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Knee replacements can reduce pain and improve function, regardless of people’s weight. Research found that few people in any weight category needed repeat surgery within 10 years. Deaths after surgery were rare in any group. The study concluded that someone’s weight alone should not rule out a referral for knee replacement surgery.

In the UK, knee replacements are common orthopaedic (bone and joint) procedures. The usual reason for them is osteoarthritis (painful and stiff joints). GPs refer people to surgeons when their knee joint is so badly worn or damaged, it is painful even at rest.

The body mass index (BMI) uses someone’s height and weight to work out if their weight is healthy. National guidelines state that GPs can refer people with osteoarthritis and a high BMI (meaning they are living with obesity or overweight) for surgery. However, in some areas, decision-makers block or delay these referrals. They may believe that people with a high BMI benefit less than others, or have more complications, from the surgery.

Previous studies have been small. This large study explored the effect of high BMI on the likely benefits of total knee replacements, the chances of needing repeat surgery, or of dying.

The study found that, irrespective of BMI, people had good outcomes from knee replacement surgery. The researchers recommend that risk assessments are not based solely on BMI.

Further information about knee replacements is available on the NHS website.

What’s the issue?

In a total knee replacement, both sides of the knee joint are replaced. Almost 1 in 10 people in the UK will need this surgery in their lifetime. The numbers are likely to rise as the population ages.

Total knee replacements can reduce pain and improve function. They are usually safe, cost-effective procedures. However, some groups of people may benefit less than others from surgery. Previous studies have suggested that those who are older, frail, have other long-term conditions or a very high BMI may have worse outcomes. They may, for example, be more likely to need repeat surgery.

UK guidance states that GPs can refer people with osteoarthritis and a high BMI for joint replacement surgery. Despite this, some areas have a local rule that prevents GPs from making these referrals. Decision-makers may be concerned that surgery is more risky for people with a high BMI, and less likely to be successful.  

However, many of the previous studies have been small, and have looked at a single outcome (repeat surgery, for example, knee pain and function, or death rate). This large study considered all 3 outcomes after total knee surgery in people with a range of BMIs.

What’s new?

The researchers looked at the records of 493,710 people who had total knee replacement surgery. Using World Health Organisation (WHO) groupings, they classed people as underweight (BMI lower than 18.5 mg/m2), normal weight (18.5 to 25 kg/m2), overweight (25 to 30 kg/m2), obese class I (30 to 35 kg/m2), obese class II (35 to 40 kg/m2) or obese class III (over 40 mg/m2).

The researchers took into account people’s age, sex, health at time of surgery, and deprivation of the area they lived in. They looked at the numbers who needed repeat surgery within 10 years, or died within 90 days. They compared people’s reports of their knee function and pain 6 months after surgery.

The study found that, after surgery:

  • few people in any group needed repeat surgery within 10 years (fewer than 1 in 20); the risk was raised in the obesity class I and II groups compared to normal weight, but it remained well within accepted levels
  • deaths within 90 days of surgery were rare; they were highest in the underweight group (10 deaths among 1,338 people) and lowest in the overweight group
  • all groups reported improved knee function and pain; differences between groups were not meaningful and improvements were only slightly lower in the overweight and all obese group

Why is this important?

This is the first study on obesity and knee replacement to look at all 3 outcomes: repeat surgery, death, and knee pain and function. It used the largest dataset on joint replacement surgery (National Joint Registry).

The results show that someone’s BMI level alone should not limit their access to knee replacement surgery. Some people with obesity had slightly poorer outcomes (more repeat surgery; less improvement in function and pain). But these differences were small and within the generally accepted range.

The findings suggest that, among people with a high BMI, surgeons are selecting the healthiest for surgery. Those who had knee replacements were probably fitter, with fewer long-term conditions, than people who did not have surgery. This selection process is effective, the researchers say. There is no need to prevent GPs referring anyone with a high BMI.

A team of different specialists (anaesthetists, surgeons, nurses and allied health professionals) should consider with a patient, their overall risk. The decision to go ahead with surgery should take the patient’s weight into account along with other risk factors, such as other long-term conditions.

Some changes occurred over the course of the study. Participants had surgery between 2005 and 2016 and could only be included if their notes recorded their BMI. In the early years especially, this sometimes did not happen. In addition, obesity and overweight have become more common in the population since 2005.

What’s next?

These findings reinforce the national guidelines for people with osteoarthritis. The conclusions are most relevant to local decision-makers who are advising GPs not to refer people with high BMI for knee surgery.

This study looked at people’s knee pain and function 6 months after surgery. The team would like to look longer-term. It is possible that BMI might influence recovery and, for example, people with a high BMI may take longer than 6 months to fully recover. Comparing scores at a later point may show different patterns. Future studies into BMI and knee surgery could also explore the cost-effectiveness of surgery. It might be that the length of hospital stay differs according to BMI.

The same database is being used to answer other questions. The lead author is involved in a similar study at the University of Bristol into the risks and benefits of hip replacement surgery.

BMI alone is not an appropriate way to decide who is suitable for surgery, the study concluded. More factors need to be considered. The ACHE study (University of Oxford), also funded by the NIHR, recently developed a tool to help GPs decide who is likely to benefit from hip or knee joint replacement.

You may be interested to read

This Alert is based on: Evans JT, and others. Obesity and revision surgery, mortality, and patient-reported outcomes after primary knee replacement surgery in the National Joint Registry: A UK cohort study. PLoS Medicine 2021;18:7.

Research on UK commissioners' policies: McLaughlin J, and others. Access to hip and knee arthroplasty in England: commissioners’ policies for body mass index and smoking status and implications for integrated care systems. BMC Health Serv Res 2023;23:77.

NICE guideline [CG177]. 2014: Osteoarthritis: care and management.

Funding: This research was funded by the NIHR Biomedical Research Centre at University Hospitals Bristol, Weston NHS Foundation Trust and the University of Bristol.

Conflicts of Interest: The study authors declare no conflicts of interest.

Disclaimer: NIHR Alerts are not a substitute for professional medical advice. They provide information about research which is funded or supported by the NIHR. Please note that views expressed in NIHR Alerts are those of the author(s) and reviewer(s) at the time of publication. They do not necessarily reflect the views of the NHS, the NIHR or the Department of Health and Social Care.

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