Differences in limb alignment and femoral mechanical-anatomical angles using two dimension versus three dimension radiographic imaging

Purpose The purpose of this study was to compare the femoral mechanical-anatomical (FMA) and mechanical femorotibial (MFT) angles in an osteoarthritic population using the 2D (two dimension) and the 3D (three dimension) EOS low-dose biplanar radiographic system (EOS). Methods FMA and MFT angles were calculated in 127 adults with osteoarthritis. In 2D, FMA angle was measured between the femoral mechanical axis and the femoral anatomical axis, and MFT angle between the femoral mechanical axis and the tibial mechanical axis. In 3D, the measurement of FMA angle consisted of identifying specific anatomical landmarks on Xrays. MFT angle was then measured between the femoral mechanical axis and the tibial mechanical axis. The distribution of 2D and 3D values was assessed in terms of means and variances. Results Mean age was 69 +/- 12 years. A total of 10% of the patients having a 3D FMA angle between 4 degrees and 7 degrees have a 2D-measured FMA over or underestimated. Particularly, FMA values tend to be underestimated in women in 2D. Finally, we found that men showed a tendency to a more varus morphology, with MFT values being significantly underestimated in 2D. Conclusions The EOS 3D reconstruction system is a reliable method to measure FMA and MFT angles in an osteoarthritic population.