A Comparison of Bony Landmarks in the Distal Femur and the Tibial Shaft: Whichlandmark Should Be Used in Total Knee Arthroplasty?

INTRODUCTION: Correct rotational alignment of the femoral component is one of the most important factors for successful total knee arthroplasty. The rotational position of the femoral component can be determined using bony landmarks, such as the transepicondylar axis, the posterior condylar axis, or the anteroposterior (AP) axis (Whiteside’s line). Rotational alignment of the femoral component can be aligned perpendicular to the tibial shaft axis when the knee is flexed at 90 degrees. Stiehl et al.proposed a tibial shaft axis method based on the results of their previous anatomical study. The anatomical study showed that, when the knee moved from extension to flexion, a perpendicular relationship was maintained from the transepicondylar axis to the mechanical axis of the tibia. However, their study did not evaluate both the surgical and the clinical epicondylar axes. Therefore, questions remain regarding which of the two axes, the surgical epicondylar axis or the clinical epicondylar axis, should be used to align the femoral component. This study evaluated the three angles between the tibial mechanical axis, and the two different transepicondylar axes (surgical epicondylar axis and clinical epicondylar axis) and the AP axis at knee flexion using magnetic resonance imaging (MRI), and compared them to the angle between the mechanical axis of the tibial and the mechanical axis of the femur at knee extension. The question considered by this study was whether the surgical or clinical epicondylar axis or the AP axis would be more reliable as a rotational landmark for maintaining constant knee alignment when moving from extension to flexion. METHODS: Alignment in knee flexion: Thirty normal knees (mean age: 38.4 years) were evaluated using MRI. Coronal view of the tibia with the distal femur was obtained in 90 degrees of knee flexion. Sections through the midline of the tibia (Figure 1) and sections through the most prominent part of both femoral condyles (Figure 2) were used for measurement. Then two sections were superimposed and the angles between the mechanical axis of the tibia and the three bony landmarks in the distal femur (a line perpendicular to the clinical epicondylar axis, a line perpendicular to the surgical epicondylar axis and the AP axis) were measured. The clinical epicondylar axis was a line connecting the most prominent point of the medial and lateral epicondyles and surgical epicondylar axis was a line connecting the most prominent point of the lateral epicondyle and the medial sulcus of the medial epicondyle (Figure 2). The AP axis was a line connecting the deepest part of the patellar groove anteriorly and the center of the intercondylar notch posteriorly. Alignment in knee extension: The angle between the mechanical axis of the femur and the mechanical axis of the tibia was measured using full-length radiograph. RESULTS: Alignment in knee flexion: The surgical epicondylar axis, the clinical epicondylar axis, and a line perpendicular to the AP axis were externally rotated from the posterior condylar axis 1.9 degrees, 5.1 degrees, and 5.9 degrees, respectively (Table 1). 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 AP axis were 3.9 degrees of varus, 0.6 degrees of varus, and 0.2 degrees of valgus, respectively (Table 2). Alignment in knee extension: Mechanical axis of the femur was in 3.0 degrees of varus relative to the mechanical axis of the tibia (Table 2).