The dramatic increase in total knee replacement utilization rates in the United States cannot be fully explained by growth in population size and the obesity epidemic

Abstract

Background: Total knee replacement utilization in the United States more than doubled from 1999 to 2008. Although the reasons for this increase have not been examined rigorously, some have attributed the increase to population growth and the obesity epidemic. Our goal was to investigate whether the rapid increase in total knee replacement use over the past decade can be sufficiently attributed to changes in these two factors.

Methods: We used data from the Nationwide Inpatient Sample to estimate changes in total knee replacement utilization rates from 1999 to 2008, stratified by age (eighteen to forty-four years, forty-five to sixty-four years, and sixty-five years or older). We obtained data on obesity prevalence and U.S. population growth from federal sources. We compared the rate of change in total knee replacement utilization with the rates of population growth and change in obesity prevalence from 1999 to 2008.

Results: In 2008, 615,050 total knee replacements were performed in the United States adult population, 134% more than in 1999. During the same time period, the overall population size increased by 11%. While the population of forty-five to sixty-four-year-olds grew by 29%, the number of total knee replacements in this age group more than tripled. The number of obese and non-obese individuals in the United States increased by 23% and 4%, respectively. Assuming unchanged indications for total knee replacement among obese and non-obese individuals with knee osteoarthritis over the last decade, these changes fail to account for the 134% growth in total knee replacement use.

Conclusions: Population growth and obesity cannot fully explain the rapid expansion of total knee replacements in the last decade, suggesting that other factors must also be involved. The disproportionate increase in total knee replacements among younger patients may be a result of a growing number of knee injuries and expanding indications for the procedure.

Fig. 1

Changes in population size by age group, 1999 to 2008. Each bar represents the percent change in population size of the given age group in the given year, compared with the population size of that age group in 1999.

Fig. 2

Changes in population size from the period of 1999 to 2000 to the period of 2007 to 2008 by obesity status and age group. Each bar represents the percent change in obese, non-obese, or total population size from the period of 1999 to 2000 to the period of 2007 to 2008 for the given age group. There is no bar for non-obese eighteen-to-forty-four-year-olds because the percent change for that group was less than zero.

Fig. 3

Total knee replacement (TKR) utilization by age group, 1999 to 2008. Each bar represents the number of total knee replacements performed in the given year, from 1999 to 2008. Different shaded regions represent different age groups. The forty-five to sixty-four-year-old age group represented 30% of total knee replacement use in 1999 and 41% in 2008. The sixty-five-years-or-older age group represented 68% of total knee replacement use in 1999 and 57% in 2008.

Fig. 4

Changes in total knee replacement utilization by age group, 1999 to 2008. Each bar represents the percent change of total knee replacements performed within the given age group in the given year, compared with the number of total knee replacements performed in that age group in 1999.