Abstract
Introduction
Providing proper rotational alignment of femoral component in total knee arthroplasty is mandatory to achieve correct kinematics, good ligament balance and proper patellar tracking. Recently functional references, like the function flexion axis (FFA), have been introduced to achieve this goal. Several studies reported the benefits of using the FFA but highlighted that further analyses are required to better verify the FFA applicability to the general clinical practice. Starting from the hypothesis that the FFA can thoroughly describe knee kinematics but that the joint kinematics itself can be different from flexion to extension movements, the purpose of this study was to analyse which factors could affect the FFA estimation by separately focusing on flexion and extension movements.
Methods
Anatomical acquisitions and passive joint kinematics were acquired on 79 patients undergoing total knee arthroplasty using a commercial navigation system. Knee functional axis was estimated, from three flexion and extension movements separately acquired included in a range between 0° and 120°. For flexion and extension, in both pre- and post-implant conditions, internal-external (IE) rotations was analysed to track any changes in kinematic pattern, whereas differences in FFA estimation were identified by analysing the angle between the FFA itself and the transepicondylar axis (TEA) in axial and frontal plane.
Results
The analysis of IE rotation (Figure 1) showed a statistically significant difference between the two paths in pre-implant condition, between 25°and 35° of flexion (p < 0.05).
The analysis of the angle between FFA and TEA showed statistical differences between flexion and extension (Figure 2) in both pre- and post-implant conditions and in both frontal and axial plane. Analogously, pre- and post-operative conditions (Figure 3) presented statistically significant difference.
Conclusions
The estimation of the functional axis changed in the frontal plane in relation to flexion and extension movements, above all considering pre-operative conditions. Therefore from a clinical point of view this study suggested to consider the only flexion movement for functional axis estimation during navigated TKA.
