Underweight Patients Are the Greatest Risk Body Mass Index Group for 30-Day Perioperative Adverse Events After Total Shoulder Arthroplasty

Taylor D. Ottesen, BS; Walter R. Hsiang, BS; Rohil Malpani, BS; Allen D. Nicholson, MD; Arya G. Varthi, MD; Lee E. Rubin, MD; Jonathan N. Grauer, MD

Disclosures

J Am Acad Orthop Surg. 2021;29(3):e132-e142. 

In This Article

Results

Cohort Characteristics

A total of 15,717 patients met the inclusion criteria, of whom 16.47% (n = 2,589) were "normal weight" with a BMI of 18.5 to 24.9 kg/m2. An additional 32.50% (n = 5,108) were "overweight" with a BMI of 25.0 to 29.9 kg/m2. Forty percent (40.00%; n = 6,287) were "obese" (BMI, 30.0 to 39.9 kg/m2), 8.89% (n = 1,398) were "morbidly obese" (BMI, 40.0 to 50.0 kg/m2), 1.39% (n = 219) were "super morbidly obese" (BMI: >50 kg/m2), and 0.74% (n = 116) were "underweight" (BMI: <18.5 kg/m2) (Table 1).

The demographics for each BMI cohort are presented in Table 1. Across all six BMI categories, the mean patient age ranged from 63 to 70 years. Most were functionally independent (range, 86.21 to 97.61%), although partial/total functional dependency was more common at both extremes of the BMI range (underweight: 13.79%, morbidly obese: 4.43%, super morbidly obese: 5.02%). Mean ASA classification ranged from 2.44 to 2.91 with higher prevalence of scores ≥3 among underweight (61.20%) obese (55.78%) and, more notably, morbidly obese (91.95%) or super morbidly obese (82.19%) patients. Smoking prevalence declined as BMI increased, dropping from 27.59% among underweight patients to 6.85% among the super morbidly obese. The risk of diabetes, in contrast, showed a stepwise increase, rising from 4.31% of underweight patients with either insulin-dependent and noninsulin-dependent diabetes mellitus (cumulative) to 33.12% of super morbidly obese patients and 31.06% of super morbidly obese patients.

Adverse Events and Obesity

The perioperative adverse events for each BMI cohort are presented in Table 2. Overall, AAE was experienced by 3.64% (n = 572) of the patients, a SAE was experienced by 2.27% (n = 357), and a MAE was experienced by 1.30% (n = 205). Furthermore, postoperative infections were experienced by 1.80% (n = 283), readmission for 2.30% (n = 361), and mortality for 0.15% (n = 23). The average operative time was 105 minutes with an interquartile range of 77 to 140 and an average length of stay of 2 days (interquartile range, 1 to 2). The varied incidence of complications for different BMI categories are shown in Table 2.

Relative to normal weight patients, underweight patients experienced a greater relative incidence of AAE within 30 postoperative days (12.07% vs 3.82%). Interestingly, super morbidly obese patients were not more likely to experience AAE (3.65% vs 3.82%), with overweight patients having the lowest incidence of adverse events across all six BMI categories (2.92%). Furthermore, underweight exhibited elevated incidence of postoperative infections and readmissions within 30 days compared with normal weight patients (6.90% vs 1.82% and 4.31% vs 2.09%, respectively) (Table 2). These incidence findings were subsequently investigated using univariate and multivariate analyses.

Univariate analyses found underweight patients to be 2.86 times more likely to experience AAE (P = 0.004), 4.10 times more likely to experience SAE (P < 0.001), and 4.01 times more likely to develop postoperative infections (P < 0.001) (Table 3). Conversely, there were no statistically increased risk of adverse events among overweight or obese patients.

Figures 1 and 2 illustrate the risk of various adverse events (any, serious, and minor) as a function of BMI. Figure 1 shows fairly level relative risk for the central portions of the graph and increased relative risk as the lower and upper BMI values. Figure 2 shows a similar trend with 30-day readmission rates and postoperative infections, whereas mortality shows significant variability.

Figure 1.

Graph demonstrating the adverse event data as a function of BMI and BMI histogram for patients undergoing total shoulder arthroplasty. Note: Left y-axis refers to the gray histogram in the figure. Right y-axis refers to the line and scatter plot of adverse event data. Horizontal lines denote relative risks of 1.0 and 2.0 as reference lines. Vertical lines bookend the stable BMI ranges. Blue squares represent serious adverse events, black circles represent MAEs, and red diamonds represent any adverse events.

Figure 2.

Graph demonstrating the binomial adverse event data as a function of BMI and BMI histogram for patients undergoing total shoulder arthroplasty. Just as in Figure 1, left y-axis refers to the gray histogram in the figure. Right y-axis refers to the line and scatter plot of adverse event data. Horizontal lines denote relative risks of 1.0 and 2.0 as reference lines. Vertical lines bookend the stable BMI ranges. In this figure, blue circles represent 30-day readmission rates, black triangles represent mortality, and red stars represent infection rates.

Given preoperative differences in demographics and comorbidities for different BMI categories, multivariable logistic regressions controlling for age, sex, BMI, preoperative functional status, smoking status, and ASA class were then performed (Table 4). After controlling for preoperative factors, underweight patients continued to exhibit significantly elevated odds of adverse events including being 2.22 times more likely to experience AAE (P = 0.034), 3.18 times more like to experience SAE (P = 0.004), and 2.77 times more likely to develop postoperative infections within 30 days (P < 0.012). No significant elevation was obsereved in adverse events or binomial adverse events for any of the other BMI categories. A forest plot illustrating differences in multivariate odds ratios (ORs) for AAE across all six BMI categories is show in Figure 3.

Figure 3.

Forest plot demonstrating the multivariate odds ratio of experiencing any adverse event for patients of varying body mass index (BMI) after a total shoulder arthroplasty.

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