Clinical and Nonclinical Determinants of Simultaneous vs Staged Bilateral Total Knee Arthroplasty

Abstract

Background

Patients requiring bilateral total knee arthroplasty (TKA) may undergo surgery either simultaneously or in a staged fashion. Contemporary trends in utilization, as well as clinical and nonclinical factors correlating with surgical selection, remain undefined.

Methods

Adult patients who underwent simultaneous or staged bilateral TKA between 2016 and 2021 were identified. Patient characteristics were assessed, including clinical (age, sex, Elixhauser Comorbidity Index) and nonclinical factors (geographic region, insurance status, and social determinants of health). Multivariate logistic regression was performed to identify predictors of undergoing simultaneous bilateral TKA vs staged bilateral TKA.

Results

Of the 138,468 patients undergoing bilateral TKA, 20,473 (14.8%) had simultaneous and 117,995 (85.2%) had staged bilateral TKA. The proportion of simultaneous procedures declined from 19.0% in 2016 to 7.1% in 2021. Simultaneous TKA patients were younger (63.9 ± 8.3 years vs 65.6 ± 8.6 years for staged; P < .001) and had a lower Elixhauser Comorbidity Index (4.5 ± 3.0 vs 4.9 ± 3.2 for staged; P < .001) than those undergoing staged bilateral TKA. Patients in the Northeast had the highest odds of simultaneous bilateral TKA (relative to Midwest: South odds ratio [OR]: 1.06; West OR: 1.09; Northeast OR: 1.87) (P < .05), while patients with Medicaid (relative to commercial insurance: OR: 0.62), and those with social (OR: 0.78) and economic (OR: 0.93) constraints had lower odds of simultaneous bilateral TKA (all P < .05).

Conclusions

The use of simultaneous bilateral TKA substantially declined between 2016 and 2021 relative to staged procedures. Surgical decisions were associated with both clinical (age, comorbidity burden, sex) and nonclinical (region, insurance, social determinants of health) factors.

Introduction

Bilateral knee osteoarthritis (OA) is common, with 25% of patients having bilateral OA at baseline and 80% of unilateral OA patients developing bilateral pathology within 12 years [1]. Patients requiring bilateral total knee arthroplasty (TKA) may undergo these surgeries simultaneously (under one anesthetic procedure) or in a staged fashion. While both approaches have their own advantages and disadvantages, [[2], [3], [4]] the relative utilization of simultaneous vs staged bilateral TKAs is not well understood.

Between 2000 and 2009, Bolognesi et al. reported that the incidence of simultaneous bilateral TKA remained stable at 3 in 10,000 person-years, while the incidence of staged bilateral TKAs tripled from 1.4-4.4 per 10,000 person-years among Medicare beneficiaries, representing a trend in the overall increased utilization of arthroplasty procedures [5]. The study did not include patients with commercial insurance–typically younger and potentially more suited for simultaneous procedures–limiting its generalizability. Using the National Inpatient Sample database, Mehta et al. [6] identified a decreasing incidence of simultaneous bilateral TKAs from 7.2%-5.6% among patients with private insurance and from 4.5%-3.1% among patients with Medicare or Medicaid Insurance between 2007 and 2016. Remily et al. [7] identified a similar trend between 2009 and 2016, with the percentage of simultaneous bilateral TKAs dropping from 5.6%-4.0%. Notably, these analyses did not account for TKAs performed in outpatient settings, an increasingly popular option [8] that may be relevant for patients undergoing staged procedures. Furthermore, these studies reported the number of simultaneous bilateral TKAs relative to the total number of TKAs in the database, capturing a heterogeneous cohort of patients with both unilateral and bilateral procedures. To date, contemporary data on the relative utilization of simultaneous vs staged bilateral TKA within the past decade remain unavailable in the literature.

