Impact of femoral component flexion on implant sizing in unicompartmental knee arthroplasty: A three-dimensional computed tomography-based simulation study

Accurate femoral component sizing is essential for successful unicompartmental knee arthroplasty (UKA). This study aimed to investigate the effect of sagittal alignment on femoral component sizing using three-dimensional (3D) simulation and to validate findings with post-operative measurements. Thirty-two knees from 29 patients undergoing UKA were analyzed using 3D computed tomography (CT)-based planning with ZedKnee software. Femoral components were virtually implanted at seven sagittal alignments (−10°, −5°, 0°, 5°, 10°, 15°, and 20°), and changes in component size, bone resection surface area, and length were evaluated. Post-operative sagittal alignment was measured relative to the femoral mechanical axis using 3D CT. Repeated measures ANOVA with Bonferroni correction was performed. Increasing flexion led to reduced component size, while extension increased size. Deviations of ±10° from neutral resulted in a one-size change. Bone resection surface area and length decreased significantly with flexion ( p ​< ​0.05), correlating with downsizing. Post-operative measurements showed a mean femoral component flexion of 1.4 ​± ​3.6° (range −3.9° to 9.7°), with 87.5% implanted within ±5° of neutral. Excessive femoral component flexion systematically reduces implant size by decreasing bone resection. Maintaining sagittal alignment within ±5° of neutral is crucial for accurate sizing. These findings emphasize the importance of precise pre-operative planning and intra-operative technique to optimize UKA outcomes. • Femoral flexion causes systematic undersizing of the unicompartmental knee arthroplasty femoral component. • ∼10° sagittal deviation changes femoral component size by one level. • Flexion reduces bone resection area, leading to implant downsizing. • Alignment within ±5° maintains consistent sizing and optimal fit.