230. RADIOGRAPHIC AND NAVIGATION STUDY OF MECHANICAL AND ANATOMIC ALIGNMENTS OF THE FEMUR ON THE LATERAL VIEW

Abstract

Purpose of the study: It has been demonstrated that navigation systems improve the quality of implantation of total knee arthroplasty (TKA). The definitions of the reference alignment for the femur are not however consensual. We wanted to define the different alignments of the femur on the lateral view, including the femoral head and comparing the alignments with those defined by the measured axes during navigated implantation.

Material and methods: We analysed 30 navigated TKA or unicompartmental prosthesis implantations. The following lines were drawn on the pre and postoperative lateral telemetric views: anatomic axis aligned on the anterior cortical of the femur, mechanical alignment n°1 (centre of the femoral head to the most distal point of the Blumensaat line), mechanical alignment n°2 (centre of the femoral head to the junction between the anterior two-thirds and the posterior third of the femoral condyles). The anatomic diaphyseal alignment was taken as the reference and the angles between this reference line and the other lines was measure. In addition, the sagittal orientation of the femoral component measured during the operation by the navigation system in relation to the n°2 mechanical alignment was noted; this orientation was also measured on the postoperative lateral telemetric views in relation to this same mechanical alignment.

Results: The mean difference between the anatomic cortical alignment and the reference was 0.3 (−1 to +). The mean difference between the n°1 mechanical alignment and the reference was −1.1 (−5 to +3). The mean difference between the mechanical alignment n°2 and the reference was 0.8 (−4 to 4). The mean intraoperative sagittal orientation of the femoral component was 0.0 (−2 to 2). The mean postoperative sagittal orientation of the femoral component was 1.1 (−4 to 6).

Discussion: The differences between the orientations of the different sagittal alignments of the femur were minimal. The cortical axis has a smaller variance and could be considered as the most reliable reference, but this alignment does not include the femoral anteversion. The difference between the sagittal orientation of the femoral component as measured by the navigation system and as measured on the postoperative x-rays was also minimal, and probably of no significance clinically.

Conclusion: The choice of the sagittal alignment of the femur is of little importance. The intraoperative navigated measurement of the sagittal orientation of the femoral component is reliable.