The relationship among three anatomic landmarks and the mechanical axis of the tibia was evaluated using magnetic resonance imaging in 30 normal knees in 15 healthy volunteers. These three relationships are calculated for flexion and extension. The angles between the tibial mechanical axis and a line perpendicular to the surgical epicondylar axis, a line perpendicular to the clinical epicondylar axis, and the anteroposterior axis were 3.9 degrees varus, 0.6 degrees varus, and 0.2 degrees valgus, respectively. The mechanical axis of the femur was 3.0 degrees varus relative to the mechanical axis of the tibia. The results of the current study suggest that the surgical epicondylar axis rather than the clinical epicondylar axis or the anteroposterior axis can maintain a more predictable orientation with respect to the mechanical axis of the tibia when moving from flexion into extension.