All national registers have found that the revision rate of UKA is about three times that of TKA. As a result, it is generally concluded that UKA have more poor results than TKA and therefore that UKA should not be used. This conclusion is probably not justified. There are many reasons why the revision rate of UKA is higher than that of TKA. Perhaps the most important is that the threshold for revision of UKA is much lower than that of TKA and therefore the higher revision rate does not necessarily suggest that UKA have worse outcomes than TKA.
Figure 10.1 shows a graph based on presented data from the Trent Regional Arthroplasty Register comparing the long term outcome of seven different total knee arthroplasties (Esler et al, 2008). As would be expected, most of the knee replacements have a survival of 90 to 95% at 15 years. However, there is a single implant with a survival rate of 100% at 15 years. This implant appears to be so much better than the other knee replacements that all surgeons should use it. However, this implant, the Sheehan knee (Fig. 10.2), is a hinged knee replacement with long stems which is no longer available because of its poor performance. Due to the size of the implant and the damage it caused when it failed, revisions were very difficult. Therefore, surgeons would try to avoid revising it even if it was loose and was causing the patient significant symptoms.
This suggests that for different types of implant there are different thresholds for revision and that these thresholds have a profound effect on the revision rate of the implant. This effect can be so large that comparison of revision rates between implants may lead to misleading conclusions.
Figure 10.1 Graph showing survival data of seven TKAs from the Trent Regional Arthroplasty Register (Esler et al, 2008).
Figure 10.2 Sheehan Total Knee Replacement (G F McCoy, N W McLeod and J R Nixon, Experience with the Sheehan knee replacement. Ulster Med J 1983; 52(1): 35-39).
There is evidence to suggest that the threshold for revision influences the comparison between UKA and TKA. The New Zealand Joint Registry (NZJR), as well as collecting data about revision, also collects Oxford Knee Scores (OKS) six months after the operation. The OKS is subcategorised into poor, fair, good and excellent (Kalairajah et al, 2005; Goodfellow et al, 2010) (Fig. 10.3). Data from the NZJR demonstrates that UKA not only have more excellent results but also fewer poor results than TKA. Therefore, the high revision rate of UKA is not because UKA have more poor results.
Figure 10.3 Proportion of UKR and TKR achieving four classes of OKS outcome.
Figure 10.4 Two year revision rates for UKA and TKA having different PROMS results at six months post-surgery (Goodfellow et al, 2010).
The NZJR also compares the six month OKS with the subsequent revision rate (NJR, 2010). We have used data derived from the NZJR to draw the graph shown in Figure 10.4 (Goodfellow et al, 2010). The graph demonstrates that, for each outcome score, the revision rate of UKA is about five times higher than that of TKA. This suggests that factors independent of outcome score increase the revision rate by five times. The most important factor is likely to be a different threshold for revision.
The most striking difference in revision rate occurs in patients who are likely to have a worse score postoperatively than preoperatively (OKS less than 20). These patients have a 10% chance of being revised if they have had a TKA and a 60% chance of being revised if they have had a UKA. This is not surprising because the revision of a UKA is usually a simple conversion to a primary TKA and the outcome of this is generally expected to be good. In contrast, a revision of a TKA is often complex, requiring the use of stems, wedges and stabilised implants and the outcome of this type of revision is known to be unpredictable. We therefore conclude that the higher revision rate of UKA is not because they have more poor results but because they have a lower threshold for revision.
Most surgeons would agree that it is an advantage of UKA over TKA that UKA is relatively straightforward to revise if there is a problem. The consequence of it being easy to revise is that the threshold for revision is lower and therefore the revision rate is higher. The higher revision rate of UKA should thus not be considered to be a problem because it is a manifestation of an advantage.
