Mechanical axis cannot be restored in total knee arthroplasty with a fixed valgus resection angle - A radiographic study

Alignment radiographs of 174 extremities in patients undergoing total knee arthroplasty were measured to determine the variability in valgus cut angle (VCA) and the anatomical factors that affect the VCA necessary to restore the mechanical axis. The VCA averaged 5.6 degrees +/- 1.0 degrees with a range of 2 degrees to 9 degrees. With the 3 different methods of defining the anatomical axis of the femur, 30% to 51% of patients required a VCA of less than 5 degrees or greater than 6 degrees. The offset and neck-shaft angle of the ipsilateral hip exert a strong effect on the VCA necessary to restore the mechanical axis. Patients with coxa valga or reduced hip offset generally require a VCA less than 5 degrees. Patients with coxa vara or increased hip offset generally require a VCA greater than 6 degrees.