Good postoperative radiographs are necessary as a baseline for comparison with later films and to allow ‘quality control’ of the surgical technique.
For these purposes, the standard methods of aligning the X-ray beam are neither sufficiently accurate, nor repeatable enough. To assess the positions of the two metal components, the X-ray beam must be centred on one component and aligned with it in two planes. The resulting projection of the other component can then be used to deduce their relative positions. We therefore suggest that fluoroscopically aligned radiographs are taken (screened X-rays). If this is not possible, reasonably good radiographs can be obtained using a digital system. Low dose images are used to adjust the position. When good alignment is achieved, a standard image is obtained.
In the anteroposterior projection, the patient lies supine on the X-ray table and the leg and the X-ray beam are manipulated under fluoroscopic control until the tibial component appears exactly end-on in silhouette. The radiograph is then taken (Fig. 9.1).
In this projection, the alignment of the beam with the flat orthogonal surfaces (horizontal tray and vertical lateral wall and keel) allows great accuracy and reproducibility.
Figure 9.1 Fluoroscopically centred anteroposterior radiograph.
In the lateral projection, the patient lies supine on the couch with the knee flexed 20°–30°. The fluoroscope is rotated through 90° so that the X-ray beam is parallel to the floor and centred on the femoral component (Fig. 9.2). (The tibial implant is not so useful in this projection because it offers no vertical surface and its horizontal surface is obscured by its lateral wall.) Therefore, the lateral projection is not as precise or as reproducible as the anteroposterior projection.
Figure 9.2 Fluoroscopically centred lateral radiograph.
Radiographs taken in this way can be repeated at any time interval in the knowledge that (at least in the anteroposterior films) the projections of the tibial component are always the same. Therefore, small changes in the relationships of the components to one another and to the bones can be detected. Furthermore, because the X-ray beam is parallel to the tibial plateau, the state of its bone/implant interface is always reliably imaged. Without properly aligned postoperative films for comparison, later radiographs are difficult, or impossible, to interpret. Subsequent radiographs should be undertaken using the same technique to allow accurate comparison.
The postoperative radiographs can be used to measure the accuracy with which the prosthesis has been implanted. As the femoral component is spherical and the bearing spherically concave, the orientation of the components are not critical. Our studies suggest that 10° of mal-orientation of the femoral component and 5° mal-orientation of the tibial component do not compromise the outcome (Gulati et al, 2009). However, the positioning of the components is important (as is the way the soft tissues are balanced but this cannot be assessed radiographically).