The timing of the second TKA among patients undergoing staged bilateral TKAs remains a topic of debate, with intervals as short as within 1 week having been proposed [[9], [10], [11]]. While some studies have found no difference in complications [12,13] or functional outcomes [14] based on the timing of the contralateral TKA, others report increased costs [15] and rates of manipulation under anesthesia [16] among patients undergoing a contralateral TKA within 90 days.

The American Academy of Orthopedic Surgeons Clinical Practice Guidelines have consensus recommendation that simultaneous bilateral TKA could be performed, versus staged (>90 days) bilateral TKA, in appropriately selected patients, but should be performed with caution. [17]. More recent recommendations from the 2024 World Expert Meeting in Arthroplasty also support this 90-day interval between surgeries [18]. With ongoing conversation about the effect of surgical timing on patient outcomes, the extent to which these guidelines are followed in practice is unknown.

Given the lack of consensus regarding simultaneous vs staged bilateral TKA for patients with severe bilateral knee OA, contemporary patterns of use and the factors influencing surgical decision-making remain poorly defined. To address these gaps in the literature, the current study utilized a large national database to evaluate recent trends in the utilization of simultaneous and staged bilateral TKA and to identify the clinical and nonclinical determinants associated with each approach.

Material and methods

Study cohort

The current study used the M165Ortho PearlDiver administrative database (Colorado Springs, Colorado). This is a large, multi-insurance database that includes data from approximately 165 million patients in the United States and has been well established for joint arthroplasty studies [[19], [20], [21], [22]]. Since the output is aggregated and deidentified, our institutional review board has found studies using PearlDiver to be exempt from review.

Patients who underwent primary TKA between January 1, 2016, and December 31, 2021, were identified using Current Procedural Terminology and International Classification of Diseases (ICD), Tenth Revision Procedure Coding System codes (Appendix A). Simultaneous bilateral TKA patients were identified as having left and right primary TKA codes entered on the same day. Patients receiving staged bilateral TKA were identified as those with a second primary TKA within 1 year of index procedure, with specification of the time interval.

All patients were active in the database for at least 1 year following their index procedure. Patients were excluded if they were <18 years old at the time of surgery, or had a history of knee fracture, neoplasm, or infection within 90 days prior to surgery.

Patient characteristics

Age, sex, Elixhauser Comorbidity Index (ECI) (a composite score derived from 30 different comorbidities, [23] listed in Appendix B), geographic region, and insurance status were extracted from the database. Social determinants of health (SDOH) were identified within 1 year prior to index surgery using ICD-Ninth and ICD-Tenth Revision codes (Appendix C) and classified using methodology previously described by Kamalapathy et al. [24] and Khalid et al. [25] These domains (social, environmental, economic, and educational) were chosen to align with the US Department of Health and Human Services “Healthy People 2030” initiative [26]. This initiative, designed to promote improvements in national health outcomes between the years 2020-2030, identified SDOH as a priority area for future research; several studies in the orthopaedic literature have since used these SDOH classifications [27,28].

Data analysis

Patient characteristics were compared between those receiving simultaneous vs staged bilateral TKA. Categorical variables were compared using univariate analysis with Pearson chi-squared tests. These analyses were followed by multivariate logistic regression to determine odds ratios (ORs) and 95% confidence intervals.

Data analysis was performed using PearlDiver’s internal software (PearlDiver, Colorado Springs, CO), with statistical significance set at P < .05. Figures were created using Microsoft Excel 16 (Microsoft Corporation, Redmond, WA) and GraphPad Prism 10 (GraphPad Software, San Diego, CA).

Results

Study cohort

A total of 138,468 patients met the inclusion criteria of having 2 primary TKA procedures within 1 year, of which 20,473 (14.8%) underwent simultaneous bilateral TKA and 117,995 (85.2%) underwent staged bilateral TKA. Over the study period, the proportion patients undergoing simultaneous TKA decreased (2016 19.0%, 2017 17.7%, 2018 15.5%, 2019 14.7%, 2020 11.1%, 2021 7.1%).

