Can rotational congruity be achieved in both flexion and extension when the femoral component is externally rotated in total knee arthroplasty?

Commercially available total knee implants were cemented onto a custom made jig which provided an axis of rotation perpendicular to the mechanical axis. The femoral component was fixed in 3° of external rotation and articulated with a tibial component implanted in either 3° of external rotation or neutral rotation. A modified femoral component having a thicker posterior lateral than posterior medial condyle was implanted in neutral rotation to articulate with a neutrally rotated tibial component. Fuji film placed in both the medial and lateral compartments was used to determine the centroids of the medial and lateral tibiofemoral contact areas and rotational congruity between the femoral and tibial components. When the femoral component was externally rotated, rotational incongruity of 3.53° occurred in extension if the tibial component was not externally rotated, and rotational incongruity of 3.23° occurred in flexion if the tibial component was externally rotated. With the modified femoral component the angle of rotational congruity remained relatively constant at approximately 0.15° throughout the flexion range and did not exceed 0.20°.