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Registry Data for Medially Stabilised Knees

Continued monitoring of registry data is an essential component of the MatOrtho® program for safe introduction of new technology. So it was reassuring to hear at this year’s ‘Forever Active Forum’ that revision rates for medially stabilised knees are lower than the familiar CR and PS type knees. Moreover that the MRK™ and SAIPH® Knees have the lowest revision rates of all medially stabilised knees!

Design Performance

Kelly Vince introduced a great session titled Design Performance at this year’s Forever Active Forum with an insightful perspective on data that matters1. With sufficient time the multicentre ‘big’ data from national registries, for example, can differentiate devices and device types and their strength is such that many surgeons change their practice according to its findings.

The session included comparison between posterior stabilised (PS) and cruciate retaining (CR) knees, presented by David Campbell and Roger Brighton. Despite confounders associated with brand differences and variation in patella resurfacing, CR knees tend to do better in the registries. David Campbell also found that the data is yet to show a universal advantage of XLPE.

The registry data for medially stabilised knees provided the highlight of the session with David Wood’s review and the panel discussion. Headline data came from the latest AOA NJRR report2 – the first registry report in the world to analyse medially stabilised knees as a distinct category.

Use of Medially Stabilised Knees

Dr Wood observed a trend in the declining use of PS and CR knees against a rise in use of medially stabilised knees since 2012 (the year that the SAIPH® became available). By the end of 2016 medially stabilised knees accounted for 7% of the Australian TKRs.

Figure 1. Primary TKR by stability (primary diagnosis OA), Figure KT17 in the NJRR2.

Given the recent rise in use, Australian registry data for medially stabilised knees is not presented for as many years as CR and PS knees. However, medially stabilised knees are not new. Finsbury (now MatOrtho®) introduced the first medially stabilised knee in 1994. The FS1000 was so-called because it had evolved from the Freeman-Samuelson Knee and its large ball-and-socket articulation provided a 1,000mm2 bearing contact area. Although large-area/low-stress surface contact remains important for longevity, the device was renamed the Medial Rotation Knee™ (MRK™) in 1998 to better describe its characteristic motion. This coincided with the availability of data from a research group led by Prof MAR Freeman which confirmed that the natural knee was medially stabilised.

Today there are 5 medially stabilised knee designs used around the world: the MRK™ and SAIPH® (MatOrtho) the Advance and Evolution (Wright/Microport) and GMK Sphere (Medacta).

National Joint Registry (NJR)

Medially stabilised knees have been a regular feature of the NJR, despite not being recognised as a separate category. Twenty-thousand medially stabilised knees have been recorded since the NJR started collecting data in 20033. The majority of those procedures used the MRK™.

The MRK™ is consistently reported with the lowest revision rate of all knees – including the most recent NJR report which shows the MRK™ has a 13-year probability of revision of just 2.88%3! It is half as likely to be revised within 13 years compared to the Advance Medial Pivot, the other medially stabilised knee with substantial use in the UK3. The MRK™ is also less likely to be revised than the PFC, which has the largest dataset3.

Figure 2 Cumulative percent probability of revision for time elapsed since surgery3.

Figure 2. Cumulative percent probability of revision for time elapsed since surgery3. The red circle indicates the 13-year time point for the MRK™.

AOA National Joint Replacement Registry (NJRR)

In the 2017 Annual Report, the AOA NJRR included comparisons of cumulative revision rates (CRR) between categories of stability and between devices within the medially stabilised category.

Comparing brands of medially stabilised knees, the SAIPH® has the lowest CRR of all medially stabilised knees, albeit at 3 years, and David Wood suggested delegates ‘watch this space’ (Figure 3, green box)!

Although it has the longest dataset of the medially stabilised knees in the NJRR (Figure 3, red box), David Wood noted that the Advance Medial Pivot was not often used in Australia. The Advance I dataset only includes 15 procedures and it had design and/or technique characteristics that led to a high proportion of failures due to tibial loosening4. Given that loosening remains a major reason for revision of all TKR designs, suitable technique and fixation design was an important design consideration5 for both the MRK™ and SAIPH®.

Figure 3Cumulative percent revision of Medially Stabilised Primary TKR (NJRR table KT18)2.

Figure 3. Cumulative percent revision of Medially Stabilised Primary TKR (NJRR table KT18)2.

An interesting observation from Roger Brighton was that the AOA NJRR had included the MRK™ within the CR category and that at 5 years it was one of the devices with the lowest CRR of all!

Figure 5 Cumulative percent revision of Medially Stabilised Primary TKR (NJRR table KT18)2

Figure 5. Cumulative percent revision of Medially Stabilised Primary TKR (NJRR table KT18)2    

When the flawed Advance design is removed from the dataset, Medially Stabilised knees have the lowest CRR of all categories.

Figure 4 Cumulative percent revision of primary TKR by stability (primary diagnosi OA, excluding Advance)

Figure 4. Cumulative percent revision of primary TKR by stability (primary diagnosis OA, excluding Advance) (NJRR table KT19)2.

Conclusions

Overall the results for medially stabilsed knees, particularly the two devices from the concept originators, are extremely encouraging. To date the SAIPH® knee safe introduction program has focussed on functional data and data from inventor and independent multicentre studies. However, the long term monitoring of the registry data is off to an exceptional start. The results are not only good in comparison to existing TKR designs, they are at the top of the tables!

  1. Vince K, Campbell D, Brighton R and Wood D. Design Performance. Session 9 of the Forever Active Forum, Melbourne, 2018.
  2. Australian Orthopaedic Association. NJRR Hip and Knee Arthroplasty, Annual Report 2017: https://aoanjrr.sahmri.com
  3. National Joint Registry. NJR 14th Annual Report, 2017: www.njrreports.org.uk
  4. Australian Orthopaedic Association. NJRR Hip and Knee Arthroplasty, Annual Report 2017: Prosthesis Investigations. https://aoanjrr.sahmri.com/annual-reports-2017
  5. Mannan and Scott. The Medial Rotation total knee replacement: a clinical and radiological review at a mean follow-up of six years. JBJS-Br; 2009; 91(6): 750-756.

 

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