Fixed distal femoral resection with a valgus cutting angle of 3° is more appropriate in intra-articular valgus deformity than juxta-articular valgus deformity in total knee arthroplasty: a retrospective single center study

Background: The traditionally recommended fixed valgus cutting angle (VCA) of 3° is used as the standard method in total knee arthroplasty (TKA) for valgus deformity. The accuracy of distal femur cutting can be affected by the type of valgus deformity because the intramedullary rod for distal femoral resection can become impinged on the femoral cortex. The purpose of this study is to analyze the accuracy and precision of a VCA of 3° in mechanical alignment in TKA for valgus deformity, based on the type of the deformity. Methods: TKAs with a fixed VCA of 3° performed in 110 intra-articular and 102 juxta-articular valgus knees were retrospectively reviewed. The age (65.8 vs. 66.1 years), sex (female: male =78/32 vs. 70/32), body mass index (26.0 vs. 26.0), and severity of deformities (valgus 12.1 vs. valgus 12.5) were not significantly different (P>0.647). The mechanical-lateral-distal-femoral angle (mLDFA) and anatomical-mechanical-axis angle (AMA-A) were evaluated. The adjusted-VCA (aVCA) was defined as the maximum angle within the range without the intramedullary rod impinging on the femoral cortex. The proportion of well-aligned femoral component (mLDFA <90°±3°) was assessed. The correlation between AMA-A-3° or aVCA-3° and postoperative mLDFA-90° was analyzed. Results: Despite the larger AMA-A in the juxta-articular group (6.2° vs. 6.8°, P<0.001), the aVCA was lesser in the juxta-articular group (5.4° vs. 4.1°, P<0.001). The mLDFA was more varus in the intra-articular group (91.7° vs. 90.6°, P<0.001) postoperatively. The proportion of well-aligned femoral component was higher in the juxta-articular group (85.3% vs. 70%, P=0.009). Postoperative mLDFA-90° was moderately correlated with aVCA-3° (r=0.301, r=387), but weakly correlated with AMA-A-3° (r=0.274, r=294) in both groups. Conclusions: Although a fixed VCA of 3° is a reliable method in achieving mechanical alignment in valgus deformity, it can be more appropriate in juxta-articular deformity than in intra-articular deformity. The aVCA is a more reasonable predictor of femoral component alignment than the AMA-A, considering the femoral cortex impingement of the intramedullary rod.