The variability in the external rotation axis of the distal femur: an MRI-based anatomical study. 
Commonly used total knee arthroplasty (TKA) systems utilising measured resection techniques default to 5°-7° valgus for the distal cut relative to the anatomical axis and 3° external rotation of the femoral component relative to posterior condylar axis (PCA).  Rotational errors of the femoral component are associated with pain, patella maltracking and a poorer outcome.  We analysed MRI scans from patients undergoing TKA using patient-specific instrumentation to assess coronal and rotational alignment from landmarks identified on the scans.  One hundred and eight scans in 59 males and 49 females were studied with age range 35-93 years (mean 67.9 years).  We found 91 % of patients had a femoral valgus angle between 5° and 7° (mean angles 5.9°), while only 24 % had an external rotation angle between 2.5° and 3.5° relative to PCA.  There was no statistical significance in rotation between males and females although outliers tended to be female.  Mean Whiteside's angle was 92.9° (87.8-98).  This study highlights the variations in external rotation between patients undergoing TKA using the PCA as a reference for rotation.  This may be a contributing factor in implant malalignment and patient dissatisfaction.