1 Introduction

Simultaneous bilateral total knee arthroplasty (SBTKA) is a well-established surgical option for patients with bilateral end-stage osteoarthritis. Although it offers benefits such as reduced total hospitalization time, single anesthesia exposure, and shorter overall rehabilitation duration, concerns remain regarding surgical accuracy, postoperative alignment, and functional outcomes on each side performed consecutively in a single session.^1–3

A critical aspect of successful total knee arthroplasty (TKA) is restoration of the mechanical axis of the lower limb. Malalignment has been associated with increased implant wear, loosening, and poorer clinical outcomes.⁴ In bilateral procedures, some evidence suggests that alignment may differ between the first and second operated knees due to surgeon fatigue, time constraints, or dominant-hand technical bias, particularly in right-handed surgeons who often begin with the right side.¹ However, the clinical relevance of such alignment discrepancies, especially their impact on patient-reported outcomes, remains controversial.

Studies comparing staged versus simultaneous bilateral TKA have yielded mixed results regarding radiological accuracy and functional outcomes. While some have reported better alignment in staged procedures,¹ others demonstrated no significant difference in long-term clinical outcomes.^2,3 Moreover, few studies have systematically examined whether subtle differences in alignment between the right and left knees in simultaneous bilateral TKA translate into meaningful differences in pain, function, or satisfaction.

The present study aims to evaluate whether the side operated first — typically the right knee in right-handed surgeons — shows superior radiological alignment compared to the contralateral knee, and whether these differences, if present, correlate with clinical outcome scores. We hypothesize that although minor radiological differences may exist due to surgical sequencing and hand dominance, these do not result in clinically significant differences in patient-reported outcomes including VAS, KSS, WOMAC, FJS, OKS, and satisfaction scores.1Are there significant differences in postoperative radiographic alignment between the right and left knees in simultaneous bilateral TKA?2Does the surgeon's hand dominance influence radiological alignment?3Are any observed radiological differences associated with functional outcome measures (VAS, KSS, WOMAC, FJS, OKS)?4Does the laterality affect patient satisfaction or joint awareness?

2 Materials and methods

2.2

2.2 Radiographic evaluation

Radiographic measurements were performed using standardized postoperative anteroposterior (AP) and lateral knee radiographs, as well as full-length standing hip-knee-ankle (HKA) orthoroentgenograms obtained at the final follow-up (minimum 2 years postoperatively). All imaging was retrieved from the hospital's PACS (Picture Archiving and Communication System) and analyzed digitally using calibrated measurement tools.

Although full-length preoperative hip-knee-ankle radiographs were not available, standard anteroposterior and lateral knee radiographs obtained preoperatively were analyzed to measure anatomical alignment parameters (aLDFA, aMPTA, tibial slope). These measurements revealed no significant side-to-side differences between the right and left knees preoperatively, suggesting comparable preoperative alignment at the joint level.

The following coronal and sagittal plane alignment parameters were evaluated for each knee:•Lateral Distal Femoral Angle (mLDFA) (Figure-1

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Fig. 1 Measurement of the lateral distal femoral angle (mLDFA).

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)•Medial Proximal Tibial Angle (mMPTA) (Figure-2

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Fig. 2 Measurement of the medial proximal tibial angle (mMPTA).

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)•Hip-Knee-Ankle (HKA) angle (Figure-3

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Fig. 3 Measurement of the Hip-Knee-Ankle (HKA) angle.

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)•Mechanical Axis Deviation (MAD) (Figure-4

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Fig. 4 Measurement of the mechanical axis deviation (MAD).

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)•Tibial Slope (posterior inclination of the tibial component) (Figure-5

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Fig. 5 Measurement of the tibial slope.

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)

Measurements were carried out independently by two experienced orthopedic surgeons who were blinded to each other's results and to the clinical outcomes. Each parameter was measured twice by each observer at different time points with at least one week interval, and the mean values were used for analysis.

