For a person who is five foot six inches, a BMI of 30 works out to 185 pounds; by contrast, that same person with a BMI of 40 would weigh 247 pounds – and that patient would be different from a surgical perspective. “To me, someone who is five foot six at 250 pounds is a much different patient and different risk profile than someone who is five foot six and 185 pounds,” he says, noting that in the US, a person with a BMI of 30 may not necessarily be viewed as obese. “Our cutoff [at the University of Michigan] for elective arthroplasty surgery at five foot six is 247 pounds.”

In the second study, researchers from the Mayo Clinic in Rochester, Minn., analyzed 8,129 patients who had 6,475 primary total knee replacements and 1,654 revision knee replacements at a large U.S. medical center between January 2000 and September 2008. The patients were stratified into eight groups based on BMI and – after taking into account age, general health and type of surgery – costs associated with their surgeries were compared.

As with the first study, increasing BMI was not associated with a higher risk of complications (in this case, within 90 days). But it was associated with significantly longer hospitals stays. Patients with a BMI less than 35 had the shortest hospital stays and lowest costs associated with the procedure. And irrespective of comorbidities or complications, increasing BMI caused longer hospital stays and higher hospitalization costs. Every five-unit increase in BMI over 30 caused hospital costs to rise about $250 to $300 for patients getting their first joint replacement and $600 to $650 for those getting a revision surgery.

“What this paper is saying is you don’t need to be super obese to have higher costs. There are costs associated with obesity even among less obese people. It’s a continuum,” says lead author Hilal Maradit-Kremers, MD, an associate professor of epidemiology at the Mayo Clinic. “The cost is there – hidden in many other things like operating room cost and room and board and the type of devices and prostheses they use.”

Dr. Maradit-Kremers says, because total knee replacements are one of the most common elective surgical procedures in the U.S., this study highlights the hidden cost of obesity for the nation’s healthcare system. Even “small” cost increases at the individual level translate to big increases in costs at the population level.

The same researchers also presented a poster at the AAOS conference on a separate but related study of nearly 9,000 patients who had primary and revision hip replacements. It shows obesity causes costs to jump during hip replacement procedures as well – even more than the jump in cost related to knee surgery – in part due to higher baseline costs. Every 5-unit increase in BMI above 30 costs $500 more in hospital costs and $900 more in 90-day costs for patients getting their first hip replacement. For patients having a revision hip replacement, every 5-unit increase in BMI above 30 costs $800 more in hospital costs and $1,500 more for 90-day costs.

Dr. Urquhart calls these studies impressive, because they take into account different degrees of obesity. “It’s nice that they were able to prove what we anecdotally have seen: the bigger the patient is, the more staff assistance you need to get them out of bed, the longer the surgery,” he says.

Dr. Urquhart says he believes the bottom line is that anyone considered obese will improve their general health and surgical outcomes by losing weight before undergoing an elective procedure. And even if patients haven’t previously been able to lose weight on their own, he says they are often able to do it if joint replacement surgery is their goal.

“It’s a really tough conversation but it’s a conversation that has to happen,” Dr. Urquhart says, noting that there is a lot of interplay between obesity and arthritis.