The coronal alignment differs between two-dimensional weight-bearing and three-dimensional nonweight bearing planning in total knee arthroplasty.

Purpose: The goal of this study is (1) to assess differences between two-dimensional (2D) weight-bearing (WB) and three-dimensional (3D) nonweight-bearing (NWB) planning in total knee arthroplasty (TKA) and (2) to identify factors that influence intermodal differences.

Methods: Retrospective single-centre analysis of patients planned for a TKA with patient-specific instruments (PSI). Preoperative WB long-leg radiographs and NWB computed tomography were analysed and following radiographic parameters included: hip-knee-ankle angle (HKA) (+varus/-valgus), joint line convergence angle (JLCA), femorotibial subluxation and bony defect classified according to Anderson. Preoperative range of motion was also considered as possible covariate. Demographic factors included age, sex, and body mass index.

Results: A total of 352 knees of 323 patients (66% females) with a mean age of 66 ± 9.7 years were analysed. The HKA differed significantly between 2D and 3D planning modalities; varus knees ( = 231): 9.9° ± 5.1° vs. 6.7° ± 4°,  < 0.001; valgus knees ( = 121): -8.2° ± 6° vs. -5.5° ± 4.4°,  < 0.001. In varus knees, HKA ( = 0.38;  < 0.0001) and JLCA ( = 0.14;  = 0.03) were associated with increasing difference between 2D/3D HKA. For valgus knees, HKA ( = -0.6;  < 0.0001), JLCA ( = -0.3;  = 0.0001) and lateral distal femoral angle ( = -0.28;  = 0.03) showed a significant influence on the mean absolute difference.

Conclusion: The coronal alignment in preoperative 3D model for PSI-TKA significantly differed from 2D WB state and the difference between modalities correlated with the extent of varus/valgus deformity. In the vast majority of cases, the 3D NWB approach significantly underestimated the preoperative deformity, which needs to be considered to achieve the planned correction when using PSI in TKA.

Level Of Evidence: Level III.