Predicting Femoral Component Rotation in Mechanically Aligned Total Knee Arthroplasty: Can Preoperative Three-Joint Alignment Radiograph Help?

Objectives: Achieving precise femoral component external rotation (FCER) is crucial in Total Knee Arthroplasty (TKA) to prevent patellofemoral maltracking, implant loosening, pain, and wear. While CT scans and MRIs help determine the rotational profile of the distal femur, their use is limited by concerns about radiation exposure and cost. This study aims to explore the potential role of a preoperative three-joint alignment radiograph in predicting the necessary FCER for mechanically aligned TKA. Methods: This retrospective cohort study was conducted on 922 patients (1,208 knees) with varus deformity who underwent primary TKA performed by a single knee surgeon between 2018 and 2020. The angular parameters from the three-joint alignment radiograph and FCER were extracted from the JRRC Knee Registry. These angular parameters included the lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), joint line congruency angle (JLCA), and varus angle (VA). FCER was determined intraoperatively based on standard anatomical criteria, including Whiteside's line, the trans-epicondylar line, posterior condylar axis, anteroposterior axis, and the assessment of the proximal tibial cut. Data analysis was performed using SPSS 26. Results: A total of 922 patients (57.9% female, 42.1% male) with a mean age of 65.39 ± 8.03 years (range: 43-90) were included in the study. The FCER was 0° in 0.33%, 3° in 83.6%, 5° in 15.23%, and 7° in 0.82% of patients. FCER showed no correlation with MPTA, LDFA, or VA, but it was significantly correlated with JLCA/LDFA, JLCA/MPTA, and JLCA/VA, with cut-offs at 0.11, 0.12, and 0.62, respectively. JLCA/MPTA emerged as the best predictor for FCER in TKA (sensitivity = 48%, specificity = 81%, and accuracy = 0.67). Conclusion: Preoperative three-joint alignment radiographs can guide the surgeon in determining the required FCER during TKA. If the ratios exceed the established cut-offs, an FCER greater than 3° may be necessary.