Background: The importance of morbid obesity as a risk factor for complications after total knee arthroplasty (TKA) continues to be debated. Obesity is rarely an isolated diagnosis and tends to cluster with other comorbidities that may independently lead to increased risk and confound outcomes. It is unknown whether morbid obesity independently affects postoperative complications and resource use after TKA.
Questions/purposes: The purpose of this study was to determine whether morbid obesity is an independent risk factor for inpatient postoperative complications, mortality, and increased resource use in patients undergoing primary TKA.
Methods: The Healthcare Cost and Utilization Project Nationwide Inpatient Sample (NIS) database was used to identify patients undergoing primary TKA from October 2005 to December 2008. Morbid obesity (body mass index≥40 kg/m2) was determined using International Classification of Diseases, 9th Revision, Clinical Modification codes. In-hospital postoperative complications, mortality, costs, and disposition for morbidly obese patients were compared with nonobese patients. To control for potential confounders and comorbid conditions, each morbidly obese patient was matched to a nonobese patient using age, sex, and all 28 comorbid-defined elements in the NIS database based on the Elixhauser Comorbidity Index. Of 1,777,068 primary TKAs, 98,410 (5.5%) patients were categorized as morbidly obese. Of these, 90,045 patients (91%) were able to be matched one-to-one to a nonobese patient for the adjusted analysis.
Results: Morbidly obese patients had a higher risk of postoperative in-hospital infection (0.24% versus 0.17%; odds ratio [OR], 1.3; 95% confidence interval [CI], 1.1-1.7; p=0.001), wound dehiscence (0.11% versus 0.08%; OR, 1.3; 95% CI, 1.0-1.7; p=0.28), and genitourinary-related complications (0.60% versus 0.44%; OR, 1.3; 95% CI, 1.1-1.5; p<0.001). There was no increase in the prevalence of cardiovascular or thromboembolic-related complications. Morbidly obese patients were at higher risk of in-hospital death after primary TKA compared with nonobese patients (0.08% versus 0.02%; OR, 3.2; 95% CI, 2.0-5.2; p<0.001). Total hospital costs (USD 15,174 versus USD 14,715, p<0.001), length of stay (3.6 days versus 3.5 days, p<0.001), and rate of discharge to a facility (40% versus 30%, p<0.001) were all higher in morbidly obese patients.
Conclusions: Morbid obesity appears to be independently associated with a higher risk for a small number of select in-hospital postoperative complications and mortality after matching for comorbid medical conditions linked to obesity. However, the independent impact of morbid obesity appears to be fairly modest, and morbid obesity did not appear to be an independent risk factor for many systemic complications. Continued research is necessary to identify the influence of associated comorbidities on early postoperative complications in morbidly obese patients after TKA.
Level of evidence: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.