Study Background:The proportion of obese patients continues to rise in the United States. The number of knee replacement (totalknearthroplasty (TKA) procedures is also elevated in younger and obese patients. The purpose of this study was to observe the effect of obesity on postoperative surgical complications, length of stay, and direct medical costs in a large sample of TKA patients. METHODS: This study included 8129 patients, of whom 6475 had primary TKA and 1654 had revision TKA, at a large US medical center from January 1, 2000 to September 31, 2008. Patients who underwent bilateral TKA within the 90-day window were excluded. Baseline patient characteristics as well as postoperative complications (infection, thrombotic events, joint instability, and peripheral fractures) were primarily collected in the original case records and joint registry. Direct medical costs were calculated using standard inflation-adjusted costs for the duration of the hospital stay as well as for the 90-day window. Patients were divided into 8 groups based on body mass index (BMI). Study endpoints included length of stay, complications during the 90-day window, and direct medical costs during the hospitalization and 90-day window. Study outcomes were compared across subgroups based on unadjusted and multifactorial risk-adjusted analyses. Linear regression analysis was used to observe the change in direct medical costs as body mass index increased. Results: 99.5% of patients had a collectable body mass index ranging from 15-73. The results showed that patients in the BMI groups of 25-30 and 30-35 had the lowest mean length of stay and direct medical costs. However, analyses adjusted for age, sex, type of surgery, and complications showed that increased BMI was associated with longer hospital stays. Analysis after adjusting for age, sex, and type of surgery showed that each 10-unit increase in BMI above 25 was associated with an increase in average direct medical costs and costs within a 90-day window of$648 and$724, respectively. This change also remained significant after adjusting for complications (for each 10-unit increase in BMI above 25, the average direct medical costs and costs within a 90-day window increased by$541 and$504, respectively. The above outcome associations also apply to patients undergoing initial TKA due to joint degeneration. For these patients undergoing initial TKA, the average direct medical costs and costs within the 90-day window increased by$575 and$510, respectively, for each 10-unit increase in BMI above 25. When adjusted for comorbidity factors, the relationship between BMI and cost was not statistically significant, suggesting that the impact of obesity on cost increases with comorbidity. CONCLUSION: In TKA, obesity did not appear to increase surgical complications within a 90-day window, but was directly related to length of stay and direct medical costs, in part because increased medical costs may have been induced by increased surgical complications due to obesity. The increase in the obese population in TKA may increase the financial burden on TKA patients.