probabilistic sensitivity analysis calculated the mean costs to be $43,146 (95% CI, $28,367 to $57,863) with 9.3 QALYs (95% CI, 7.3 to 10.9 QALYs) gained for simultaneous bilateral total knee arthroplasty compared with $65,101 (95% CI, $34,091 to $85,244) and 9.4 QALYs (95% CI, 7.4 to 10.9 QALYs) gained for staged procedures. Within the 95% CI, there was a large distribution of potential incremental cost and incremental effectiveness values.

The results of the analysis with the matched sample differed from the results of the unmatched sample, which suggests that these populations do differ. Using the matched sample, the complication rates and the mortality rate were higher in the staged group. The minor complication rate was 9.84% for the staged group compared with 6.57% for the simultaneous group. The major complication rate was 2.55% for the staged group compared with 1.43% for the simultaneous group. The in-hospital mortality rate was 1.97% for the staged group compared with 1.92% for the simultaneous group (Table II).

The total costs derived from the matched sample were higher than the unmatched sample. The estimated mean cost was $72,233 for a staged bilateral total knee arthroplasty compared with $43,401 for simultaneous bilateral total knee arthroplasty. The QALYs gained were 9.287 for staged procedures and 9.305 for simultaneous procedures. Given the higher costs and slightly lower QALYs gained for staged bilateral total knee arthroplasty, the simultaneous bilateral total knee arthroplasty strategy dominated the staged strategy. In the Monte Carlo probabilistic sensitivity analysis, the mean cost for simultaneous bilateral total knee arthroplasty was $43,406 (95% CI, $29,002 to $57,421) with an effectiveness of 9.3 QALYs gained (95% CI, 7.4 to 11 QALYs). The mean cost for staged bilateral total knee arthroplasty was $72,277 (95% CI, $52,232 to $92,872) with an effectiveness of 9.27 QALYs gained (95% CI, 7.2 to 11 QALYs).

Our two-way sensitivity analysis (see Appendix) tests the robustness of our results by (1) doubling the inpatient costs for simultaneous bilateral total knee arthroplasty while cutting in half the staged bilateral total knee arthroplasty costs, (2) doubling the rehabilitation costs for simultaneous bilateral total knee arthroplasty while cutting in half the rehabilitation costs for staged bilateral total knee arthroplasty, and (3) doubling the probability of complications and mortality for simultaneous bilateral total knee arthroplasty while cutting in half the rehabilitation costs for staged bilateral total knee arthroplasty. For the costs, in all cases, simultaneous bilateral total knee arthroplasty had lower total costs on average and higher QALYs, which implies that simultaneous bilateral total knee arthroplasty dominates staged bilateral total knee arthroplasty over a broad range of scenarios. When changing the probability of complications in favor of staged bilateral total knee arthroplasty, staged bilateral total knee arthroplasty produced modestly higher total QALYs gained, but the incremental cost-effectiveness ratios were all larger than $1.5 million per QALY gained, still favoring simultaneous bilateral total knee arthroplasty.

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Discussion

Although prior literature shows that both staged and simultaneous bilateral total knee arthroplasty are cost-effective relative to no surgical intervention33, and that simultaneous bilateral total knee arthroplasty is less costly than staged bilateral total knee arthroplasty, there are conflicting findings regarding the safety and effectiveness of staged or simultaneous procedures3,6,11,14-16,18-20,23,24. Furthermore, there has been no economic evaluation, to our knowledge, comparing staged and simultaneous procedures, in which costs and outcomes have been considered jointly.

