Obesity and Its Effect on Outcomes in Total Joint Arthroplasty
Understanding how obesity may influence outcomes in total hip and total knee arthroplasty patients is important in order to lower the risk of complications in this population. Does that also mean there should be a BMI cutoff for total joint arthroplasty patients?
Alex Gaukhman, MD; Jenna Bernstein, MD; Ran Schwarzkopf, MD, MSc; and James Slover, MD, MS
Unlike other major health risks such as tobacco use, which is declining, the prevalence of obesity continues to rise: As of 2016, an estimated 39.8% of adults in the United States were classified as obese. [1,2] The incidence of obesity increases as patients age, and a high body mass index (BMI) is an independent risk factor for development of osteoarthritis of the hip. 
Understanding the effects of obesity on outcomes in total joint arthroplasty (TJA) patients is important in order to lower the risk of complications in this population.
The Literature on BMI and TJA
The influence of obesity on the complications and outcome of TJA has been well described in the literature. A large meta-analysis and systematic review of 20 studies showed an increased overall infection rate in obese patients (BMI of 30 or more) undergoing total knee arthroplasty (TKA), with an odds ratio of 1.90.  Revision for any reason occurred more often in obese patients, with an odds ratio of 1.30. 
Wagner et al  found that reoperation and implant revision increased with higher BMI. They also found an increased risk of dislocation within the first 6 months after total hip arthroplasty (THA) in patients with a higher BMI. 
The risk of infection typically increases due to associated comorbidities such as diabetes mellitus, potentially longer operative times, and a large soft tissue envelope. Excess adipose tissue has been shown to decrease perioperative tissue oxygenation similar to smoking.  Despite supplemental oxygenation, increased adiposity leads to tissue hypoxia, which is especially pronounced during surgery. 
Morbid obesity, defined as a BMI of 40 kg/m2 or more, has also been shown to substantially increase the risk of periprosthetic complications. Meller et al  showed that morbidly obese patients had a significantly elevated risk of complications, with a 2-fold or higher risk of hip dislocation and wound dehiscence. There was also an increase in death, periprosthetic joint infection (PJI), acute renal failure, and knee revisions, with a hazard ratio ranging between 1.5 to 2.0. 
The risk of failure after a 2-stage component exchange is also increased in obese compared with non-obese patients. Houdek et al  evaluated outcomes of 2-stage revision surgery for infection in morbidly obese patients (BMI of 40 or more) compared with a matched cohort of non-obese patients (BMI <30). Morbidly obese patients had greater rate of reinfection (18% vs 2%), revision (42% vs 11%), and reoperation for any reason (61% vs 2%).  Patients who are identified as “super obese” – BMI of 45 to 60 kg/m2 – have an odds-ratio of 8.44 for developing in-hospital complications. 
A systematic review by Haynes et al  analyzed 17 studies to find an association between BMI and THA outcomes. Three studies demonstrated detrimental effect on outcomes with BMI over 30, and 8 studies showed no difference. 
The Swedish National Knee Arthroplasty Registry investigated the correlation of height, weight, BMI, and revision rates in TKA patients. Higher weight and BMI were associated with an increased rate of revision due to infection.  The tallest heights were associated with higher rates of revision for all reasons other than infection, disproving a clear association between increasing BMI and complications after TJA. 
Should There Be a BMI Cutoff for TJA?
Despite the aforementioned complications, patients should be able to undergo TJA regardless of their BMI. In 2016, the prevalence rates of pre-obesity and obesity exceeded 60%.  Most of these individuals are unable to lose weight in the long term. 
Obesity is often a socioeconomic issue, with data from the US Census Bureau showing that obesity rates in poorer counties are 145% higher than in wealthy counties.  A study based on data from the National Institutes of Health found that increased density of fast food restaurants was associated with increased risk of obesity, especially predominant in low-income and black urban neighborhoods, where access to healthier food options (and the ability to purchase them) may be limited.  To restrict surgery among lower-income individuals based on BMI is to impose further disparity of care.
Effective weight loss tools do not exist. Denying patients surgery based on weight denies them a chance to improve their function. A randomized controlled trial by Gandler et al  showed that fewer than half of study participants could lose more than 5% of their body weight at 12 months on a dietitian-led weight management program. Even with a rigorous program, this would not be enough weight loss to impact the risk of PJI.
There is some support for bariatric surgery as the only real option for weight loss when compared with counseling and education. Yayac et al  reviewed the effectiveness of bariatric surgery for the reduction of PJI risk in TJA patients and found mixed data, with some studies showing a trend towards lower infection rates after bariatric surgery and some studies showing no difference in infection rates. Many of these studies were small, and all were retrospective; they do not provide enough data to justify denying obese patients the opportunity to undergo TJA.
Among obese TJA patients, significant improvement in functional outcomes has been shown to be equivalent to, or to even exceed, that of non-obese patients. Li et al  found that higher BMI was associated with more pain at baseline but greater postoperative pain relief after THA or TKA, concluding that obesity in itself should not be a deterrent to undergoing TJA.
In a retrospective review of patients with BMI greater than 40 versus BMI less than 40, Halawi et al  found no difference in outcomes in THA patients. Morbidly obese TKA patients did not improve as much in the first 6 months postoperatively, but by 1 year, they had similar improvements to the non-morbidly obese group. 
Obesity is a comorbidity affecting all aspects of the individual, contributing to numerous systemic and local medical complications. Despite the risks, obese patients with end-stage hip or knee osteoarthritis may still require surgical treatment if they have exhausted conservative, non-operative regimens. Much like other comorbidities that require management prior to elective surgery, patients who are obese require optimization in order to reduce the risk of complications. Patients must be counseled appropriately and educated about the potential complications.
BMI cutoffs may have clinical value in guiding treatment of TJA patients. However, using an absolute value to dictate care without assessing the entire individual may deny significant pain control and improvement in quality of life to obese patients who may nonetheless qualify for TJA.
Alex Gaukhman, MD, and Jenna Bernstein, MD, are NYU/ISK Adult Reconstruction Fellows at NYU Langone Health, New York, New York. Ran Schwarzkopf, MD, MSc, and James Slover, MD, MS, are from the Division of Orthopedics, Adult Joint Reconstruction, at NYU Langone Health, New York, New York.
Disclosures: The authors have no disclosures relevant to this article.
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