Fig. 7

Projections of the cylindrical femur axis onto the tibial plateau for different flexion angles (extension: light grey, flexion: dark grey) for the different implants of a single knee specimen. In the native knee the characteristic posterior femoral rollback can be observed with increasing knee flexion. This rollback is much more pronounced laterally than medially. This movement is partially imitated by the cruciate ligament retaining (GCR) and posterior stabilized (GPS) primary knees. In both systems, however, a real rollback is not observed medially, but rather the lateral rollback pivots around a medial turning point. In the rotating hinge knee (RSL) and the total hinge knee (SSL) no femoral rollback can be measured, but the axis position remains constant through all flexion angles. Of note, the sagittal axis position is located more ventrally than in the primary conditions