Which implants have been identified as ‘outliers’ by the NJR over the years and which of these are still being used?

Peter Howard and Tim Wilton

Peter Howard is an Orthopaedic Consultant in Derbyshire. He is Past President of the BHS and Chair of Implant Scrutiny Committee for the NJR.

Tim Wilton is a Consultant Orthopaedic Surgeon at Department of Orthopaedics, Royal Derby Hospital, Derby, with specialist interests in knee and hip replacement surgery. He is Past President of the BOA and BASK and is Medical Director of the NJR.

The NJR reports on the outcomes of implants used in primary arthroplasty and identifies any with (statistically) significantly higher than expected revision rates.

This article is intended to inform surgeons about implants that have been seen at outlier status, and particularly those still in use. By ‘outlier’ status we mean that the device has twice the expected revision rate at that particular time as compared to the average implant of its class. The lower limit of the relevant confidence interval has to pass this threshold before the implant is deemed an ‘outlier’ in which case we can reasonably say that the implant is ‘at least’ twice as likely as expected to require revision.

There are currently 15 knee replacement implants that have been notified to the MHRA as having outlier status when used in primary cases. These all appear in the Annual Report 2022 Table 4.4 and of these we can see that eight named implants have not been used in the NJR for several years. The other seven implants are still in use and we draw the attention of all surgeons to these implants.

It will be seen that many of these have only been shown to have outlier status when they are used without primary resurfacing of the patella. The higher revision rate with these implants could therefore reasonably be expected to be partially or, in some cases, completely resolved if the patella is routinely resurfaced at the time of the primary surgery. This applies to the Journey 2 BCS, Genesis 2 Oxinium and RHK prostheses, although the latter appears

no longer to be being used. In the case of the Journey 2 BCS the company has written to surgeons suggesting that they carefully consider resurfacing the patella as the results do seem to be so significantly affected by whether the patella is resurfaced or not. We thoroughly endorse the suggestion that this particular implant has very much higher revision rates when the patella is not resurfaced and suggest it would seem unwise to disregard that advice.

In contrast, the E-motion bicondylar knee has only been shown to have outlier status when the patella is resurfaced at the primary operation.

The Endo-model modular rotating hinge and the Smiles (METS hinged/linked) knee are likely to be used in different cases to standard bicondylar knees and outlier status may largely reflect the complexity of the cases in which these are used. Surgeons should still be aware of this higher failure rate and therefore should consider whether they really need to use these in a primary case unless there are clear indications that such a device is the best implant choice.

It might be that the ACS Knee is also effectively discontinued: it has been used on only three occasions in the past three years. The knee has only been used in relatively small numbers but the high revision rate applies equally to the overall figures for the implant and to the one higher volume user, so perhaps it is reasonable to say that it should no longer be used.

The Genesis 2 Oxinium PS knee is possibly dealt with somewhat harshly in this analysis, in that the patients for whom this implant is chosen tend to be much younger and possibly more active. The relevant analysis is not adjusted for age, gender or indication for surgery, unlike some other analyses which are performed. The results by brand in the Annual Report show that those having an Oxinium knee are, on average, twelve years younger than those having a CoCr knee of the same design. A special NJR sub-group analysis was commissioned by the company to investigate this and it was clear that much of the difference between the Genesis 2 Oxinium PS and the Genesis 2 CoCr PS revision rates were no longer seen after adjustment for these parameters.

All of the above have been reported on the basis of the PTIR method, which relies on the performance of an implant over the whole of its period of usage. Implants in the NJR may be subjected to closer scrutiny under certain conditions, such as when reports are received from surgeons concerned about the performance of certain variants, or when a device seems to have a very specific mode of failure. Kaplan- Meier analysis of revision rate is then performed, using the average for the implant group as the ‘expected’ value, and if necessary followed up with other statistical tests.

If a variant is found to be significantly outside the expected range, then this is also reported to the company and the MHRA. As a consequence

of this type of analysis the Committee has reported two specific variants in the NexGen total knee replacement brand. These are two combinations of the ‘Option’ stemmed cemented (non-precoat) tibial implant specifically when used in conjunction with the posterior stabilised Flex femoral components (Flex and Flex GSF). These have borderline outlier status but have several times the expected rate of revision for aseptic loosening of the tibia. The same tibial component has good revision rates with many of the other femoral components and in particular with CR variants.

In hips we report on hip stems, cups, and combinations thereof. These all appear in the Annual Report 2022, Tables 4.1 – 4.3. Of the 13 hip stems notified to the MHRA, just two remain in use. There have been 11 cups reported of which three are still in use, and 34 stem-cup combinations of which five remain in use. 34 of the reported variants or combinations involved significant proportions of metal-on-metal bearings, and all these are now discontinued.

Of those implants still in use where MoM was not a contributor, the SPII stem is no longer an outlier, and the Aura II cementless is a new notification in 2022. The Delta One TT and the Trabecular Metal Revision Shell are both implants most likely to be used in complex primary cases, but surgeons should again be aware of this higher failure rate and therefore should consider whether it really is necessary to use these in a primary case and whether

there are clear indications that such a device is the best implant choice. The Bimentum dual mobility is again an implant that would be expected to have specific and somewhat different indications to most THRs, but there is a trend for increasing popularity of these bearing construct types. Surgeons therefore need to consider carefully whether they are using these special devices because there is a specific indication in a given case, or whether they are using them ‘in case’ of a complication, such as dislocation, whether or not it may be a problematic complication in their own practice.

The ACE dual mobility cup is newly reported this year. It is recently introduced and has relatively low numbers and the outlier status could therefore be due to the relatively high impact of small numbers of revisions. Nevertheless, it is included in the outlier list as it meets the statistical criteria in spite of the wide confidence intervals observed.

All of the stem-cup combinations reported in table 4.3 that are still in use involve either a stem and/or cup that has also been reported to the MHRA.

If surgeons have concerns over the performance of these or other implants that they have used or may be thinking of using they should contact the authors as there is detailed information available about all implants used in the NJR – Tim Wilton: Timothy.wilton@njr.org.uk. •