Three-Dimensional Morphology of the Distal Part of the Femur Viewed in Virtual Reality

The morphologic shape of the distal part of the femur dictates the shape, orientation, and kinematics of prosthetic total knee replacement. Traditional prosthetic designs incorporate symmetric femoral condyles with a centered trochlear groove. Traditional surgical techniques center the femoral component to the distal part of the femur and position it relative to various bone landmarks. However, failure patterns documented in retrieval studies1,2, case series3, and kinematic studies demonstrate how traditional designs and surgical techniques reflect a poor understanding of distal femoral morphology and knee kinematics. It has been shown that the flexion/extension axis of the knee is fixed within the femur and that the articular surfaces of the condyles are circular in profile4,5. Ligament length patterns are significantly altered by abnormal axial alignment when a hinged knee brace is used6. It is expected that a malaligned femoral component would have the same effect in total knee arthroplasty. The purpose of this report is to demonstrate, with conventional images and with interactive animations in virtual reality, the three-dimensional shape of the naturally asymmetric distal part of the femur, with illustration of the sulcus axis of the trochlear groove and the flexion/extension axis of the condyles relative to conventional axes (mechanical, anatomic, epicondylar, and posterior condylar). Correlations between the morphologically determined rotation axes and experimentally determined kinematic axes are illustrated. Eighty-five mummified cadaveric knees were measured with a stereotactic micrometer (Fig. 1)7. The location and orientation of the sulcus were obtained by repeated horizontal passes of the stereotactic stylus over the distal part of the femur, beginning at the top of the articular surface and progressing down to the intercondylar notch (Fig. 2). With each horizontal pass, the lowest depression of the trochlea (sulcus) was identified by …