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Background: The volume of primary joint replacements performed in the United States increased rapidly over the past twenty years, but the growth rate of total knee arthroplasties exceeded that of total hip arthroplasties. The aim of this study was to identify the key contributing factors behind this differential growth rate. Methods: We compiled longitudinal data on total hip arthroplasty and total knee arthroplasty volume, length of hospital stay, and in-hospital mortality from the Nationwide Inpatient Sample; we calculated reimbursement using information available in the Federal Register and Centers for Medicare we determined trends in body mass index from Behavioral Risk Factor Surveillance System findings; and we estimated the size of the surgical workforce based on membership data from the American Academy of Orthopaedic Surgeons. These sources each contained at least ten years of data, ending in 2009. Data sources were analyzed and were compared to identify supply-side and demand-side factors contributing to the more rapid growth observed in total knee arthroplasty. Results: Of the factors examined, body mass index played the most substantial role in increasing demand for total knee arthroplasty above that of total hip arthroplasty, with younger individuals affected to a greater degree. More rapid growth in utilization of total knee arthroplasty over total hip arthroplasty in individuals with a body mass index of ≥25 kg/m2 was responsible for 95% of the differential increase in total knee arthroplasty over total hip arthroplasty volumes. Hospital and physician reimbursement, length of stay, and in-hospital mortality did not improve more for total knee arthroplasty than total hip arthroplasty. The surgical community responded to additional demand primarily by increasing per-physician output. Conclusions: Growth in total knee arthroplasty volume has far outpaced that of total hip arthroplasty among those with a body mass index of ≥25 kg/m2 but not for those with a body mass index of <25 kg/m2. The magnitude of this effect will continue to expand if the proportion of Americans with a body mass index of ≥25 kg/m2 continues to increase. Changes in hospital and physician reimbursement, length of stay, and in-hospital mortality did not contribute to this differential growth rate.
Derman et al. (2014) studied this question.