The effect of obesity on direct medical costs in total knee arthroplasty.

BACKGROUND

Obesity prevalence continues to rise in the United States. We sought to examine the effect of obesity on length of hospital stay and direct medical costs in a large cohort of patients who underwent total knee arthroplasty.

METHODS

The study included 8129 patients who had undergone 6475 primary total knee arthroplasties and 1654 revision total knee arthroplasties at a large U.S. medical center from January 1, 2000, to September 30, 2008. Patients with bilateral procedures within ninety days following the index admission were excluded. Data on clinical and surgical characteristics and complications were obtained from the original medical records and the institutional joint registry. Patients were classified into eight groups based on their body mass index at the time of surgery. Direct medical costs were calculated in 2010 U.S. dollars by using standardized, inflation-adjusted costs for services and procedures billed during hospitalization and the ninety-day window. Study end points were hospital length of stay and direct medical costs. End points were compared across the eight body mass index categories in both unadjusted and multivariable risk-adjusted analyses. Linear regression models were used to determine the cost impact associated with increasing body mass index and obesity accounting for comorbidities and complications.

RESULTS

Body mass index data were available for 99.5% of patients and ranged from 15 to 73 kg/m2. Length of stay and the direct medical costs were lowest for patients with body mass index values in the normal to overweight range. Increasing body mass index was associated with significantly longer hospital stays and costs. Every 5-unit increase in body mass index beyond 30 kg/m2 was associated with approximately $250 to $300 higher hospitalization costs in primary total knee arthroplasty and $600 to $650 higher hospitalization costs in revision total knee arthroplasty. These estimates persisted after adjusting for comorbidities or complications.

CONCLUSIONS

Obesity is associated with longer hospital stays and higher costs in total knee arthroplasty. The effect of obesity on costs appears to be independent of obesity-related comorbid conditions and complications.