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Partial knee replacement surgery improves pain and function similarly to total knee replacement in people with osteoarthritis that affects only a single compartment of the knee. Partial knee replacement surgery is also cheaper.

In this NIHR-funded trial of 528 people with osteoarthritis affecting only one compartment of the knee, those who had partial knee replacement saw at least as much improvement as those who had a total joint replacement. Their care also cost about £900 less over five years, making partial knee replacement more cost-effective.

Partial knee replacement did not lead to a greater need for further surgery, a surprise finding which contrasted with previous evidence. However, the need for further surgery is uncommon and the study may not have been large enough to capture data on this.

Longer-term follow up of the trial is underway. However, the current results suggest that partial knee replacement could be offered more often to eligible patients as a first-line option, assuming that surgeons experienced with the technique are available.


Why was this study needed?

Knee replacement is a common operation to treat severe knee osteoarthritis that has not been resolved by other treatments. More than 300,000 knee replacements were carried out in the UK between 2015 and 2017.

Some people have damage to the knee joint on only one side (uni-compartment osteoarthritis) which means they could consider either a partial or total knee replacement. There has been insufficient evidence about which operation works best for these people.

At present, fewer than 9% of knee replacements are partial. However, a recent study of registry data from England suggested that partial knee replacement could be more cost-effective than total knee replacement.

The current trial was intended to fill the gap in the evidence and inform practice.


What did this study do?

TOPKAT (Total Or Partial Knee Arthroplasty Trial) was a randomised controlled trial carried out at 27 sites across the UK, involving 68 surgeons and 528 patients.

The sites recruited people who were being considered for knee replacement, who had osteoarthritis of the medial compartment of the knee. This meant they would be suitable for either partial or total knee replacement. People were randomly assigned to one or other operation.

Amongst the 528 people randomised, 44 people had a knee replacement using the technique they had not been assigned to. This was either because of patient choice or surgeon decision once surgery was underway. For example, partial knee replacement was not possible if the arthritis was more widespread than expected.

Participants were followed up for five years and checked annually.

The results should be relevant to UK hospitals, assuming they have surgeons with sufficient expertise in partial knee replacement.


What did it find?

  • Both groups of patients had much-improved knee pain and function, assessed by the 48-point Oxford Knee Score. After five years, people who had total knee replacement had an 18 point improvement and people who had partial knee replacement had a 19 point improvement. A 5-point difference is considered clinically significant, so the two procedures were similar for this outcome.
  • The study’s cost-effectiveness analysis found that partial knee replacement was more effective in terms of quality of life, resulting in 0.24 additional quality-adjusted life-years (QALYs) over five years. It was also less expensive, with care costing £910 less over the five years of follow-up.
  • Average hospital stay was longer for those who underwent total knee replacement (4.3 days) than those who had partial knee replacement (3.2 days).
  • The proportion of people who had a re-operation was similar in both groups. Re-operation rates were 6% among the partial knee replacement group and 8% among the total knee replacement. In both groups, 4% of people needed a revision of their knee replacement.


What does current guidance say on this issue?

The guideline on osteoarthritis published by NICE in 2014 includes recommendations on referral for consideration of joint replacement. However, it does not include guidance on which type of joint replacement device or technique is recommended. This guideline is being updated, with the update due to be published in August 2021.

In addition, a NICE guideline on primary joint replacement of the hip, knee or shoulder is in development and is expected to be published in March 2020.


What are the implications?

The results of the study imply that partial knee replacement can be offered with confidence for people with single compartment disease considering knee replacement. Offering partial knee replacement as a first choice may be better value for the NHS as it reduces costs, mainly because of shorter hospital stays.

Questions remain about the revision and re-operation rate for partial knee replacement over the longer term, and results from the planned 10-year follow-up of this trial will be of interest.

Surgeons would need to be fully trained and experienced in partial knee replacement in order to be able to replicate the results of this study.


Citation and Funding

Beard D, Davies L, Cook J, et al. The clinical and cost-effectiveness of total versus partial knee replacement in patients with medial compartment osteoarthritis (TOPKAT): 5-year outcomes of a randomised controlled trial. Lancet 2019; 394(10200):746-56.

The study was funded by the NIHR Health Technology Assessment Programme (project number 08/14/08).



Evans J, Whitehouse M. Partial versus total knee replacement for knee osteoarthritis. Lancet 2019; 394(10200):712-3.

NICE. Joint replacement (primary): hip, knee and shoulder. GID-NG10084. London: National Institute for Health and Care Excellence; 2019.

NICE. Osteoarthritis: care and management. CG177. London: National Institute for Health and Care Excellence; 2014.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre


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Revision is a later operation to replace the original knee implant with a new device, often as a result of device failure, infection, or persistent pain.

Re-operation refers to any later operation on the implanted knee, which could be a revision or other surgery.



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