To assess the reliability and consistency of the radiographic measurements, interobserver agreement was evaluated using the intraclass correlation coefficient (ICC) for each parameter, with values ≥ 0.75 indicating excellent agreement, in accordance with established guidelines.⁵

3 Results

A total of 89 patients (178 knees) who underwent simultaneous bilateral total knee arthroplasty were included in the final analysis. The mean age at the time of surgery was 66.37 ± 7.01 years, and the mean postoperative follow-up duration was 39.97 ± 12.92 months. The cohort consisted of 14 men and 75 women patients. The average body mass index (BMI) was 36.59 ± 5.90 kg/m² (Table-1). All patients had undergone the procedure in a single surgical session, with standardized perioperative protocols and postoperative rehabilitation applied to both knees equally.

Clinical outcomes were evaluated using validated patient-reported outcome measures (PROMs) and objective functional scores. There were no statistically significant differences between the right and left knees in terms of postoperative range of motion (115.94 ± 16.25° vs. 115.67 ± 14.90°, p = 0.7697) or pain levels measured by the Visual Analog Scale (1.32 ± 1.71 vs. 1.57 ± 1.74, p = 0.2292). Similarly, patient satisfaction scores, rated on a 0–10 scale, did not differ significantly between sides (8.68 ± 1.65 vs. 8.29 ± 1.72, p = 0.1581) (Table-2).

Functional outcomes were comparable across knees. The mean Knee Society Score was 86.20 ± 12.79 on the right and 84.91 ± 13.05 on the left (p = 0.4763), while the KOOS score averaged 87.61 ± 12.83 vs. 86.59 ± 13.17 (p = 0.2991). WOMAC scores showed a slightly better outcome on the right (11.92 ± 12.52) compared to the left (14.62 ± 14.09), but this difference did not reach statistical significance (p = 0.0807). The Forgotten Joint Score was 86.61 ± 16.47 for the right knee and 85.26 ± 15.97 for the left (p = 0.4685). Lastly, the Oxford Knee Score was similar between sides (42.04 ± 7.12 vs. 41.74 ± 6.61, p = 0.5495) (Table-2). Collectively, these findings support the hypothesis that both knees, despite potential differences in surgical technique or surgeon preference, yielded clinically equivalent outcomes in terms of pain relief, function, and patient satisfaction.

Radiological assessments were performed using standardized long-leg weight-bearing radiographs obtained during the final follow-up visit. Most alignment parameters showed no statistically significant differences between the right and left knees. The mean mechanical axis deviation (MAD) was 7.84 ± 16.52 mm on the right and 11.01 ± 20.01 mm on the left (p = 0.1125), and the medial proximal tibial angle (MPTA) was 88.34 ± 3.30° versus 88.35 ± 3.25° (p = 0.5399), respectively. Tibial slope measurements were also similar, averaging 6.79 ± 3.56° on the right and 6.70 ± 3.67° on the left (p = 0.5262) (Table-3).

However, two alignment parameters showed statistically significant differences between sides. The hip-knee-ankle (HKA) angle was significantly greater on the left (5.00 ± 3.86° vs. 3.74 ± 3.22°, p = 0.0087), and the lateral distal femoral angle (LDFA) was also higher (91.51 ± 3.89° vs. 90.20 ± 2.83°, p = 0.0009), indicating increased varus alignment in the left knees (Table-3). These differences, while statistically significant, remained within clinically acceptable ranges and did not translate into differences in functional outcomes between the sides.

In summary, our comparative evaluation of simultaneously implanted right and left total knee arthroplasties yielded the following findings in response to our research questions:(1)Radiographic alignment differed significantly between sides, with HKA and LDFA measurements indicating more neutral alignment on the right knee.(2)This asymmetry aligns with our hypothesis regarding surgical sequencing and hand dominance favoring the right (dominant) side.(3)Despite these radiological differences, no statistically significant differences were found in patient-reported clinical outcomes including VAS, KSS, WOMAC, KOOS, FJS, OKS, satisfaction, or ROM.(4)These results indicate that mild coronal plane asymmetries in mechanical alignment do not adversely affect early to mid-term functional performance or subjective joint awareness.

