Background: Downsizing of femur is often considered during TKA to increase the flexion space or to reduce the mediolateral overhang of the femoral component. This leads to an obvious opening up of flexion space by 2-3 mm. However, the effect of downsizing of the femur component on knee extension and final sagittal correction are unknown. This CAS based prospective study aims to find out change in knee extension caused as a result of downsizing the femur component through objective data from computer navigation.
Methods: Out of a total 1811 CAS knees operated between 2020 and 2022. 152 knees required downsizing of which 112 knees were included after applying exclusion criteria. The patients had average BMI of 28.2 ± 4.6 kg/m and average age of 62.5 ± 8 years.
Results: The average preoperative deformity was fixed flexion of 7.7 ± 4.3 (0.5-14) and varus of 8.6 ± 2.7 (3-13). The average difference of deformity in sagittal plane in extension between larger and downsized femur trial was 5.8° which was statistically significant (p = 0.001). Knee achieves more extension after downsizing possibly because of decrease in metallic volume in the joint capsule and soft tissue envelope. There was significant correction in coronal plane alignment (1°) on downsizing as the soft tissue envelope on the medial side relaxed due to decreased impingement (p = 0.001).
Conclusion: Surgeons should be aware that downsizing of the femur may increase the knee extension by 5.8° and this can be a strategy to increase both gaps specially when there is significant mediolateral overhang of the femoral component.
Level of evidence: Type IV.
Keywords: Computer navigation; Femur downsizing; Knee extension; Total knee arthroplasty.
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