Using short knee radiographs to predict the coronal alignment after TKA: Is it an accurate proxy for HKA on full-length images?

BACKGROUND: The postoperative clinical outcomes has been extensively demonstrated to correlate with the coronal alignment after total knee arthroplasty (TKA). However, in different studies, either the hip-knee-ankle angle (HKA) on a full-length radiograph or the femorotibial angle (FTA) on a short knee film was used to categorize the postoperative coronal alignment. Meanwhile, several different FTA ranges were regarded as neutral alignment in different studies. As a result, it is still unknown that how FTA on short knee films and HKA related to each other. The FTA may be able to become an accurate proxy of HKA to predict the coronal alignment. The purpose of this study was to explore the correlation between the FTA and the HKA after TKA and to find the most accurate FTA range. METHODS: About 223 patients were included in this study and standard weight-bearing short knee films as well as full-length radiographs were acquired. The pre- and postoperative FTA, as well as the postoperative anatomical lateral distal femoral angle (aLDFA) and anatomical medial proximal tibial angle (aMPTA) were measured on short knee films by two orthopedic surgeons independently. On full-length films, the pre- and postoperative FTA, the pre- and postoperative HKA, as well as the postoperative mechanical lateral distal femoral angle (mLDFA) and mechanical medial proximal tibial angle (mMPTA) were also recorded by two other surgeons independently. Pearson correlation analysis was performed to compare FTA and HKA, aMTPA and mMTPA, aLDFA and mLDFA, respectively. RESULTS: The postoperative FTA and HKA had a good correlation (r = 0.86). The agreements were reached 82.7%, 71.0%, and 68.2% of all patients using three previously reported FTA ranges. When analyzing the independent alignment of the tibial tray and the femoral component, 84.1% and 57.9% of all patients was reached an agreement on the classification. CONCLUSIONS: On most occasions, the consistence between the FTA and HKA in assessing the coronal limb alignment of the lower extremity and the tibial component is satisfactory. However, the postoperative full-length film is still needed to evaluate accurately the coronal alignment of the femoral component.