The mediolateral position of the femoral component controls the mediolateral position of the bearing on the tibia. The main 6 mm hole for the peg of the femoral component should be placed centrally in the femoral condyle (Fig. 6.11(c)). If the femoral component is positioned in this manner and the vertical tibial cut is made just medial to the apex of the spine, the bearing should be close to, but not jammed against, the wall. A common mistake is for the vertical saw to slide down the tibial spine and the cut is made too far medially. If this is the case, the bearing can jam against the vertical wall. To prevent this, a trial reduction of the tibial template and the trial femoral component with a trial bearing should be made, prior to preparing the slot for the keel of the tibial component. If the bearing is found to jam against the wall, the vertical saw cut should be moved 2 mm further laterally. Occasionally, due to the femoral component being sited too far medial relative to the tibial component, the bearing is some distance from the wall. The risk here is that the bearing would rotate and dislocate. This is now unlikely to occur because of the asymmetric design of the bearing and therefore should be accepted. The only exception is if the vertical tibial cut has in error been made lateral to the medial spine in which case the tibial component should be moved medially a millimetre or two.
Rotation and ‘attitude’
The articular surface of the femoral component is a segment of a sphere (with its centre at the tip of its peg). The position of a complete spherical surface does not alter when it rotates around its centre and so, in some sense, the rotational alignment of the femoral implant is an irrelevance. Figure 6.15 shows, for instance, that rotational alignment of the femoral component about the axis of its peg does not alter the position of the bearing in the flexed knee. Nevertheless, because the femoral component is an incomplete sphere, its ‘attitude’ is altered by rotation in any of the three planes. The attitude of the component affects (1) the availability of its limited articular surfaces and (2) its appearance on radiographs. However, because the contact area is so large (about 6 cm2), some malalignment of the component is unlikely to compromise the contact area. We have found that ±10° malalignment does not compromise the outcome (Goodfellow & O’Connor 1992).
Figure 6.15 Femoral component attitude: rotation about the axis of its peg (knee flexed to 110°). We consider angular alignment up to 10° acceptable.
The alignment of the femoral component is controlled by the femoral drill guide, which is linked to an intramedullary rod (see Fig. 7.15). The linkage is such that the femoral component is implanted about 10° flexed and 7° varus relative to the diaphyseal axis. It is therefore flexed about 10° and in neutral varus/valgus relative to the mechanical axis. The linkage does not control the mediolateral position of the femoral component. This has to be selected manually so that the drill hole is central on the condyle.
The femoral component is flexed 10° relative to the bone for a number of reasons. It increases the contact area between the femoral component and the bearing in high flexion (Fig. 6.16). It also facilitates accurate balancing of the knee. In order to assess ligament tension without the influence of the posterior capsule, the extension gap is measured at 20°. Ultimately the flexion gap should be measured 90° flexed relative to this at 110°. If adjustment of the extension gap is not to influence the flexion gap, then the femoral peg, along the direction of which the extension gap is adjusted, should be parallel to the tibial component in about 110° flexion. If the tibial component is flexed 7° relative to the tibial axis, this is best achieved by flexing the femoral component about 10°.
Figure 6.16 Radiographs showing increased contact area in flexion with a flexed femoral component.
The reason why the femoral component is at 7° varus relative to the diaphyseal axis is that its peg is then approximately parallel to the mechanical axis. The intramedullary rod is very thin so therefore does not accurately control orientation, indeed errors of 3 – 5° are commonly seen. However, due to the spherical nature of the femoral component, these are of no consequence (Fig. 6.15). This would not be true were the surface of the femoral component cylindrical.