The purpose of this study was to determine which surgical intervention is most cost-effective when bilateral total knee arthroplasty is indicated. The Markov model includes the most commonly reported in-hospital complication events. A large sample of bilateral and unilateral total knee arthroplasties was extracted from the nationally representative NIS data set. The probabilities of complications and mortality for both simultaneous bilateral total knee arthroplasty and two unilateral total knee arthroplasties were derived from this large data set. Therefore, we believe these values to be precise, reliable, and internally and externally valid. The costs associated with each case were also derived from the NIS data set and encompassed all costs associated with the surgery and hospitalization. The costs of simultaneous bilateral total knee arthroplasty were considerably lower than those of staged bilateral total knee arthroplasty. The reported risk of higher complication rates associated with simultaneous bilateral total knee arthroplasties may have influenced the surgeon and patient’s decision to select a staged strategy, especially in older patients with compromised health status. To account for any differences in the health status between the staged and simultaneous cohorts, we performed propensity score matching to attempt to control for confounding variables such as sex, age, comorbidities, severity of illness, type of insurance, and hospital type. The matched sample led to strikingly different results: simultaneous bilateral total knee arthroplasty was clearly favored on both the cost and outcome dimensions when simultaneous bilateral total knee arthroplasty patients were matched with similar patients on measurable dimensions. Although the average results are clear, it is important to note the wide variation in the incremental cost-effectiveness ratios as illustrated by the Monte Carlo probabilistic sensitivity analysis.

Much of the objection to simultaneous bilateral total knee arthroplasty arises from the perception of higher complication and mortality rates, but the nationally representative data used in this study indicated lower complication rates for simultaneous bilateral total knee arthroplasty for both the matched and unmatched samples and lower mortality for simultaneous bilateral total knee arthroplasty in the matched sample. The most commonly reported and serious medical complications following bilateral total knee arthroplasty are deep-vein thrombosis, pulmonary embolism, cardiovascular events, and mortality (Table VII). In a contemporary meta-analysis, Restrepo et al. reported no difference in the risk of deep-vein thrombosis following simultaneous bilateral total knee arthroplasties compared with unilateral procedures7. In 2003, Bullock et al. reported a 4.4% rate of deep-vein thrombosis in 512 staged bilateral total knee arthroplasties compared with a 1.6% rate of deep-vein thrombosis in 255 simultaneous bilateral total knee arthroplasties23. Earlier studies, with small sample sizes, noted higher rates of deep-vein thrombosis following staged bilateral total knee arthroplasty than those following simultaneous bilateral total knee arthroplasty11,14,24. Pulmonary embolism is a serious risk following bilateral total knee arthroplasty. The pulmonary embolism risk following bilateral total knee arthroplasty ranges from 0% to 5.6%3,11,14,15,19,23,24. With use of Medicare claims data, Barrett et al. reported that the pulmonary embolism risk is 80% higher for simultaneous procedures than unilateral procedures3. These authors concluded that the sum of the risks for each staged procedure may equal or may exceed the risk of simultaneous procedures. Restrepo et al. reported higher odds of pulmonary embolism following a simultaneous procedure compared with a single unilateral procedure7. Hutchinson et al.19 and Bullock et al.23 found slightly higher pulmonary embolism rates following simultaneous bilateral total knee arthroplasty compared with staged bilateral total knee arthroplasty. Conversely, Forster et al.15 and Jankiewicz et al.14 found slightly higher rates following staged procedures.

There is also conflicting evidence when comparing cardiac events between simultaneous and staged procedures. Ritter et al.20 found similar rates of cardiovascular events when comparing 2050 simultaneous bilateral total knee arthroplasties (1.5%) with 152 staged bilateral procedures (1.3%). Bullock et al.23 found that there was a 3.9% rate of cardiovascular events following simultaneous bilateral total knee arthroplasty compared with a 0.78% rate in the staged group. In contrast, other studies11,19,24 found a higher risk of cardiovascular events for staged procedures ranging from an increased risk of 1.7% to 8%.

The majority of studies comparing mortality rates following simultaneous and staged bilateral total knee arthroplasty have reported higher rates for simultaneous procedures. Stefánsdóttir et al.6 compared the mortality rate of 1139 simultaneous bilateral total knee arthroplasties with that of 3432 staged procedures. The thirty-day mortality rate was 7.53 times higher for the simultaneous group. In a smaller study, Mangaleshkar et al.16 found that the mortality rate was 7.4% for simultaneous procedures compared with 0% for staged bilateral total knee arthroplasties. A meta-analysis by Hu et al.17 noted an overall threefold increase in mortality rates of simultaneous bilateral total knee arthroplasties compared with staged bilateral total knee arthroplasties. Conversely, Ritter et al.20 found similar mortality rates for simultaneous procedures (0.68%) and staged procedures (0.66%).

