Total knee replacement surgery should not depend solely on body mass index

  • 8 September 2022

Researchers suggest that access to total knee replacements should not be limited solely because of a patient’s body mass index (BMI). A study supported by the Bristol BRC found patients with high BMIs were not significantly more at risk of worse outcomes than those with normal or low BMIs after a total knee replacement.

One in 10 people in the UK will need a total knee replacement during their lifetime but in some parts of the UK, referral to knee surgeons is restricted because of a high BMI. The study team wanted to analyse the impact of BMI on surgical outcomes to see whether there was any evidence to back this approach up.

They looked at data held by the National Joint Registry for total knee replacements that had taken place between 2005 and 2016. They focused on three outcomes: the number of people needing repeat surgery within 10 years, the number of people dying within 90 days of surgery and pain levels and function after surgery. Patients with a raised BMI were compared to those with a “normal” BMI.

Researchers found patients in the “overweight” and “obese” groups were less likely to die within 90 days of the procedure, than those with a “normal” BMI. Patients with increased BMIs were more likely to need follow-up surgery after 10 years, but the overall number of patients needing an additional procedure was within accepted levels.

All patient groups reported experiencing less pain and some improvement in function six months after the procedure. This improvement was slightly lower among those in the “overweight” and “obese” categories but even these differences were not seen as meaningful.

Overall, the study team did not find any evidence to support restricting total knee replacement referrals solely because of raised BMI. The levels of risk associated with poorer outcomes were acceptable, even if they were slightly increased for patients in the “overweight” or “obese” groups.

Jonathan Evans, lead author on the study, said:

“The key message is not that people of any BMI should have knee replacement surgery. It’s more that the filtering should be done by a multi-professional team of anaesthetists, surgeons and nurses in hospital. The decision is far more complex than a BMI on its own.

“This study included only those people who had a knee replacement. That means others with a high BMI may have been filtered out by a surgeon and an anaesthetist, who thought surgery was not appropriate. These people were not captured by our analysis. But we showed that if people with a high BMI get to have surgery, they do just as well as others.

“In fact, people with a high BMI were less likely to die than people with a low BMI. That is probably because this group of people are self-selected. They might have a high BMI but still be fit. And so they might do better than someone who is slim, but unfit.”


Obesity and revision surgery, mortality, and patient-reported outcomes after primary knee replacement surgery in the National Joint Registry: A UK cohort study

Jonathan Thomas Evans, Sofia Mouchti, Ashley William Blom, Jeremy Mark Wilkinson, Michael Richard Whitehouse, Andrew Beswick, Andrew Judge