The Obesity Paradox: Body Mass Index Complication Rates Vary by Gender and Age Among Primary Total Hip Arthroplasty Patients.

Abstract Background High body mass index (BMI) has long been recognized as a risk factor for post-operative complication among total hip arthroplasty (THA) patients. However, recent studies showed mixed results in the effect of high BMI on surgical outcomes. Our study is to examine the association of pre-operative BMI with complication incidence, stratified by age and sex. Methods We queried the ACS NSQIP database to identify patients who underwent elective primary THA between 2012-2016. We examined the associations between BMI as a continuous and a categorical variable and risk of 30-day postoperative complication, using two multiple polynomial logistic regression models. We also created predictive plots to graphically assess the relationship between BMI and complication by sex and age. Results 117,567 eligible patients were included in the analyses. The predictive probability of all-type postoperative complications showed a U-shaped relationship with continuous BMI values (range: 10-65 kg/m2). The lowest complications risks occurred in patients with BMI between 35-40. Females had higher complication rate than males across all BMI values. This U-shaped relationship was only observed among patients younger than 60 years old, while the associations appear to be inversely linear among patients aged greater than 60 years. Conclusion Our results suggest that the current theory of a linear association between BMI and complication risk may not apply to elective primary THA. Strict BMI cutoffs may not minimize risk, especially among patients over 60 years old. Orthopaedic surgeons should factor in patient-specific variables of age and sex when determining acceptable surgical risk given a particular BMI value.