On the anteroposterior projection, the femoral component usually has the appearance of a ‘bucket seat’ (Fig. 9.6), with the component internally rotated relative to the tibia. The explanation for this results from the obligatory ‘screw home’ mechanism of the knee in extension. The components are implanted in a neutral position relative to each other in flexion. The radiograph is taken in extension. With extension there is internal rotation of the femur (see Figs. 3.2 – 3.4) and thus of the femoral component relative to the tibial component. The femoral component should be parallel to the mechanical axis, but ±10° of varus/valgus positioning is acceptable (see Fig. 6.15).
Figure 9.6 See text for details
On the lateral projection, the fixation peg should be flexed to the long axis of the femur (Fig. 9.7). With the introduction of a two peg femur and the microplasty technique, we aim for the femoral component to be flexed by 10° (±10°) to the femoral axis.
Figure 9.7 Femoral component flexed relative to the femoral axis.
The bone/implant interfaces of the femoral component are not as readily seen as those of the tibial implant. The inner surface of the inferior facet of the femoral component is concave, so that interface is hidden by the metal. The only interface that is readily imaged is at the flat posterior facet. If the radiographic technique has resulted in a true lateral silhouette, that interface is visible. It should present a thin parallel-sided layer of cement, with shallow penetration of the dense femoral bone. The central peg of the implant should appear solidly cemented in the drill hole.
Meniscal bearing
On the anteroposterior projection, the mediolateral position of the bearing on the tibial plateau is deduced from the position of the radio-opaque markers. In the current bearings, there is a line across the front and two small balls at the back corners (Fig. 9.6). The bearing should lie with its lateral edge 2–3 mm away from the vertical wall of the tibial implant. Its position is explained by remembering that the bearing was positioned 1 mm from the wall with the knee flexed. The radiograph is taken with the knee extended, and extension causes the bearing to glide not only forward on the plateau but also medially, away from the wall (Chapter 6, Fig. 6.17).