4 Discussion

This prospective observational study evaluated the clinical and radiological symmetry of simultaneously implanted right and left total knee arthroplasties (TKAs) in a cohort of 89 patients. The primary hypothesis was that, although coronal alignment might differ slightly due to surgical side preference or dominance, the clinical outcomes between the knees would remain equivalent. The findings confirmed this hypothesis: while functional outcomes—including pain relief, range of motion, patient satisfaction, and multiple validated scoring systems—were statistically indistinguishable between the sides, radiographic analysis revealed significantly greater varus alignment in the left knees, as evidenced by higher HKA and LDFA values. These results support the notion that subtle asymmetries in mechanical alignment may occur during simultaneous bilateral TKA procedures, potentially reflecting technical nuances, but do not appear to influence short/mid-term patient outcomes.

Previous studies evaluating simultaneously performed bilateral TKA have reported mixed findings regarding the presence and clinical impact of side-to-side asymmetries. Ersen et al. observed long-term functional equivalence between the first and second knees in simultaneous bilateral TKA, despite potential intraoperative differences during sequential implantation.² Similarly, Huang et al. demonstrated that although early postoperative satisfaction may transiently differ between knees, objective outcome measures tend to converge over time, supporting the reliability of bilateral implantation strategies.³ Our findings are consistent with these reports, further affirming that postoperative function remains comparable between limbs, regardless of minor variations in alignment.

Our study is in agreement with these findings, reinforcing that simultaneous bilateral TKA yields clinically equivalent outcomes across both limbs. However, unlike most prior studies, we conducted a detailed analysis of radiographic alignment parameters and identified statistically significant differences in the coronal plane specifically higher LDFA and HKA values on the left side, indicating a subtle varus deviation. To our knowledge, such a comprehensive side-by-side analysis of mechanical alignment in bilateral TKA, with a focus on the dominant hand of the surgeon and the side of initial approach, remains underrepresented in the literature. This radiographic asymmetry, although statistically significant, did not translate into any detectable difference in clinical function or patient satisfaction. Thus, our study adds novel data by suggesting that side-specific alignment variations may exist even in standardized simultaneous procedures, yet remain clinically silent in the short to mid-term.

The observed side-to-side difference in coronal alignment—specifically higher LDFA and HKA values in the left knees—may be explained by factors inherent to the surgical setup and surgeon behavior in simultaneous bilateral TKA. Given that most surgeons are right-handed and typically begin the procedure on the right side, greater precision and comfort may be achieved during the initial implantation. As fatigue increases or intraoperative focus shifts during the second knee, subtle deviations in component positioning may occur. Hiranaka et al. have previously noted that the sequence of knee arthroplasty may influence certain technical aspects, such as cementing quality and alignment accuracy, particularly in longer or bilateral procedures.¹⁰

Additionally, the ergonomic challenges of bilateral TKA—such as surgical exposure, limb positioning, and instrument angulation—may vary between sides, potentially introducing asymmetry even under standardized protocols. Our findings support the hypothesis that the side operated first (in our cohort, predominantly the right side) may benefit from more consistent mechanical alignment, though this advantage does not appear to affect postoperative functional recovery.

The absence of clinically significant differences between the right and left knees—despite measurable coronal alignment discrepancies—supports a growing body of evidence suggesting that mild post-TKA malalignment does not necessarily impair functional outcomes. For instance, Parratte et al. demonstrated that deviations within ±3° from neutral alignment did not compromise implant survival or patient-reported outcomes over long-term follow-up.^11–13 Additionally, Hadi et al., in a systematic review analyzing over 2200 TKAs, found that 73 % of comparisons involving malalignment parameters showed no association with PROM scores, concluding that while malalignment may be present, its impact on early clinical results is generally minimal.¹⁴ Our data align with these findings: despite statistically significant differences in HKA and LDFA angles, there was no detectable impact on pain, function, or satisfaction at mid-term follow-up. Thus, this work adds to the understanding that the clinical “safe zone” for alignment may be broader than traditionally believed, and minor asymmetries should not overshadow functional recovery in simultaneous bilateral TKA.

