For patients with severe arthritis, joint replacement is considered when more conservative treatments have failed. Because patients with obesity have a higher risk of complications during and after surgery, some surgeons, hospitals, and insurance companies have adopted body mass index cutoffs as a basis for deciding whether to offer patients these elective surgeries. But some experts argue that these cutoffs are arbitrary, exclude patients who can still benefit from the surgery, and can increase disparities in care.
“By enforcing cutoffs in general, you’re losing the ability for each surgeon to determine who they want to operate on,” said Daniel Wiznia, MD, assistant professor of orthopedic surgery at Yale University, New Haven, Conn. He is on the leadership committee of the Movement Is Life Caucus, a nonprofit group focused on eliminating disparities in musculoskeletal health. “For every surgeon, it’s up to them to decide if they feel comfortable doing the surgery,” he noted in an interview. “My guidance for that would be, don’t just say no because of the number – look at the patient’s entire medical profile.”
According to the Centers for Disease Control and Prevention, nearly 42% of adults in the United States have a BMI over 30, and 9.2% of adults have a BMI over 40. This excess weight puts additional stress on joints: When a person is walking, experts estimate that the force on the knees can be two to three times someone’s body weight. Over time, this pressure can wear down the cartilage on joints.
As a result, people who are overweight or obese are more likely to develop osteoarthritis and to need joint replacements. According to a Canadian study, patients with a BMI of 30-35 are 3.4 times as likely to require a hip replacement and are 8.5 times as likely to require a knee replacement compared to individuals with a BMI in the “healthy weight” range. With a BMI above 40, individuals were 8.5 times more likely to need a hip replacement and were 32.7 times as likely to need a knee replacement.
More complications, greater expense
While there are no universally recommended BMI cutoffs for joint replacement surgery, it is not uncommon for institutions to require that patients have a BMI below a certain value (usually 35-40) to proceed with surgery. A 2013 survey of physicians from the American Association of Hip and Knee Surgeons found that 52% of surgeons required a BMI below 40 to qualify for surgery.
One of the main reasons for these cutoffs is the elevated risk of complications during and after surgery. Research suggests that obesity is associated with higher rates of wound dehiscence, prosthetic joint infection (PJI), and revision total joint arthroplasty. One 2016 study suggests that patients with a BMI of 35-39.9 are twice as likely to experience PJI compared to patients with a BMI below 35. For patients with a BMI of 40 or higher, PJI is four times as likely.
Another study found that patients whose BMI is 35-40 and who undergo total joint arthroplasty have a 6.4-fold greater risk of deep incision infection. For those with a BMI over 40, that rises to a 12.9-fold increased risk compared to patients with a BMI of 18.5-25. Patients with obesity tend to have other comorbidities that can increase the risk of complications during surgery, such as type 2 diabetes, coronary artery disease, and chronic kidney disease.
Because of the increased risk of complications, health care costs tend to be higher for patients with obesity. The growing popularity of bundled health payments can discourage operating on patients who are more likely to experience complications, such as patients with high BMIs, noted Dr. Wiznia.
Research suggests that minorities and people with lower socioeconomic status are disproportionately affected by these cutoffs. According to the CDC, among non-Hispanic Black Americans and Hispanic Americans, rates of obesity are higher than among their White counterparts, and these patients are less likely to undergo joint replacement. Strictly enforcing this eligibility criterion can worsen those disparities. A study involving 21,294 adults over age 50 from the National Health and Nutrition Examination Survey (NHANES) found that requiring a BMI of under 35 for total joint arthroplasty resulted in Black patients being 39% less likely to be eligible for surgery than White patients. And individuals with an annual household income under $45,000 were 19% less likely to qualify for surgery than those with a household income above $45,000.