AAOS Now

Published 4/1/2018

BMI Affects Mortality Risk After Revision TKA

Patients with very low or very high BMI are at most risk
The impact of obesity on complications after primary total knee arthroplasty (TKA) is well known. Less understood, however, are the effects of obesity on the long-term mortality risk in patients undergoing revision TKA.

Researchers at the Mayo Clinic sought to assess the association between body mass index (BMI) and mortality risk in a large cohort of patients undergoing revision TKA. The results of their study were presented in Scientific Poster P0668, “Body Mass Index and Mortality Risk Following Revision Total Knee Arthroplasty,” which was on display at the AAOS 2018 Annual Meeting.

The study included 4,735 patients who had undergone one or more revision TKA procedures at the clinic between 1986 and 2015. The mean age of the patients was 68 years; 47 percent were men. Follow-up continued until the patient died or until Oct. 2, 2017.

At the time of the initial TKA surgery, patients were categorized by their BMI as underweight (< 18.5 kg/m2), normal weight (18.5 kg/m2–24.9 kg/m2), overweight (25.0 kg/m2–29.9 kg/m2), obese (30.0 kg/m2–39.9 kg/m2), and severely obese (≥ 40 kg/m2).

Normal weight served as the reference group in determining the risk associated with other BMI groups in revision TKA. Risk was based on standardized mortality ratios (SMR) and using age- and sex-adjusted multivariable Cox regression models. Expected mortality rates were based on the total U.S. population mortality rates.

Weight matters
The overall age- and sex-adjusted mortality in the patient population studied was significantly higher than that in the general population (SMR = 1.06; 95 percent confidence interval [CI]: 1.02, 1.11), indicating that revision TKA patients had a higher risk of death compared to their peers of similar age and sex in the general U.S. population.

As shown in Table 1, excess mortality was observed in all BMI categories, but particularly among underweight patients (HR = 1.70; 95 percent CI: 1.02, 2.83), and among severely obese patients (HR = 1.38, 95 percent CI: 1.13, 1.70).

Increasing age and male sex were significant predictors of mortality. When adjusted for age and sex, the risk of death among underweight patients was almost two-fold that of normal weight revision TKA patients. In contrast, patients in the overweight-obese range (BMI 25 – 39.9 kg/m2) did not experience an increased risk of death compared with normal weight patients.

The results of the study revealed that, compared to the general population, patients who underwent revision TKA had a higher mortality rate and the patient’s BMI had a significant impact on this mortality. Underweight patients experienced the highest mortality rate, followed by severely obese patients.

The authors of Scientific Poster P0668 are Mario Hevesi, MD; Hilal Maradit-Kremers, MD, MSc; Jie J. Yao, BA; Megan M. O’Byrne MA, Dirk R. Larson MS, Daniel J. Berry, MD; and David G. Lewallen, MD.