Should we reduce distal femoral valgus cut by two degrees on radiograpchic measurement to avoid overcorrection of alignment in TKA? —A comparison study with radiographic VCA and real 3D-CT VCA

Background: Restoration of target alignment is one of the main goals of total knee arthroplasty (TKA). Radiograph is the world-wide accepted method to obtain valgus cut angle (VCA) which determine distal femoral cut before TKA, although its accuracy is questionable. The aim of this study was to investigate the deviation between radiographic VCA and real 3D-CT VCA in order to adjust the radiograph-based preoperative plan for better alignment target. Methods: A retrospective collection of 360 patients with constitutional varus and knee osteoarthritis. Both radiographic VCA and 3D VCA (CT based) were measured and the difference between them were calculated. Then corrected the radiographic VCA value according to the direction of deviation, each time by 1°, to ensure that the target alignment was within 0°-3° varus range. Finally, it was investigated whether age, gender, side, BMI, and preoperative alignment had an effect on the deviation of radiographic VCA to verify whether the correction was widely applicable. Results: More than 91% of the radiographic VCA were overestimated and one third were overestimated by more than 1° ( P < 0.001). Reducing the radiographic VCA by 2° could ensure that more than 96% of the target alignment was within 0°-3° varus range. The deviations were not statistically correlated with age, gender, BMI, side, and preoperative alignment ( P >0.05). Conclusions: Using the radiographic VCA reduced by 2° as a reference for distal femoral resection could ensure that more than 96% of patients with constitutional varus had a TKA target alignment in 0°-3° varus range.Level of evidence Ⅲ.