Effect of distance between the feet on knee joint line orientation after total knee arthroplasty in standing full-limb radiographs

Purpose: Although knee joint line orientation (KJLO) after total knee arthroplasty (TKA) has been emphasized as an important factor that can affect postoperative knee kinematics, the effect of foot position on KJLO has not been fully understood. This study aimed to (1) identify the anatomical and positional factors that determine KJLO after TKA, and (2) determine the effect of foot position on KJLO after TKA. The hypothesis of this study was that the post-TKA KJLO would change depending on the distance between the feet, as well as the coronal implant positions. Methods: A total of 92 radiographs from 46 patients who underwent TKA were retrospectively reviewed. Two postoperative standing full-limb anteroposterior radiographs taken with the feet in different positions (with both feet in contact with each other or shoulder width apart) from each patient were evaluated. The correlation between KJLO after TKA and possible anatomical and positional factors, including leg length, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), ankle joint line orientation (AJLO), mechanical tibiofemoral angle (mTFA), and distance between the feet, were analyzed, and the KJLO equation was computed using multiple linear regression. KJLO was also compared among different combinations of valgus or varus alignment of the femoral and tibial components. Results: LDFA, MPTA, AJLO, and distance between the feet were identified as determinants of KJLO after TKA, and the distance between the feet was strongly correlated with KJLO. Based on the KJLO equation (KJLO [°] = 107.548 − 0.441 × LDFA [°] − 0.832 × MPTA [°] + 0.093 × AJLO [°] + 0.037 × ITD [mm]), KJLO changes by 3.7° per 100 mm of distance between the feet. The KJLO of patients with valgus femoral and varus tibial components was more parallel to the ground than those with other combinations. Conclusion: KJLO after TKA was strongly affected by the distance between the feet when taking full-limb radiography, and the KJLO changed by 3.7° per 100 mm of distance between the feet. To assess the KJLO after TKA reproducibly, standardization of the distance between the feet is necessary. Level of evidence: IV.