Joint Replacements: Should There Be BMI Cutoffs?

Lucy Hicks

December 05, 2022

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 (BMI) 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.

Dr Daniel Wiznia

"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, an assistant professor of orthopedic surgery at Yale School of Medicine, New Haven, Connecticut. He is also 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 with Medscape Medical News. "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 (CDC), 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. Additionally, 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, healthcare 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 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.

BMI No Better Than Other Risk Factors

Although high BMI is independently associated with a higher risk of complications, the increased risk of complications conferred by a BMI at or above 40 is similar to or lower than those of other comorbidities that surgeons generally accept, said Nicholas Giori, MD, PhD, a professor of orthopedic surgery at Stanford University, California, and chief of orthopedic surgery at the VA Palo Alto Health Care System. These other comorbidities include age older than 75, hypertension that requires medication, and insulin-controlled diabetes. "The independent risk of just having the diagnosis of insulin-dependent diabetes is actually comparable to the independent risk of having obesity by itself," he told Medscape, "and all of us operate on [patients with] diabetes."

Dr Nicholas Giori

Also, there is no BMI at which the risk of complications suddenly increases, according to the American Academy of Orthopaedic Surgeons. "It's a rising complication rate as you go into higher BMIs," Giori said. "If you operate on someone with a BMI of 39 vs 41, you're not going to find that much of a difference [in risk]." But if a medical system enforced a hard BMI cutoff of 40, one patient would qualify for surgery while the other would be barred.

Weight Not as "Modifiable" as Previously Thought

Weight is often considered a "modifiable factor" for a person considering undergoing total joint arthroplasty, but research suggests that the issue is more complicated. "Obesity is tricky, because some people are successful [in weight loss]," said Giori. Those tend to be the more memorable stories. "But a large majority have a really hard time losing substantial weight — enough to make a difference in risk," he continued.

A study conducted in North Carolina found that restricting patients with a BMI over 40 from having elective total joint arthroplasty procedures until their weight was optimized did not result in successful weight loss. Only 20% of patients who originally presented with a BMI above this limit eventually underwent surgery after 2 years, and fewer than half of these patients had achieved a BMI of less than 40 at the time of their surgery. A third of all patients in the study did not return to the orthopedic office after their first visit.

"To hold a hard cutoff when it's very, very hard to modify...is essentially telling people that they are not going to ever have surgery," Giori said; "I think that can be unfair to some patients."

Dr Benjamin Stronach

Bariatric surgery is often suggested for patients with obesity who have not experienced successful weight loss with diet and lifestyle changes alone, but bariatric surgery comes with its own complications. Research on outcomes from total joint arthroplasty among patients with who have lost weight with bariatric surgery has yielded mixed results. "I rarely push anyone hard to go that route but present it as an option for certain patients," said Benjamin Stronach, MD, an orthopedic surgeon at the University of Arkansas for Medical Sciences, in Little Rock. He usually brings up bariatric surgery with patients with a BMI in the high 40s or higher to gauge their interest. If patients are already considering weight loss surgery, his office provides referrals.

But even bariatric surgery does not result in successful long-term weight loss for every patient, Stronach said. He's seeing more and more patients who come for consultations after having undergone bariatric surgery 10 to 15 years ago. These patients lost a significant amount of weight, but then gained the weight back. He noted that bariatric surgery can be very successful for some patients who adhere to their postbariatric regimen. "We typically see fairly impressive results in the short term," he said.

Patients With Obesity Benefit From Joint Replacement

Although patients with obesity are at higher risk for complications from joint replacement surgery, research suggests that these patients can still benefit greatly from these surgeries and that these surgeries remain cost-effective. Some studies have found that patients with obesity tend to have worse outcomes after surgery than patients who are not obese, but often, patients with high BMIs are starting from a lower point, with greater joint pain and limited mobility, Giori said. But the improvements — that is, net change in measured outcomes — can be greater for obese patients.

