We have always been interested in how the OUKA has been performing in the national registers and how its revision rate compares with that of other UKA. We have, however, found this data difficult to interpret, as it is very contradictory. For example when the Swedish Knee Arthroplasty Register (SKAR) first reported the results of the OUKA in 1995 (Lewold et al, 1995), it was performing very badly with a revision rate about twice that of the Marmor. As SKAR thought they had identified a bad implant, they wrote to all the Swedish surgeons using the OUKA advising them to stop using it. Ten years later (Table 10.1), in 2005, the OUKA was the best performing UKA in SKAR. This clearly demonstrates that registers cannot reliably achieve their primary aim of identifying poorly performing implants.
Table 10.1 UKA tabulated in order of their revision rate, with the lowest at the top, in the Swedish (SKAR), Australian (AOANJRR) and New Zealand (NZJR) joint registries in 2005. The comparator or comparators are shown in italics. * indicates that the revision rate is significantly different from the comparators.
Table 10.1 shows the results of UKA performed in Australia and New Zealand as well as in Sweden in 2005. The OUKA was not only the best in Sweden but also in New Zealand. It was, however, significantly worse than the three comparators used in Australia (comparators selected because they were the best performing implants done in large numbers). More strikingly, the Repicci was the best in Australia and worst in Sweden. Clearly registers cannot reliably compare different implants. The reason for these apparently contradictory results is that the results of a joint replacement depend not only on the implant but also the indications and surgical technique.
Registers collect very little data about indications and technique so cannot adjust for these. Surgical experience is in some ways a surrogate for indications and technique. We were able to obtain data from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) to try and help explain why the comparator implants were doing significantly better than the OUKA. We found that the OUKA was being done in hospitals doing smaller numbers than the hospitals doing the comparators. When adjustments were made for this, the significant difference in results disappeared.
For completeness, although it is difficult to interpret, we present the outcomes reported in the most recent registry reports. SKAR does not report the revision rate of the OUKA, but it is not significantly different from their comparator (Swedish Knee Arthroplasty Register, 2013). They acknowledge that the results of the OUKA have improved with teaching. In the NJR the survival rate was 88% at 10 years. In ANJR the survival rate was 85% at 10 years (Australian Orthopaedic Association, 2013). They acknowledge that the high revision rate was due to surgeons implanting small numbers. In the Danish Knee Arthroplasty Register (DKAR) the five-year survival is 90% (Danish Knee Arthroplasty Register, 2010). The register acknowledges that survival is not a good endpoint with which to compare UKA and TKA. In the NZJR, the revision rate of the cemented OUKA is 1.37/100 component years (New Zealand Joint Registry, 2014). The cementless OUKA (see p. 206, Clinical results of cementless Oxford UKA) has the lowest revision rate of any UKA at 0.72/100 component years.
