Three distal femoral axes have been described to aid in alignment of the femoral component; the Trans Epicondylar Axis (TEA), the Posterior Condylar Axis (PCA) and the Antero Posterior (AP) axis. Our aim was to identify if there was a reproducible relationship between the axes which would aid alignment of the femoral component. This is the first study compare all three distal femoral axes with each other using magnetic resonance imaging (MRI) in a Caucasian population. Our sample group represents real life patients awaiting total knee arthroplasty (TKA), as opposed non-arthritic or cadaveric knees. We identified the relationship between these rotational axes by performing MRI scans on 89 patients awaiting TKA with patient-specific instrumentation. Measurements were taken by two observers. Patients had a mean age of 62.5 years (range 32–91). 51 patients were female. The mean angle between the TEA and the AP axis was 92.78° with a standard deviation of 2.51° (range 88° – 99°). The mean angle between the AP axis and the PCA was 95.43° with a standard deviation of 2.75° (range 85° – 105°). The mean angle between the TEA and the PCA was 2.78° with a standard deviation of 1.91° (range 0° – 10°). We conclude that while there is a reproducible relationship between the differing femoral axes, there is a significant range in the relationship between the femoral axes. This range may lead to greater inaccuracy than has previously been appreciated when defining the rotation of the femoral component. There is most variation between the PCA and the AP axis. The TEA's relationship with the PCA and AP appears important in defining rotation. Due to the well accepted difficulty in defining the TEA intra-operatively, there may be a role for patient-specific instrumentation in TKA surgery with pre-operative MRI.
Three distal femoral axes have been described to aid in alignment of the femoral component; the Trans Epicondylar Axis (TEA), the Posterior Condylar Axis (PCA) and the Antero Posterior (AP) axis. Our aim was to identify if there was a reproducible relationship between the axes. Hopefully this will aid the surgeon to more accurately judge the rotation of the femoral cutting block by using the axes with the least variation. This is the first study compare all three distal femoral axes with each other using magnetic resonance imaging (MRI) in a Caucasian population awaiting total knee arthroplasty (TKA). We identified the relationship between these axes by performing MRI scans on 89 patients awaiting TKA with patient-specific instrumentation. Measurements were taken by two observers. Patients had a mean age of 62.5 years (range 32–91). 51 patients were female. The mean angle between the TEA and AP axis was 92.78°, standard deviation (SD) 2.51° (range 88°–99°). The mean angle between the AP axis and PCA was 95.43°, SD 2.75° (range 85°–105°). The mean angle between the TEA and PCA was 2.78°, SD 1.91° (range 0°–10°). We conclude that while there is a reproducible relationship between the differing femoral axes, there is a significant range in the relationship between the femoral axes. This range may lead to greater inaccuracy than has previously been appreciated when defining the rotation of the femoral component. There is most variation between the PCA and the AP axis. Most systems have a cutting block with 3° of external rotation from the PCA and this would be parallel to the TEA in the majority, but not all, cases in this series. This data suggests that if the surgeon is to pick two axes to reference from, one should include the TEA.