This study has several limitations that should be acknowledged. First, although all surgeries were performed by the same right-handed surgeon using a standardized technique, subtle intraoperative variations—such as differences in soft tissue tension or exposure difficulty between the right and left knees—may have influenced alignment outcomes. Second, the study design is prospective observational cohort in nature, which may introduce selection and information biases, despite rigorous inclusion and exclusion criteria.

Third, while a minimum two-year follow-up period was ensured for all patients, longer-term radiological and functional assessments are required to determine the durability and clinical impact of any alignment discrepancies. Fourth, patient satisfaction was measured using a non-validated numeric rating scale from 0 to 10. Although this method is commonly used in clinical settings and provides pragmatic insight, it lacks formal psychometric validation.

Fifth, and perhaps most importantly, preoperative full-length standing hip-knee-ankle radiographs (orthoroentgenograms) were not available for the study cohort. As a result, we were unable to assess or control for potential preoperative alignment differences between the limbs using mechanical axis measurements. However, standard preoperative anteroposterior and lateral knee radiographs were available and analyzed to assess anatomical alignment parameters (aLDFA, aMPTA, tibial slope). These measurements revealed no significant differences between sides preoperatively, partially mitigating this limitation by suggesting comparable preoperative joint-level alignment. Nonetheless, the inability to assess full-limb alignment remains a limitation when interpreting the origin of postoperative differences.

Finally, the study population was limited to patients operated at a single institution by a single surgeon, which, while ensuring procedural consistency, may affect the generalizability of the findings to broader clinical practice.

Despite these limitations, the study possesses notable strengths, including a well-powered cohort, prospective data collection post-study registration, and comprehensive radiological analysis. To our knowledge, this is one of the few studies to examine mechanical alignment asymmetry in simultaneous bilateral TKA with direct reference to surgical laterality. The finding that radiographic deviations do not translate into clinical disparity provides valuable insight into the tolerability of alignment variation in real-world practice. Future multicenter or robotic-assisted studies may further clarify the implications of surgical side dominance in TKA alignment and outcomes.

5 Conclusion

In this prospective cohort study of patients undergoing simultaneous bilateral total knee arthroplasty, we observed that although the left knees exhibited a statistically greater varus alignment—reflected in higher HKA and LDFA values—this asymmetry did not translate into differences in pain, function, range of motion, or patient-reported outcome scores. These findings support the clinical tolerability of minor alignment discrepancies between limbs in well-executed bilateral procedures. Importantly, our data suggest that surgical side preference and ergonomic factors may subtly influence radiological alignment, even when operative protocols are standardized. As such, awareness of side-related variability should inform both surgical planning and expectations in bilateral TKA, particularly when performed by a dominant-handed surgeon. Further investigation is warranted to assess whether these alignment variations have long-term biomechanical or prosthetic implications.

Credit author statament

Muhammed Furkan Darılmaz: Conceptualization; Data curation; Investigation; Methodology; Writing – original draft.

Mustafa Bulut: Formal analysis; Writing – review & editing.

Hakan Yolaçan: Project administration; Writing – review & editing.

Serkan Güler: Supervision; Writing – review & editing.

Ethical approval and consent to participate

This prospective observational cohort study was conducted at the Department of Orthopaedics and Traumatology, Aksaray University Training and Research Hospital. The study protocol was approved by the Institutional Ethics Committee of Aksaray University (Protocol: SAGETİK156, Approval Number: 2025/008, Date: January 16, 2025).

Consent for publication

Appropriate consent from the patient was obtained per institutional protocol and guidelines.

Funding

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.