The staged cohort was stratified by the interval between the first and second procedures. The yearly proportion of patients undergoing staged TKA increased across all time categories: 0-6 weeks (2016 9.0%, 2017 9.1%, 2018 9.7%, 2019 9.8%, 2020 9.8%, 2021 10.2%), 6-12 weeks (2016 11.0%, 2017 11.5%, 2018 11.1%, 2019 12.4%, 2020 12.3%, 2021 13.8%), 12 weeks-6 months (2016 29.6%, 2017 29.5%, 2018 30.5%, 2019 31.1%, 2020 34.0%, 2021 34.7%), and 6-12 months (2016 31.4%, 2017 32.2%, 2018 33.2%, 2019 32.0%, 2020 32.9%, 2021 34.2%). These findings demonstrate a shift toward longer staging intervals over time (Fig. 1).

Figure 1.

Trends in proportion of patients receiving simultaneous vs staged bilateral TKA within 1 year of index surgery from 2016 to 2021.

Among patients undergoing staged procedures, the median time to contralateral surgery was 145 days (interquartile range 88-237). A total of 29,824 patients (25.3%) underwent contralateral surgery within 3 months, while 88,171 patients (74.7%) underwent contralateral surgery at an interval of greater than 3 months.

Patient characteristics

Patients undergoing simultaneous bilateral TKA were younger (63.9 ± 8.3 years relative to 65.6 ± 8.6 years for staged), healthier (ECI 4.5 ± 3.0 for simultaneous relative to 4.9 ± 3.2 for staged), and more likely to be male (45.9% relative to 37.9% for staged) compared to those undergoing staged procedures (P < .001 for all). There were significant differences in the breakdown of geographic region and insurance status between the 2 groups (P < .001 for both). Fewer patients in the simultaneous cohort reported experiencing social (0.3% vs 0.5%, P = .003) and economic (9.7% vs 11.1%, P < .001) disadvantages in the year prior to surgery compared to those in the staged cohort (Table 1).

Table 1.

Demographic characteristics of patients undergoing either simultaneous or staged (within 1 year) bilateral TKA.

Factor	Simultaneous	Staged	P value
N (%)	20,473 (14.8%)	117,995 (85.2%)
Age (mean, SD)	63.9 (8.3)	65.6 (8.6)	<.001
<50	866 (4.2%)	4134 (3.5%)
50-59	5235 (25.6%)	24,199 (20.5%)
60-69	8973 (43.8%)	48,819 (41.4%)
70-79	5082 (24.8%)	36,672 (31.1%)
≥80	317 (1.5%)	4171 (3.5%)
Sex			<.001
F	11,075 (54.1%)	73,226 (62.1%)
M	9398 (45.9%)	44,769 (37.9%)
ECI (mean, SD)	4.5 (3.0)	4.9 (3.2)	<.001
0-2	5709 (27.9%)	28,857 (24.5%)
3-5	8626 (42.1%)	46,764 (39.6%)
6-8	4159 (20.3%)	26,498 (22.5%)
≥9	1979 (9.7%)	15,876 (13.5%)
Region			<.001
Midwest	5321 (26.0%)	35,802 (30.3%)
South	6390 (31.2%)	41,712 (35.4%)
West	2745 (13.4%)	17,411 (14.8%)
Northeast	5943 (29.0%)	22,428 (19.0%)
Insurance			<.001
Commercial	14,415 (70.4%)	76,938 (65.2%)
Medicaid	491 (2.4%)	4103 (3.5%)
Medicare	5292 (25.8%)	34,264 (29.0%)
SDOH
Social	70 (0.3%)	592 (0.5%)	.003
Environmental	<11 (<0.05%)	57 (0.05%)	.698
Economic	1984 (9.7%)	13,054 (11.1%)	<.001
Educational	83 (0.4%)	565 (0.5%)	.172

Bold value indicates statistical signfiicance with P <0.05.

