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Fig. 1 Rates of clinically significant (>5%) BMI loss at follow-up
Source: Cochrane N, et al., “Total Hip Arthroplasty in Morbidly Obese: Does a Strict Body Mass Index Cutoff Yield Meaningful Change?”

AAOS Now

Published 3/25/2022
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Ariel DeMaio

Study Questions Whether Strict BMI Cutoff Yields Meaningful Changes in Post-THA Outcomes

Obesity is a significant risk factor in the development of osteoarthritis, and as the number of patients with obesity in the United States continues to increase, so does the demand for total hip arthroplasty (THA). Body mass index (BMI) cutoffs (typically >40 kg/m2) have been established to optimize post-THA outcomes and minimize the risk of complications, but according to a report being presented at the AAOS 2022 Annual Meeting, BMI alone should not be used as the sole reason to deny THA.

The study found that age, female sex, and Elixhauser Comorbidity Index score were more often independent predictors of unfavorable acute postoperative outcomes, as reported the researchers, led by Niall Cochrane, MD, from the Department of Orthopaedic Surgery at Duke University School of Medicine. Dr. Cochrane will present these findings on Thursday during the Annual Meeting.

Fig. 1 Rates of clinically significant (>5%) BMI loss at follow-up
Source: Cochrane N, et al., “Total Hip Arthroplasty in Morbidly Obese: Does a Strict Body Mass Index Cutoff Yield Meaningful Change?”
Table 1 Adjusted odds ratios for acute postoperative outcomes
Source: Cochrane N, et al., “Total Hip Arthroplasty in Morbidly Obese: Does a Strict Body Mass Index Cutoff Yield Meaningful Change?”

“For total knee arthroplasty, the consensus in current literature is that morbid obesity, defined as a BMI ≥40, is the threshold for which most perioperative complications, including infection and revision rates, appear to increase considerably,” explained Dr. Cochrane and coauthors. “However, the data for THA are mixed, and there is much less consensus on a threshold above which complications increase.”

In their analysis, Dr. Cochrane and colleagues retrospectively reviewed outcomes in 80 patients with BMI ≥40 who underwent THA at a single academic center. The patients were then propensity score–matched according to age, sex, race, Elixhauser Comorbidity Index score, and tobacco use, which created two additional cohorts: 240 patients with obesity (BMI ≥30 to <40) and 240 patients with regular weight to overweight (bmi ≥18.5 to><30).>

The authors hypothesized that morbidly obese patients would have satisfactory results in the following three categories:

  • survivorship free of infection and all-cause revision
  • acute postoperative outcomes, including discharge disposition and readmissions
  • postoperative change in BMI

The mean age for the entire cohort was 59 years (range, 19–88), and 56.6 percent of all patients were female.

When looking at incidence of periprosthetic joint infection (PJI) during the mean follow-up of 3.9 years, the PJI rate was 1.2 percent in the BMI >40 cohort, compared with 4.0 percent in the BMI 30–40 cohort and 1.9 percent in the BMI <30 cohort (>P = 0.21). After adjusting for patient age, sex, race, and Elixhauser score, there was no significant association between obesity and PJI. Only Elixhauser score was a significant predictor of PJI (P <0.01), the authors reported. elixhauser score also significantly associated with all-cause revision, but obesity was not.>

Morbid obesity was predictive of longer hospital length of stay and facility discharge, but the authors noted that age, female sex, and non-White race were also predictive of those outcomes (Table 1). Post-hoc analyses found no significant difference among morbidly obese, obese, overweight, and normal-weight patients in terms of either of those outcomes.

At final follow-up, a higher percentage of morbidly obese patients had clinically significant (>5 percent) BMI loss; however, the finding was not significant (Fig. 1).

“This [study] is the first and largest single-center study to compare a cohort of morbidly obese patients to two matched cohorts of patients based on age, sex, race, Elixhauser comorbidity score, and tobacco use,” the authors noted. “Many previous studies looking at arthroplasty in an obese population were registry-based and could not effectively control for comorbid conditions that affect the obese patient population.”

Still, they acknowledged that the results of the study should be interpreted with certain limitations in mind. “This study was a retrospective review of each cohort and comes with the inherent disadvantages of this study design. Further prospective trials should be completed to truly compare outcomes between the cohorts,” wrote Dr. Cochrane and colleagues. In addition, a relatively small number of THAs were performed in the morbidly obese patient population at the academic center studied, which has a BMI cutoff in place that can be waived based on surgeon discretion. “In addition, our matching was not done on a 1:1:1 ratio, as patients were propensity score–matched to determine our final cohort,” they added, which potentially decreases the specificity of patient matching.

“Our data set provides strong, unique evidence that, although morbid obesity may increase resource utilization in the acute postoperative period, it alone is not a significant predictor for PJI or revision arthroplasty,” Dr. Cochrane and coauthors concluded. “This [finding] draws into question the previously recommended guidelines of denying a patient THA based on BMI alone. There may be downstream healthcare costs that the system would otherwise incur due to sustained high BMI if a morbidly obese patient is denied a THA, and these should be further evaluated.”

The study will be presented as Paper 491 Thursday at 4 p.m. in Room S406b.

Dr. Cochrane’s coauthors of “Total Hip Arthroplasty in Morbidly Obese: Does a Strict Body Mass Index Cutoff Yield Meaningful Change?” are Sean Patrick Ryan, MD; Billy Insup Kim, MD; Mark Wu, MD; Jeffrey A. O’Donnell, MD; Samuel Secord Wellman, MD; and Thorsten M. Seyler, MD.

Ariel DeMaio is managing editor of AAOS Now. She can be reached at ademaio@aaos.org.