We believe that our study with use of population complication and mortality rates in this economic analysis is an improvement on prior work in several ways. First, we used multiple years of hospital discharge data, providing nearly 25,000 simultaneous bilateral total knee arthroplasties and approximately 382,000 unilateral total knee arthroplasties. The large number of observations allowed for greater precision in calculating complication and mortality rates. Second, our computation of complication rates for staged bilateral total knee arthroplasty included five different possible outcomes for each complication type and assumed that patients had both knees replaced, which allowed us to compare staged and simultaneous groups. This methodology allowed for a direct comparison of the bilateral replacement options. Third, we used propensity score matching to match each patient who had undergone a simultaneous bilateral total knee arthroplasty with a patient who had undergone a unilateral procedure with similar characteristics and to compute complication rates and costs based on a matched sample. Given arguments that different patient types are more amenable to different procedures, this methodology allowed us to compare similar patients. The matching suggests that, in a matched sample, on average, simultaneous outcomes are better and costs are lower with simultaneous bilateral total knee arthroplasty.

Our model had limitations that are inherent to all Markov models. First, the accuracy of the calculations was dependent on the accuracy of the data inputs25,35-37. There may be errors related to the NIS database that may misclassify procedures and complications. However, evidence suggests an acceptable degree of coding accuracy using administrative claims data for total joint arthroplasty research35. Other model inputs such as anesthesia costs, rehabilitation costs, and health state utilities were derived from the literature. Second, other longer-term postoperative complications, such as total knee arthroplasty revision or unforeseen systemic events, were assumed to be equivalent and were not included in the decision tree. Additionally, societal costs such as wages lost for patients and caregivers were not modeled. In terms of complication and mortality rates, an additional caveat is in order. We used observational data; patients were not randomly assigned to undergo simultaneous or staged bilateral total knee arthroplasty. The lack of randomization is important given that physicians may specialize in the surgical approach that best suits their skill set. However, if surgeons are choosing staged over simultaneous bilateral total knee arthroplasty because of concern about patient outcomes on average, our study suggests that these surgeons may rethink their practice strategies.

In conclusion, the results of this cost-utility analysis indicate that simultaneous bilateral total knee arthroplasty is more cost-effective on average than staged bilateral total knee arthroplasty. For a patient electing staged bilateral total knee arthroplasty when simultaneous bilateral total knee arthroplasty is a reasonable treatment option, there is a high likelihood of higher costs with lower gains in QALYs. For the sample matched on patient characteristics, this Markov model estimated higher minor and major in-hospital complication rates for staged bilateral total knee arthroplasty; in-hospital mortality rates were also slightly higher for staged bilateral total knee arthroplasty. Individuals who undergo staged bilateral total knee arthroplasty face the risk of complications twice, in addition to incurring the costs of two surgical procedures and two hospitalizations. In an unmatched analysis, in which mortality rates for staged bilateral total knee arthroplasty were lower than those for simultaneous bilateral total knee arthroplasty, the incremental cost-effectiveness ratio for staged bilateral total knee arthroplasty compared with simultaneous bilateral total knee arthroplasty was $590,474 per QALY, which is a highly unfavorable incremental cost-effectiveness ratio, in our currently resource-constrained health-care environment38. Researchers have suggested that acceptable incremental cost-effectiveness ratios for the United States should be no higher than $328,874 per QALY38. These data can provide a broad framework to guide clinical decision-making when bilateral total knee arthroplasty is indicated. Preoperative health status, social support, and patient and surgeon preference, unique to any clinical scenario, should be considered when selecting a surgical strategy to treat end-stage bilateral knee osteoarthritis.

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Appendix Cited Here...

A description of the computation of staged bilateral total knee arthroplasty costs and complication rates, a figure showing a decision tree for simultaneous total knee arthroplasty, and tables showing the differences between simultaneous and staged bilateral total knee arthroplasty groups before and after matching, the sensitivity analysis for inpatient and rehabilitation costs, the sensitivity analysis for the probability of complications and mortality, and the codes for complications are available with the online version of this article as a data supplement at jbjs.org.

Investigation performed at the University of North Carolina at Charlotte and OrthoCarolina Research Institute, Charlotte, North Carolina

A commentary by Boris Bershadsky, PhD, is linked to the online version of this article at jbjs.org.

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