BMI as a barrier to orthopedic surgery doesn’t make sense for patient health

I am getting bent out of shape over surgeons telling patients they cannot get a knee replacement because they are above a certain BMI cutoff.

Here’s a familiar scenario: A patient with a high body weight and BMI limps down the hall to my exam room. “How have you been?” I ask. He sighs. “I still can’t get a new knee. The other doc says I have to I lose 50 pounds before I can go under the knife!” So, this guy, a former college athlete, has been told he can’t get surgery until he loses weight, but chronic pain, immobility, and stress all make it much harder to lose weight. He has been advised to see me, an endocrinologist, to find out how to reduce his weight and BMI so he is eligible for surgery. I tell him that medications approved for weight loss have the best effect when combined with regular physical activity and a healthy diet.

To summarize: No new knee until he loses weight, and harder to lose weight with the bum knee. What the Joseph Heller is going on here? Why is there a knee-jerk response by many orthopedists to deny joint replacement surgery to people above a certain BMI (usually 35)?

The most commonly cited rationale for BMI cutoffs is that higher BMIs correlate with worse surgical outcomes. What this argument implies is that people with high body weight are unhealthy and unlikely to have a good outcome due to their weight alone. There is little consideration paid to cardiometabolic health or history of physical activity; all eyes are on that number on the scale that gets factored into a problematic mathematical formula that calculates BMI. Sure, in general, those with high body weight might have worse post-operative outcomes for a lot of reasons, including medical comorbidities that increase risk of post-operative complications or worse long-term outcomes. Orthopedic surgeons presumably don’t want their personal scorecards to include a high number of patients with post-operative complications or suboptimal outcomes. And many insurance companies refuse preauthorization for surgeries for patients with a BMI of more than 40. So, they tell lots and lots of people to shuffle off to other doctors who specialize in medical weight management.

But I have good news for my orthopedist friends: They may not need to worry so much about their stats. A very large 2021 retrospective study from the U.K. reviewed outcomes of nearly 500,000 total knee replacements between 2005-2016. The study compared those with “normal” versus elevated BMI and looked at the number of patients who died within 90 days, the number of who needed a revision surgery after 10 years, and changes between pre-operative and six-month post-operative Oxford Knee Score (a 12-question assessment of pain, mobility, and ability to do activities of daily life). Bottom line: Those with elevated BMI did not do significantly worse following surgery and as such, the authors concluded that restricting surgery based on BMI alone is not based on clinical evidence. Similarly, as STAT has reported, high BMI as a roadblock to top surgery among transgender individuals has recently been reconsidered.

This makes sense to me, because BMI itself is a flawed metric, as the American Medical Association recently acknowledged. The equation for calculating BMI dates back to the 19th century and is based on body weights of white, European men. BMI therefore cannot be applied universally across races and ethnicities, between sexes, or even over the life span. BMI takes into account only height and weight, not body composition, and neither body weight nor BMI take into account cardiometabolic health.

BMI is best applied to populations, not individuals, to generally classify obesity; it is one metric among many that should be used to assess a person’s overall health and readiness for surgery. Can we really say that a patient with a BMI of 36 who is eating a high-quality diet, bikes 10-20 miles a day, and engages in strength and flexibility training twice a week is less fit for surgery than someone with a BMI of 30 who drinks three sugar-sweetened beverages a day, smokes, and is completely sedentary? Additionally, a quick look at some stats of the New England Patriots shows that several offensive and defensive linemen have BMIs above 35, a few as high as 40 and 41. I’m pretty sure these gentlemen are not told they need to lose weight before they go to the OR.

To be sure, there are medical procedures in which there are appropriate hard and fast BMI or weight cutoffs. For example, CT scanners and MRI machines have weight limits, and certain diagnostic procedures are difficult with high body weight, especially if it is distributed mostly around the abdomen. Some procedures, such as an ablation to treat atrial fibrillation, are riskier with BMIs over 35, as higher body weight is associated with higher risk for recurrence.

I suppose orthopedists who send patients to me figure, what’s the harm in delaying surgery? Like most referring doctors, orthopedists probably think that the newest class of medications that are approved for weight loss, including the GLP-1 receptor agonists Wegovy and Saxenda and the newest dual GLP1/GIP agonist Zepbound, are the bee’s knees and will result in swift, dramatic weight loss.

The reality is that these medications are in increasingly short supply, very often not covered by insurance, and cost over $1,000 per month out of pocket. They also have many side effects and contraindications, and some bodies simply don’t respond to them. I have many patients waiting for orthopedic surgery who are already engaged in the trifecta that should produce weight loss: a high-quality diet, as much physical activity as possible, and medication for weight loss. Many patients do not lose enough weight to get the surgery and continue to see me every several months, as severe orthopedic pain continues to make it difficult for them to move.

I wish the orthopedic and medical communities could be on a joint mission to see people as entire individuals, not a number on a scale, when deciding who gets what treatment and when. A big step in the right direction would be to eliminate BMI cutoffs and instead have better direct communication between orthopedists and medical specialists, such as cardiologists or endocrinologists, to risk stratify each person as a person, not a number.

This piece was updated to include information about insurance companies and preauthorization

Jody Dushay, M.D., MMSc, is an endocrinologist at the Beth Israel Deaconess Medical Center and assistant professor of medicine at Harvard Medical School. She is also the founder and director of Wellpowered, a comprehensive wellness and weight management program.