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This is a plain English summary of an original research article. The views expressed are those of the author(s) and reviewer(s) at the time of publication.

Replacing one side of the knee, unicompartmental knee replacement, may result in shorter hospital stays, fewer short-term complications, faster recovery and better knee function than total knee replacements. However, about 8% need revision by five years - about two to three times the rate with total knee replacements.

People with pain and immobility from osteoarthritis that cannot be controlled by other treatments may be referred for knee surgery. If the damage is only to one side of the knee, partial knee replacement may be an option. Recent reviews had shown that partial knee replacements may be a better use of NHS resources, being both clinically and cost-effective.

This study suggests that partial knee replacement also has better outcomes that are important to patients, though slightly more people need subsequent revision surgery

Why was this study needed?

Knee replacements are commonly performed for people with osteoarthritis.

An estimated 25% to 47% of patients who are suitable for joint replacement in the UK have osteoarthritis in one compartment of the knee and potentially could receive either implant. Total knee replacements involve all three components: the medial, lateral and patellofemoral compartments. When only one of these compartments is replaced, this is called a unicompartmental partial knee replacement.

In 2017, there were 112,836 knee replacement procedures undertaken in England and Wales: 9% of them were unicompartmental. Previous research has shown that partial knee replacements can save costs and improve the quality of life for patients compared with total knee replacements. National guidelines do not recommend a particular approach.

Most previous studies have focused on the clinical benefits of partial versus total knee replacement rather than outcomes such as recovery time and mobility. This study aimed to provide an account of all available evidence on outcomes.

What did this study do?

This systematic review analysed 60 studies comparing partial and total knee replacements by three groups: randomised controlled trials (seven studies); data from large national joint registries and multicentre databases (17 studies); and large cohort studies (36 studies). The researchers held workshops with patients to identify the most important outcomes to study. Each data group was then analysed separately for each outcome.

A limitation of this approach is that it uses observational registry studies and cohort studies, which are subject to selection bias, with randomised controlled trials (RCTs). There is also possible bias introduced by the surgeons' skill levels and preferences for different surgical procedures. But the researchers have looked at the three different study types separately, and the large numbers do give more precision.

What did it find?

  • All three groups of data showed that hospital stays were shorter after a partial knee replacement than after a total knee replacement. The differences in length of hospital stay between partial and total knee replacement groups were -1.20 days (95% confidence interval [CI] -1.67 to -0.73) according to RCTs, -1.43 (95% CI -1.53 to -1.33) in joint registries, and -1.73 (95% CI -2.30 to -1.16) based on cohort studies.
  • Fewer short-term complications were seen with partial knee replacements. The differences were small and only reported in a few studies for cardiac events, stroke, blood clots in the leg or lung and risk of deep infections requiring long-term antibiotics or further surgery.
  • A limited amount of data on kneeling ability after the operation suggests that patients who had a partial replacement had better results (risk ratio 0.53, 95% CI 0.28 to 1.01). There was no difference in pain scores, but better overall functional improvements were reported by patients after a partial knee replacement according to patient-reported outcome measure (PROM) scores (standardised mean difference -0.58, 95% CI -0.88 to -0.27 for registries, and -0.29, 95% CI -0.46 to -0.11 for cohort studies).
  • Reoperation rates were similar, however more revisions were needed following the partial knee replacement at five, ten and 15 years. The large registry studies with most revisions showed that at five years the revision rate after partial knee replacement (8.8%) was around 2.5 times more likely than after total knee replacement (3.2%, risk ratio 2.50 [95% CI 1.77 to 3.54]).
  • Patients identified the rate of recovery and return to work or sport as important factors in their choice of operation. Very few studies reported on this outcome, but two cohort studies reported a sooner return to work after a partial knee replacement (mean difference -0.96 weeks, 95% CI -1.31 to -0.61). Two cohort studies suggested that return to sport was sooner after the partial knee replacement (mean difference -5.24 weeks, 95% CI -6.84 to -3.64).

What does current guidance say on this issue?

NICE guidelines on the management of osteoarthritis cover patient criteria for referral for joint surgery but do not include recommendations on a preferred surgical procedure.

A NICE guideline specifically on knee replacements is due to be published in March 2020 and will consider the clinical effectiveness of partial knee replacement.

What are the implications?

This study supports previous systematic reviews that show that despite higher revision rates, patients having partial knee replacement have shorter hospital stays, quicker recovery periods, and fewer complications. If revision is needed, it is less likely to be required to treat deep infection, which is more of a risk in total knee replacement.

This study focuses on outcomes that are important to patients and as such will be useful in supporting them and their surgeons in discussing preferences for surgery.

The authors note that hospital revision rates show a link with the volume of unicompartmental surgery undertaken in a hospital. Lower revision rates are achieved in centres doing more of this type of surgery implying that training and experience are important determinants of outcomes.

Citation and Funding

Wilson HA, Middleton R, Abram SGF et al. Patient-relevant outcomes of unicompartmental versus total knee replacement: systematic review and meta-analysis. BMJ 2019;364:l352.

No separate funding was supplied for this study.



Burn E, Liddle AD, Hamilton TW et al. Choosing between unicompartmental and total knee replacement: what can economic evaluations tell us? A systematic review. Pharmacoecon Open. 2017;1:241–53.

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

HQIP. 15th Annual Report 2018: National Joint Registry for England, Wales, Northern Ireland and the Isle of Man. London: Healthcare Quality Improvement Partnership; 2018.

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