A Prospective Multicenter Analysis of the Mid-term Outcomes of Component Size Variation in Simultaneous Bilateral Total Knee Arthroplasty: Does One-Size Difference Matter?

Background In simultaneous bilateral total knee arthroplasty (bTKA), surgeons often encounter scenarios where femoral or tibial component sizes differ between limbs. While component sizing is routinely individualized, the functional and survivorship implications of such asymmetry remain insufficiently studied. A clearer understanding is clinically relevant, as patient anatomy, bone stock, and soft-tissue balance often necessitate asymmetric sizing, yet evidence guiding its safety and outcomes is limited. This prospective multicenter study evaluated whether inter-limb differences in component sizing influence postoperative function, patient-reported outcomes, or revision risk in bTKA. Methods Patients undergoing cemented bTKA with standardized implants (metal femoral components, metal-backed tibial baseplates, and ultra-high-molecular-weight polyethylene inserts) were prospectively enrolled across 10 high-volume centers. Patients were stratified into Subset A (identical component sizes in both knees) and Subset B (different component sizes between knees). Clinical and functional outcomes were assessed preoperatively and postoperatively at regular intervals up to three years. Endpoints included range of motion (ROM), Knee Society Score (KSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and 36-Item Short-Form Health Survey (SF-36). Radiographic assessment evaluated alignment, fixation, and implant stability. Minimal clinically important difference (MCID) thresholds were used to confirm the clinical relevance of improvements. Results A total of 166 patients (332 knees) were analyzed, with 140 in Subset A and 26 in Subset B. Baseline characteristics were comparable between groups. Both subsets demonstrated significant improvements in ROM and functional scores from baseline to three years (p<0.001 for within-group changes). Subset A achieved higher final ROM (123.6° vs. 118.1°; p<0.001) with less variability, and KSS were also significantly greater in this group (p<0.05). Subset B, despite inter-knee size differences, showed improvements similar to Subset A in WOMAC scores and demonstrated higher values in selected SF-36 domains, including physical functioning, role limitations, and emotional well-being (p<0.05). No implant-related complications, revisions, or radiographic failures were reported in either group during the three-year follow-up, and radiographs confirmed stable and well-aligned prostheses. Conclusion A one-size difference in femoral or tibial components between knees does not compromise clinical or functional outcomes in bTKA. While symmetric sizing may provide slightly greater ROM and KSS improvements, minor asymmetry is not detrimental and may even be associated with enhanced patient-reported quality-of-life domains. These findings support the practice of individualized component selection and reassure surgeons that small inter-limb differences in implant sizing do not negatively impact mid-term function or survivorship.