On multivariate analysis, younger age (relative to age <50: 60-69 OR 0.82; 70-79 OR 0.61; ≥80 OR 0.35), lower ECI (relative to ECI 0-2: 3-5 OR 0.96; 6-8 OR 0.84; ≥9 OR 0.68), and male sex (relative to female: OR 1.38) were associated with simultaneous TKA. Simultaneous TKAs were most common in the Northeast region (relative to Midwest: South OR 1.06; West OR 1.09; Northeast OR 1.87) (P < .05). Patients with Medicaid insurance (relative to commercial: OR 0.62) and those with social (OR 0.78) and economic (OR 0.93) SDOH disadvantage (all P < .05) had lower odds of undergoing simultaneous TKA (Table 2 and Fig. 2).

Table 2.

Odds of undergoing simultaneous relative to staged (within 1 year) bilateral TKA.

Factor	OR (95% CI)	P value
Age
<50	Ref
50-59	0.99 (0.92, 1.07)	.866
60-69	0.82 (0.76, 0.88)	<.001
70-79	0.61 (0.57, 0.67)	<.001
≥80	0.35 (0.30, 0.40)	<.001
Sex
F	Ref
M	1.38 (1.34, 1.43)	<.001
ECI
0-2	Ref
3-5	0.96 (0.93, 1.00)	.037
6-8	0.84 (0.81, 0.88)	<.001
≥9	0.68 (0.65, 0.72)	<.001
Region
Midwest	Ref
South	1.06 (1.02, 1.10)	.006
West	1.09 (1.03, 1.14)	.001
Northeast	1.87 (1.79, 1.94)	<.001
Insurance
Commercial	Ref
Medicaid	0.62 (0.56, 0.68)	<.001
Medicare	1.00 (0.96, 1.03)	.841
SDOH
Social	0.78 (0.60, 0.99)	.048
Environmental	0.86 (0.38, 1.71)	.686
Economic	0.93 (0.89, 0.98)	.007
Educational	0.96 (0.75, 1.20)	.702

Bold value indicates statistical signfiicance with P <0.05.

Figure 2.

Odds of undergoing simultaneous relative to staged bilateral TKA. Black bars represent statistically significant associations (P < .05), while gray bars indicate variables that were not statistically significant (P > .05).

Discussion

In a large contemporary cohort of bilateral TKA patients, staged procedures overwhelmingly predominated, and the use of simultaneous surgery was found to steadily decline between 2016 and 2021. This trend may reflect the increasing popularity of ambulatory surgical centers for primary TKA, [29] as these settings are better suited for patients undergoing unilateral procedures. Most staged cases were performed at intervals of 90 days or longer, reflecting a practical shift that parallels current guideline recommendations. Patients selected for simultaneous TKA were generally younger and healthier, but nonclinical factors—including insurance status, geographic region, and documented SDOH disadvantage—strongly correlated with surgical selection. These findings emphasize that bilateral TKA decision-making in current practice is driven not only by medical risk profiles but also by system- and patient-level constraints that may affect safety, access, and postoperative recovery planning.

Representing the most recent published cohort analysis, we show that 15% of patients underwent simultaneous surgeries, while 85% had staged surgeries within 1 year, consistent with previous estimates [12]. Over the 2016-2021 study period, the proportion of simultaneous bilateral TKAs decreased relative to staged bilateral TKAs. The distribution of surgeries showed approximately 3-quarters of patients received their second TKA at an interval of 90 days or greater, suggesting closer adherence with published guidelines over time [17,30].

Younger, healthier, and male patients were more likely to undergo simultaneous bilateral TKA, as supported in other publications [12,31,32]. Notably, several nonclinical factors were also found to be strongly associated with surgery selection. Patients in the Northeast region were found to have 87% higher odds of undergoing simultaneous surgeries compared to those in the Midwest, suggesting significant variation in clinical practice by geographic region. This finding has not yet been described in the literature. Surveys of arthroplasty surgeons have shown experience during fellowship training to be a primary driver of future practice patterns [33,34]. Many orthopaedic surgeons stay to practice in the same geographic region in which they completed their training, [35,36] potentially explaining how regional variation in clinical practice develops and persists.

