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New Zealand National Joint Registry Eleven Years Data Jan 1999 – 2009

The total number of registered joint arthroplasties at 31.12.2009 was 132510 which had been performed on 99,104 individual patients of which 11409 (11.5%) died during the 11 year period. The number of observed component years contained within the Registry has now reached well over 500,000 years.

The increase of 15,885 registered joints for 2009 compared to the 15,311 in 2008 represents a overall annual gain of 3.7% which is significant when compared to the 0.38% increase for 2008.There were increased registrations for all arthroplasty categories when compared to 2008 registrations, except for elbows which fell by 15%.The biggest increase was 16% for unicompartmental knees which reversed the trend of the previous two years.

There are now approximately 63,000 hip arthroplasties in the registry with an overall revision rate of 0.66 per 100 observed component-years (ocys) with a 10-year prosthesis survival of 93.1%.

The annual percentage of uncemented hip arthroplasties continues to rise and in 2009 reached almost 52%. This rise is at the expense of fully cemented hips which last year fell to 14% of total compared to 56% in 1999.

Hybrid arthroplasty remains relatively static at 34%. As in previous years when the 3 types of hip fixation are analysed against the four age bands: under 55 years, 55-64 years, 65-74 years, and greater than 75 years, it shows that the uncemented arthroplasty has a significantly higher revision rate (p<0.05) in all except the under 55 age band.

The data also shows that overall the hybrid hip has the lowest revision rate across the 4 age bands. However, the KM curves for the 3 types of arthroplasty continue to converge and at ten years prosthesis survival is 93.19%, 93.51% and 92.94% respectively for cemented, uncemented and hybrid hips. If this trend continues uncemented hips may demonstrate lower revision rates over the next 5-10 years.

Revision rates for individual hip component combinations as well as for individual components for which there are a minimum of 250 primary procedures have been calculated.

The Corail/Pinnacle, Twinsys uncem /Selexys, Spectron/ Duraloc and Elite plus/Duraloc have revision rates significantly higher (p<0.05) than the overall rate of 0.66/100 ocys. The first two combinations were among the top ten for 2009 and should therefore be flagged. Ten of the 32 Corail/ Pinnacle revisions had had the primary procedure at the same hospital and when these are deleted the revision rate is no longer significant.

The ASR cup is one component with a significantly higher revision rate
that has also been noted in other Registries and has now been withdrawn from the market . However, the New Zealand revision rate is not as high as has been reported by others.

Overall the hip revision rate noted above and the ten year prosthesis survival of 93.10% are among the best for similar joint registries around the world. A similar situation applies to knee prostheses with the overall revision rate 0.53/100 ocys, (95% confidence interval; 0.50, 0.56) and the ten year survival of 95.63% again among the best for international Joint Registries.

The revision rates for the various bearing surfaces used in primary hip arthroplasty i.e. metal on plastic, metal on metal, ceramic on plastic, ceramic on metal, ceramic on ceramic have been further analysed this year with respect to head size and acetabular type. For head sizes =< 28mm the ceramic on ceramic articulation had a significantly higher revision rate and for head sizes >28mm the metal on metal articulation had a significantly higher revision rate. Overall the metal on plastic articulation has a significantly lower revision rate than the other combinations.

Although uncemented knee arthroplasty represents just 4.5% of all primary knee arthroplasties it has a significantly higher revision rate (P<0.05) than either fully cemented or hybrid in which the tibial component is cemented and the
femoral component uncemented. Analyses have confirmed that it is the loosening of the uncemented tibial component that is mainly responsible for the increased revision rate.

With regard to unicompartmental knee arthroplasty the main feature for 2009 was the doubling of the number of implanted uncemented Oxford prostheses which also topped the prosthesis usage list. The minimally invasive approach for the uni-compartmental knee arthroplasty remains popular and in 2009 was again used in 37% of procedures. Despite the oxinium uni being reported as having a very high revision rate in previous reports 3 further ones were implanted during 2009. Nine out of 33 have been revised.

Once again we have compared the deep infection revision rates within six months of the primary procedure for primary hip and knee arthroplasty against theatre environment. Six months has been chosen as infection within this time period is highly likely to have been introduced at the time of surgery. This year’s analyses again demonstrate that for primary hip and knee arthroplasty there was 3 times the risk for revision for deep infection when the primary procedure was carried out in a laminar flow theatre compared to a conventional theatre. The use of space suits also significantly increases the risk of revision for deep infection in both conventional and laminar flow theatres. As noted in last year’s editorial an in depth investigation of these findings was being undertaken and a paper has been
accepted for publication in the British Journal of Bone and Joint Surgery.