Introduction. Several in vitro and in vivo studies have found correspondence between transepicondylar axis (TEA) and functional flexion axis (FFA) in healthy subjects. In addition some studies suggest that the use of FFA for rotational alignment of femoral implant may be more accurate than TEA. Ostheoarthritis (OA) may modify limb alignment and therefore flexion axis, introducing a bias at different flexion ranges during kinematic acquisition. In this study we want to understand whether OA affects somehow the FFA evaluation compared to TEA and whether the FFA could be considered a usable reference for implant positioning for osteoarthritic knees. Methods. We included a group of 111 patients undergoing TKA. With a navigation system, we recorded intraoperative kinematic data in three different ranges of motion (0°-120°; 35°-80°; 35°-120°). We compared the difference in orientation of FFA (computed with the mean helical axis method) in the three ranges as also the difference with the TEA on frontal and axial planes. The correlation of preoperative limb deformity with FFA and TEA was also performed. Results. In OA patients an average difference of −2.8° ± 5.0° between TEA and FFA was found on frontal plane, while on axial plane results showed a difference of 0.6° ± 4.7°. No statistical difference was found among the three ranges in axial view whereas some difference was found in frontal view (p<0.0001). Angle between TEA and FFA was not correlated with limb alignment on axial plane, while it was, even if poor, in frontal plane. Conclusions. In pathological knees there is the same correspondence between TEA and FFA both in frontal and axial plane and preoperative limb alignment does not correlate with orientation of FFA and TEA. Results are in
