Knee functional flexion axis in osteoarthritic patients: comparison in vivo with transepicondylar axis using a navigation system

No study, up to now, has examined the effect of arthritis on pathologic subjects using functional flexion axis (FFA). The purpose of this study is to understand whether arthritis affects somehow the FFA evaluation and to assess whether the FFA could be considered a usable reference for implant positioning for osteoarthritic knees. Using a navigation system, FFA orientation was evaluated intraoperatively (computed with the mean helical axis method) in three different ranges of motion (0A degrees-120A degrees; 35A degrees-80A degrees; 35A degrees-120A degrees) and in two different planes (coronal and axial), for 111 osteoarthritis patients undergoing total knee arthroplasty. The results were compared with a control group of 60 patients that underwent ACL reconstruction. The angle between the transepicondylar axis (TEA) and FFA was computed. Results showed in arthritic knees on frontal plane, an average difference between TEA and FFA of -2.8A degrees A A +/- A 5.0A degrees while on axial plane it was 0.6A degrees A A +/- A 4.7A degrees. No statistical difference was found between the three ranges in axial view, whereas some difference was found in frontal view (P < 0.0001). The TEA-FFA angle was not correlated with limb alignment on axial plane, while it was, even if poor, in frontal plane. In the control group, in frontal and in axial view, no statistical difference was found for the angle between TEA and FFA. FFA can be used as reference for implant positioning in axial plane also in pathologic knees, while for the frontal plane further investigations are required.