The geometry of the tibial implant and the method of fluoroscopically controlled radiography that we have employed to image it, have provided abundant evidence of the radiographic appearances of the bone/cement/implant interface under that component. The radiolucent ‘line’ seen on a radiograph is the image of a thin layer of relatively lucent material that can be seen only if the X-ray beam is parallel to it (O’Connor et al, 1982). The degree of accuracy required cannot be regularly achieved without the use of screened alignment (Fig. 9.9).
Figure 9.9 Two fluoroscopic images of the same knee taken one after the other. In the lower film the X-ray beam was tilted 2°, and the complete radiolucent line beneath the medial part of the plateau has disappeared.
Cemented components
The almost ubiquitous appearance of radiolucency beneath the tibial components of the biocompartmental cemented Oxford Knee (Phase 1) when using the fluoroscopic technique was reported in 1984 (Tibrewal et al, 1984). A radiolucent line was observed under at least one of the tibial components in 77 of 80 knees (96%) in which radiodense cement had been used. Most of the radiolucencies were incomplete. The most common site was medial to the keel in the medial implants and lateral to the keel in the lateral implants. Radiolucency was also common around and under the keel. The radiolucent line was usually no more than 1 mm thick and none exceeded 3 mm.
The radiodense line
A striking feature of the radiographs was the presence of a thin radiodense line in the bone immediately adjacent to the radiolucencies. It was present in all but three of the 77 knees with radiolucent lines. It was also present, and more readily seen, in all the 11 knees in which the components had been fixed with radiolucent cement (precluding the demonstration of the radiolucent line, the presence of which was inferred). In all these knees, a thin bone shell completely surrounded both tibial components, and the bone trabeculae could be seen inserting into it, as they do into the normal subchondral bone plate.
Time of appearance
The radiolucent and the radiodense lines appear at the same time, usually between 6 and 12 months after the arthroplasty. Once developed, they do not progress.
Natural history
Although the study referred to above was of bicompartmental Phase 1 Oxford prostheses (and some prototype devices implanted before late 1978) (Tibrewal et al, 1984), our subsequent experience has confirmed nearly all the conclusions drawn from it. The incidence of radiolucencies in cemented unicompartmental replacement with the Phase 2 and 3 implants is somewhat lower (75%) than that mentioned above which referred to knees not compartments. No correlation has ever been found between clinical symptoms and the presence of radiolucency, and Röntgen stereometric analysis (RSA) studies have shown no association with the rate of subsidence of the tibial component (Kendrick et al, 2015).
In a study of 26 knees examined radiographically 1 year and 10+ years after OUKA (Phases 1 and 2), 21 had partial or complete radiolucent lines around the tibial implant Fig. 9.10) (Weale et al, 1999).
Figure 9.10 Physiological radiolucent and radiodense lines at (a) 1 year and (b) 10 years after implantation.
All but one were ≤1 mm thick. Only two had progressed between the early and late reviews, and only one of these was 2 mm thick.
Berger et al. (1999) reported an incidence of 49% partial or complete radiolucency around the tibial component of the Miller–Galante unicompartmental implant at 3–7 years review. None of them progressed after the third year and there was no instance of loosening of a component. The radiographs were not screened for alignment, which may explain the lower incidence. An RCT (56 knees in 48 patients) looking at fixed (Miller-Galante) versus mobile UKA (OUKA) showed a significantly lower incidence of radiolucent lines at the bone/implant interface with the mobile as compared to fixed UKA (8% vs. 37% respectively, p<0.05) at two-year follow up (Li et al, 2006).
Significance
We conclude that radiolucent lines around the tibial component of the cemented OUKA are the rule not the exception, and that the radiographic technique mainly determines how frequently they are observed. They are probably as common around the femoral component, but are more difficult to demonstrate there. They do not appear to be the cause of symptoms nor evidence of loosening of the component. Therefore, we refer to the radiographic appearances described above as ‘physiological radiolucency’. They can usually be distinguished from the pathological lucency that surrounds an infected or a loose component by thickness and the presence of the radiodense line. The physiological lucent line is almost always <2 mm thick and defined by a thin radiodense bone plate; the pathological lesion is thicker, and the margins of the radiolucent zone are characteristically ill defined.