Posterior Cortical Axis: A New Landmark to Control Femoral Component Rotation in Total Knee Arthroplasty.

Rotation errors of the femoral component are held responsible for occurrences such as instability in flexion and midflexion, patellar maltracking, and arthrofibrosis following total knee arthroplasty. However, in many cases, the epicondylar axis cannot be reliably identified due to bone defects or metal artifacts on computed tomography, so alternative landmarks are necessary to evaluate the femoral component rotation. The current study sought to determine the relationship of the posterior cortical bone and the anterior cortical bone in relation to the epicondylar axis. In this retrospective study, 398 consecutive patients who had undergone magnetic resonance imaging of the knee joint were included. The average angle between the posterior cortical bone and the epicondylar axis was 7.3°±3.3°. When the posterior cortical bone was used as the reference, the average absolute error was 2.6°±2.1°. In comparison, the average angle between the anterior cortical bone and epicondylar axis was 10.4°±4.5°. When this reference was used, the average absolute error was 3.6°±2.8°. The posterior cortical bone is a more consistent landmark than the anterior cortical bone is for intra- or postoperative approximation of the epicondylar axis. This appears to be due to the flat geometry of the posterior cortical bone compared with the elliptical form of the anterior cortical bone of the distal femur. In practice, an external rotation of the femoral component of 7° in relation to the posterior cortical bone is to be recommended. [Orthopedics. 2017; 40(3):188-190.].