Does Operative Time Modify Obesity-related Outcomes in THA?
Authors: Abella Maveric K. I. L. , Angeles John P. M. , Finlay Andrea K. , Amanatullah Derek F. 
Published: April 2023

Abstract

Background
Most orthopaedic surgeons refuse to perform arthroplasty on patients with morbid obesity, citing the higher rate of postoperative complications. However, that recommendation does not account for the relationship of operative time (which is often longer in patients with obesity) to obesity-related arthroplasty outcomes, such as readmission, reoperation, and postoperative complications. If operative time is associated with these obesity-related outcomes, it should be accounted for and addressed to properly assess the risk of patients with obesity undergoing THA.

Questions/purposes
We therefore asked: (1) Is the increased risk seen in overweight and obese patients, compared with patients in a normal BMI class, associated with increased operative time? (2) Is increased operative time independent of BMI class a risk factor for readmission, reoperation, and postoperative medical complications? (3) Does operative time modify the direction or strength of obesity-related adverse outcomes?

Methods
This retrospective, comparative study examined 247,108 patients who underwent THA between January 2014 and December 2020 in the National Surgical Quality Improvement Project (NSQIP). Of those, emergency cases (1% [2404]), bilateral procedures (1% [1605]), missing and/or null data (1% [3280]), extreme BMI and operative time outliers (1% [2032]), and patients with comorbidities that are not typical of an elective procedure, such as disseminated cancer, open wounds, sepsis, and ventilator dependence (1% [2726]), were excluded, leaving 95% (235,061) of elective, unilateral THA cases for analysis. The NSQIP was selected due to its inclusion of operative time, which is not found in any other national database. BMI was subdivided into underweight, normal weight, overweight, Class I obesity, Class II obesity, and Class III obesity. Of the patients with a normal weight, 69% (30,932 of 44,556) were female and 36% (16,032 of 44,556) had at least one comorbidity, with a mean operative time of 86 ± 32 minutes and a mean age of 68 ± 12 years. Patients with obesity tend to be younger, male, more likely to have preoperative comorbidities, with longer operative times. Multivariable logistic regression models examined the effects of obesity on 30-day readmission, reoperation, and medical complications, while adjusting for age, sex, race, smoking status, and number of preoperative comorbidities. After we repeated this analysis after adjusting for operative time, an interaction model was conducted to test whether operative time changes the direction or strength of the association of BMI class and adverse outcomes. Adjusted odds ratios (AOR) and 95% confidence intervals (CIs) were calculated, and the interaction effects were plotted.

Results
A comparison of patients with Class III obesity to patients with normal weight showed that the odds of readmission went from 45% (AOR 1.45 [95% CI 1.32 to 1.59]; p < 0.001) to 27% after adjusting for operative time (AOR 1.27 [95% CI 1.01 to 1.62]; p = 0.04), the odds of reoperation went from 93% (AOR 1.93 [95% CI 1.72 to 2.17]; p < 0.001) to 81% after adjusting for operative time (AOR 1.81 [95% CI 1.61 to 2.04]; p < 0.001), and the odds of a postoperative complication went from 96% (AOR 1.96 [95% CI 1.58 to 2.43]; p < 0.001) to 84% after adjusting for operative time (AOR 1.84 [95% CI 1.48 to 2.28]; p < 0.001). Each 15-minute increase in operative time was associated with a 7% increase in the odds of a readmission (AOR 1.07 [95% CI 1.06 to 1.08]; p < 0.001), a 10% increase in the odds of a reoperation (AOR 1.10 [95% CI 1.09 to 1.12]; p < 0.001), and 10% increase in the odds of a postoperative complication (AOR 1.10 [95% CI 1.08 to 1.13]; p < 0.001). There was a positive interaction effect of operative time and BMI for readmission and reoperation, which suggests that longer operations accentuate the risk that patients with obesity have for readmission and reoperation.

Conclusion
Operative time is likely a proxy for surgical complexity and contributes modestly to the adverse outcomes previously attributed to obesity alone. Hence, focusing on modulating the accentuated risk associated with lengthened operative times rather than obesity is imperative to increasing the accessibility and safety of THA. Surgeons may do this with specific surgical techniques, training, and practice. Future studies looking at THA outcomes related to obesity should consider the association with operative time to focus on independent associations with obesity to facilitate more equitable access.

Level of Evidence
Level III, therapeutic study