On the anteroposterior projection, the vertical cut should be just medial to the apex of the medial spine. The component should appear approximately perpendicular (±5°) to the tibial axis (Fig. 9.3).
Figure 9.3 See text for details
The medial margin should always reach to the medial tibial cortex and may overhang a little (no more than 2 mm as greater overhang may cause soft tissue irritation). The bone/implant interface should show a complete cement layer with a few millimetres penetration into the bone. (Cement penetration is deeper laterally and around the keel than medially, where the subchondral bone is less porous.) If the vertical saw cuts have been made too deep (increasing the risk of fracture), they may be outlined by opaque cement or show up as vertical lucent lines. (The horizontal saw cut may undermine the tibial eminence laterally but this has no serious consequences.)
With cementless implants, often a narrow radiolucency may be present on the immediate post-operative radiograph. It is typically partial and suggests incomplete seating of the tibial component. This is of no consequence as, with the passage of time, the component will settle and the radiolucency will disappear.
Figure 9.4 Post-operative radiograph (a) of a cementless tibial component showing a radiolucent zone at the interface which had disappeared by the time the one-year radiograph (b) was obtained.
On the lateral projection, the tibial component should slope downwards and backwards at about 7° (±5°) to the tibial axis (Fig. 9.5). The posterior edge of the component should reach to the posterior cortex but should not overhang. Overhang implies that the posterior cortex was damaged when the groove for the keel was excavated. Extra care must be taken whilst preparing the keel slot in female patients with small bones.
