Abstract
Introduction
Several methods, based on both functional and anatomical references, have been studied to reach the goal of a proper knee kinematics in total knee arthroplasty (TKA). However, at present, there is still a large debate about which is the most precise and accurate method to achieve the correct rotational implant positioning. One of the main methods already used in TKA to describe the tibiofemoral flexion-extension movement, based on a kinematic technique, thus not influenced by the typical variability related to the identification of anatomical references, is called “functional flexion axis” (FFA) method. The purpose of this study was to determine the repeatability in estimating knee functional flexion axis, thus evaluating the robustness of the method for navigated total knee arthroplasty.
Methods
Passive kinematic and anatomical acquisitions were performed with a commercial navigation system on 87 patients undergoing TKA with primary osteoarthritis. Knee FFA was estimated, before and after implant positioning, from three flexion-extension movements between 0° and 120° (Figure 1). The angle between Functional Flexion Axis and an arbitrary clinical reference, the transepicondylar axis (TEA), was analysed in frontal and axial view (Figure 2). Repeatability Coefficient and Intraclass Correlation Coefficient (ICC) were estimated to analyse the reliability and the agreement in identifying the axis.
Results
The analysed angle between FFA and TEA presented differences between pre- and post-operative conditions only in the frontal plane (from −8.3° ± 5.5° to −2.8° ± 5.3°) (p < 0.0001). There was good intra-observer reliability and agreement. Repeatability coefficient ranged between 4.4° (3.7° – 4.9°) and 3.4° (2.9° – 3.8°), 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°.
Conclusions
The present study demonstrated that total knee arthroplasty affected the estimation of FFA only in the frontal plane. The FFA method has good repeatability both in pre- and post-operative conditions, thus it can be considered for clinical purposes, including navigated knee arthroplasty, as well. Since the FFA is surgeon-independent and avoids the bias due to anatomical landmarks identification, this could potentially provide an alternative technique to plan implant positioning in total knee arthroplasty.
