Analysis of knee functional flexion axis in navigated TKA: identification and repeatability before and after implant positioning

Purpose Providing correct rotational alignment of femoral component in total knee arthroplasty (TKA) is mandatory to achieve correct kinematics, good ligament balance and patellar tracking. The purpose of this study was to evaluate potential clinical applications of functional flexion axis (FFA) by analysing the differences between pre- and post-implant placement. This evaluation was supported by the analysis of repeatability, assessing the robustness of the proposed method. Methods Anatomical acquisitions and passive kinematics were acquired on 87 patients undergoing TKA using a commercial navigation system. Knee FFA was estimated, before and after implant positioning, from three flexion-extension movements between 0 and 120 degrees. The angle between FFA and transepicondylar axis was analysed in frontal and axial planes. Repeatability coefficient and intraclass correlation coefficient (ICC) were used to analyse the reliability and the agreement in identifying the axis. Results The analysed angle presented differences between pre- and post-operative conditions only in the frontal plane (from -8.3 +/- 5.5 degrees to -2.8 +/- 5.3 degrees) (p<0.0001). There was good intraobserver reliability and agreement. Repeatability coefficient ranged between 4.4 degrees (3.7-4.9 degrees) and 3.4 degrees (2.9-3.8 degrees), the ICC between 0.87 (0.83-0.91) and 0.93 (0.90-0.95) and the standard deviation ranged between 1.3 and 1.0 degrees. Conclusions The present study demonstrated that TKA affected the estimation of FFA only in the frontal plane. This method reported good repeatability, demonstrating its usefulness for clinical purposes particularly to evaluate rotational positioning of the femoral component in the axial plane.