Patients with Medicaid had 38% lower odds of simultaneous bilateral TKA compared to those with commercial insurance. Mehta et al. previously demonstrated that patients with Medicare/Medicaid insurance were less likely to undergo simultaneous bilateral TKAs than patients with commercial insurance [6]; however, Medicare and Medicaid coverage were not distinguished from each other in the analysis. After controlling for other patient factors in the multivariate analysis, the current study found a significant difference between Medicaid and commercial insurance, but no significant difference between Medicare and commercial insurance. Reimbursement may factor into surgeons’ willingness to offer simultaneous bilateral TKA in some cases. When performed as simultaneous bilateral procedures, the second TKA is reimbursed at 50% the rate of the first [6,37]. This may be most disadvantageous when the patient has Medicaid insurance, which reimburses less on average than both Medicare and commercial insurance [38].

Finally, patients with documented economic and social SDOH disadvantage were less likely to undergo simultaneous bilateral TKAs. While prior studies have demonstrated worse outcomes for SDOH-disadvantaged patients following unilateral TKA, [31,39,40] the impact of these factors on bilateral procedures has not been previously studied and warrants further evaluation. Simultaneous bilateral TKA entails a more intensive recovery course and higher likelihood of requiring inpatient rehabilitation, which carries substantial cost burden [32,41]. For patients with limited postoperative support, this may make the procedure less feasible. These considerations highlight the importance of preoperative SDOH screening, which may allow surgeons to provide more individualized counseling on the relative risks and benefits of simultaneous vs staged procedures and connect patients with supportive resources when appropriate. Building on the current study, additional research is needed to better understand the impact of SDOH on postoperative outcomes among bilateral TKA patients.

The current study has several limitations. Due to its retrospective nature, causative relationships cannot be determined, and it is impossible to know which factors drive surgical decision-making for individual patients. As with any administrative study, there may be errors in the coding. Simultaneous bilateral TKAs were captured using ICD procedure codes, which specify laterality, but are not typically used for outpatient procedures. Only 3% of all simultaneous bilateral TKAs are performed in an outpatient setting, [42] so while the prevalence of simultaneous surgeries may be an underestimate, the overall effect is expected to be minimal. Among simultaneous TKA patients, it was not possible to distinguish between those undergoing one vs two anesthetic events in the same day, since this information is not reported in the database. Finally, while the use of SDOH codes has expanded significantly in recent years, [28] there are undoubtedly patients with disadvantages that were not captured by ICD coding, due to inconsistent screening and/or documentation practices. As this set of codes is expected to be more specific than it is sensitive, the prevalence of SDOH disadvantage is likely higher in reality than what is captured in the dataset.

Conclusions

The utilization of simultaneous bilateral TKA declined between 2016 and 2021, relative to staged bilateral TKA. Treatment approach was associated with both clinical (age, comorbidity burden, sex) and nonclinical (insurance status, SDOH, region) factors, highlighting the multifactorial nature of this decision-making. Future studies may build on these findings to further explore the impact of SDOH on postoperative outcomes.

Conflicts of interest

J.N. Grauer: North American Spine Society Journal Editor-in-chief, Journal of the American Academy of Orthopaedic Surgeons Deputy Editor, and Past North American Spine Society Board Member. I. Molloy: Paid consultant for Smith & Nephew. All other authors declare no potential conflicts of interest.

For full disclosure statements refer to https://doi.org/10.1016/j.artd.2026.102025.

CRediT authorship contribution statement

Gwyneth C. Maloy: Writing – original draft, Investigation, Conceptualization. Wesley Day: Writing – original draft, Investigation. Scott J. Halperin: Writing – review & editing, Writing – original draft, Methodology. Wasif Islam: Writing – review & editing, Methodology. Jonathan N. Grauer: Writing – review & editing, Supervision, Resources. Ilda B. Molloy: Writing – review & editing, Supervision, Conceptualization.