title: The Role of Overweight and Obesity in Relation to the More Rapid Growth of Total Knee Arthroplasty Volume Compared with Total Hip Arthroplasty Volume.
abstract: 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 & Medicaid Services databases; 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/m2was 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/m2but 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/m2continues to increase. Changes in hospital and physician reimbursement, length of stay, and in-hospital mortality did not contribute to this differential growth rate.
authors.affiliation: Department of Orthopaedics, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for P.B. Derman: dermanp@hss.edu.
authors.forename: Peter B
authors.initials: PB
authors.lastname: Derman
authors.affiliation: Department of Orthopaedics, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. E-mail address for P.B. Derman: dermanp@hss.edu.
authors.forename: Peter D
authors.initials: PD
authors.lastname: Fabricant
authors.affiliation: Health Care Management Department, The Wharton School, University of Pennsylvania, 3641 Locust Walk, Philadelphia, PA 19104.
authors.forename: Guy
authors.initials: G
authors.lastname: David
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000191:economics / Q000706:statistics & numerical data*; D019645:Arthroplasty, Replacement, Knee / Q000191:economics / Q000706:statistics & numerical data*; D040343:Behavioral Risk Factor Surveillance System; D005260:Female; D017052:Hospital Mortality; D006801:Humans; D007902:Length of Stay / Q000706:statistics & numerical data; D008137:Longitudinal Studies; D008297:Male; D008484:Medicaid; D006278:Medicare; D008875:Middle Aged; D009765:Obesity / Q000453:epidemiology*; D050177:Overweight / Q000453:epidemiology*; D010818:Practice Patterns, Physicians' / Q000706:statistics & numerical data*; D014481:United States / Q000453:epidemiology