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The NJR Accountability and Transparency Model

NJR Accountability and Transparency Model

FAQs regarding hospital outlier status for revision

Could you explain what constitutes an outlier?

The definition we use is ‘Outcome data for a surgeon, unit or implant brand that falls outside of acceptable control limits.’ In other words, if we use revision rates as an example, the observed revision rate is statistically higher than what is expected.

Mr Peter Howard, Orthopaedic Surgeon and Chair of the Committee responsible for this area of work:

Peter Howard,
Hospital outlier FAQ

‘When a hospital is identified as an outlier, it triggers closer scrutiny and audit of that hospital’s performance. There is an agreed process to enable appropriate action to be taken to review units which fall below expected performance thresholds.

The National Joint Registry’s approach to monitoring hospital performance and outcomes ensures that hospitals are informed of their outcomes monitored in a clear and robust manner. This process is reassuring for patients because if there are any problems these will be identified and addressed.’

Why are some hospitals on the outlier list? And why are the actual figures for them not published?

These hospitals are on the list because they have been identified as outliers, so their standardised revision rate is above the control limits. They have been contacted and will already have implemented action plans to address their outcomes. The raw revision rate is less helpful as it is much fairer to adjust the revision rate according to patient physical status and indication for surgery, which is what we do when preparing these comparative charts.

We publish the revision ratio for all hospitals on our surgeon and hospital website here:
NJR Surgeon and Hospital Outcomes Website – The National Joint Registry (

Why do some hospitals have higher revision rates?

There will always be hospitals with higher revision rates than others because this is worked out using a comparative scale. This has to be viewed in the context that the UK has lower revision rates than most other countries. However, our aim is to help all units achieve optimal outcomes.

Mr Tim Wilton – Medical Director, NJR:

Tim Wilton NJRSC,
Hospital outlier FAQ

‘The purpose of the registry is to provide outcome data to hospitals and surgeons so that they can review
this and see how they can improve on performance. It is a continuous improvement programme and as a result of this work revision rates overall have halved in the last decade.

There are many potential reasons why a hospital may be found to have a higher revision rate than expected – these can include a younger patient age group, the type of implant used, infection and/or co-morbidities and the physical environment of the facilities – and the reason for drawing this matter to their attention is to encourage an urgent and thorough review of the hospital’s practice.

There is an agreed process to enable appropriate action to be taken to review units which fall below expected performance thresholds.’

Why are there consistently around ten hospitals on the published outliers list? Would it not be assumed that treatment/ops have improved over time and fewer hospitals should be on the outliers list?

This is the nature of comparative analysis. Essentially, we are identifying outliers as compared to the average performance of all other hospitals. A small number of units would appear outside the control limits by chance alone. If overall performance improves, then the average performance improves, as well as the control limits so if all hospitals improved by the same amount, the proportion of outliers would remain approximately the same. Outcomes are expected to vary between hospitals but this data and notification process facilitates continuous quality improvement. This therefore facilitates continual quality improvement.

Some of these hospitals might be on the list for some time – is there a reason for this?

Unlike acute medical conditions, outcomes for joint replacement surgery generally happen a long time after the initial surgery. We measure the revision rates at 5 years and 10 years. Any improvements that are implemented by hospitals after we first identify them as an outlier will consequently take some years to have any measurable effect.

It is therefore common for outlier hospitals to remain outliers for a number of years, until the impact on newly operated patients is seen.

Were there any changes during the pandemic with regard to revision rates?

COVID-19 has had a significant effect on the delivery of orthopaedic services – delivery of routine joint replacement surgery was stopped in many places for over a year. We saw a drop of around 50 per cent of pre-pandemic volume in hip and knee surgery and in the second year a recovery to about 70 to 85 per cent of pre-pandemic volumes. A higher proportion of revision surgery has been performed over those two years, over the number of primary operations, to alleviate pain for those needing it most or to address urgent issues.

The effect of this will be different in different hospitals and the full effects of this have yet to be seen, but it is certainly likely that the effects on some hospitals will result in increased revision rates as a statistical effect, rather than a clinical one. Some patient populations were relocated to be treated in alternative facilities such as at private hospitals, away from main hospitals with many operational COVID wards. This means that some hospitals will have accumulated a greater number of revision cases and more high-risk primary cases, without this being offset with more routine primary surgery.

Alert and Alarm Unit & Unit Process data analysis and risk adjustment

The following factors are used in calculating the rates of revision for both units and surgeons and for hip and knee:

Data is adjusted for age, sex and indication for original primary (osteoarthritis versus other indications and specifically for hip trauma).

Repeat analyses are done for various subgroups that may change but currently include:

  • Exclusion of withdrawn implants, three years after withdrawal
  • Primaries within five years only (i.e. within five-year interval prior to cut point for data analysis)
  • Subgroups of hips, namely cemented, uncemented and hybrid stemmed, non-metal-on-metal hips, all metal-on-metal hips and all resurfacings
  • Subgroups of knees, namely cemented, uncemented/hybrid, total, unicondylar and patellofemoral

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