"Several studies have shown equal or greater improvements in validated outcome scores, function, and satisfaction compared with non-obese patients after surgery," authors write in a recent review article in which they discuss how to optimize joint replacement surgery for patients with obesity. The article, published in the November 2022 issue of the Journal of the American Academy of Orthopaedic Surgeon (JAAOS), is part of a collection of review articles by the Movement Is Life Caucus.

Encourage Weight Loss, but Look Beyond the Number

Rather than adhering to strict BMI cutoffs, some experts urge surgeons to consider the patient as a whole and to evaluate each individual's overall health and potential risk. Giori generally considers high BMI as just another comorbidity when assessing a patient's overall risk. "For a person who only has a high BMI but is otherwise healthy, I see no reason not to go ahead and schedule that person for surgery, because reducing the patient's BMI will not substantially reduce the patient's complication risk, and a delay in surgery may adversely affect the patient's quality of life and ability to earn a living." he said.

"If someone is between a BMI of 40 and 45, we are definitely going to have a discussion about weight," Stronach said. He generally counsels against surgery for any patient with a BMI at 45 or above. He generally wants patients to have a BMI below 40 before surgery but considers individual cases for exceptions. "We will still move forward at times with someone with a BMI of 41, as an example, who is otherwise healthy," he said. Similarly, if a patient has lost a significant amount of weight (eg, the patient's BMI was reduced from 50 to 41), the patient is actively engaged in improving their health, and surgeons believe the patient has significantly reduced their risk, "a lot of time, we're not going to draw a line in the sand right at [a BMI of] 40," he said.

While using a BMI of under 35 or 40 as a guideline when starting to work with patients is reasonable, working toward a weight loss of 5% to 10% of total body weight is another goal to consider, authors advise in the JAAOS obesity review article. Research suggests that even a 5% reduction in overall body weight can reduce surgical complications and can improve a patient's glucose and lipid levels and cardiac profile. Referrals to dieticians and weight loss programs, as well as behavioral counseling, can also be useful in initiating weight loss and keeping patients engaged in the process, the authors write.

Consider a Patient's Comorbidities

Many patients with obesity also have comorbidities, such as type 2 diabetes and hypertension, that can also be optimized for surgery so as to lower a patient's overall risk profile. For patients with diabetes, achieving an A1C of 8% or lower can be a reasonable goal and can reduce risk. "We've found that an HbA1C level of 8% or less is something that virtually all diabetics (though not everybody) can reach, and it's something that can be reached in a reasonable amount of time," Giori said. Preoperative use of beta-blockers, continued use of ACE inhibitors or angiotensin receptor blockers, and behavioral modifications can improve a patient's cardiac health before surgery.

Malnutrition can also be a correctable problem for patients, regardless of BMI. In the Movement Is Life collection of optimization articles, experts recommend that orthopedists screen for malnutrition with blood tests for albumin, vitamin D, transferrin, and total lymphocyte count. Patients with malnutrition should be screened for food insecurity, experts advise, and surgical candidates with deficiencies can be given supplements of omega-3 fatty acids, arginine, and protein shakes.

Surgeon Comfort and Shared Decision-making

Wiznia emphasized that the patient and surgeon need to discuss the risks of surgery, concerns about potential complications, and how a complication could affect the patient's life moving forward. "Ultimately, the surgeon needs to make the decision [of whether or not to proceed [with surgery] with the patient," he said, "but not every surgeon is going to feel comfortable operating on these patients, and not every medical institution is going to have the equipment and the investments to support surgeons doing it."

Giori agreed that surgeons should only proceed with surgical cases they feel comfortable with. Certain surgeons may decide not to operate on individuals with higher BMIs because of the potential complications and can refer these patients to more specialized care centers. Operating on larger patients is more difficult and requires surgical skills and expertise that the surgeon may not have, he noted. "What I do object to is a system-wide BMI cutoff ― for example, if an insurance company won't pay for you to have a joint replacement, regardless of where you go or who your surgeon is," Giori added. "I think that's wrong, because it's not patient centered and it's basically excluding people from having a life-altering operation."

Giori and Wiznia report no relevant financial relationships. Stronach is a consultant for DJ Orthopaedics, Johnson & Johnson, and MiCare Path.

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