Prevalence of Primary and Revision Total Hip and Knee Arthroplasty in the United States From 1990 Through 2002

Kurtz, Steven PhD; Mowat, Fionna PhD; Ong, Kevin PhD; Chan, Nathan PhD; Lau, Edmund MS; Halpern, Michael MPH, MD, PhD

Journal of Bone & Joint Surgery - American Volume: July 2005 - Volume 87 - Issue 7 - p 1487–1497
doi: 10.2106/JBJS.D.02441
Scientific Articles

Background: The purpose of this study was to quantify the procedural rate and revision burden of total hip and knee arthroplasty in the United States and to determine if the age or gender-based procedural rates and overall revision burden are changing over time.

Methods: The National Hospital Discharge Survey (NHDS) for 1990 through 2002 was used in conjunction with United States Census data to quantify the rates of primary and revision arthroplasty as a function of age and gender within the United States with use of methodology published by the American Academy of Orthopaedic Surgeons. Poisson regression analysis was used to evaluate the procedural rate and to determine year-to-year trends in primary and revision arthroplasty rates as a function of both age and gender.

Results: Both the number and the rate of total hip and knee arthroplasties (particularly knee arthroplasties) increased steadily between 1990 and 2002. Over the thirteen years, the rate of primary total hip arthroplasties per 100,000 persons increased by approximately 50%, whereas the corresponding rate of primary total knee arthroplasties almost tripled. The rate of revision total hip arthroplasties increased by 3.7 procedures per 100,000 persons per decade, and that of revision total knee arthroplasties, by 5.4 procedures per 100,000 persons per decade. However, the mean revision burden of 17.5% for total hip arthroplasty was more than twice that for total knee arthroplasty (8.2%), and this did not change substantially over time.

Conclusions: The number and prevalence of primary hip and knee replacements increased substantially in the United States between 1990 and 2002, but the trend was considerably more pronounced for primary total knee arthroplasty.

Clinical Relevance: The reported prevalence trends have important ramifications with regard to the number of joint replacements expected to be performed by orthopaedic surgeons in the future. Because the revision burden has been relatively constant over time, we can expect that a greater number of primary replacements will result in a greater number of revisions unless some limiting mechanism can be successfully implemented to reduce the future revision burden.

1 Exponent Inc., 3401 Market Street, Suite 300, Philadelphia, PA 19104. E-mail address for K. Ong: kong@exponent.com

2 Exponent Inc., 149 Commonwealth Drive, Menlo Park, CA 94025

3 Exponent Inc., 1800 Diagonal Road, Suite 355, Alexandria, VA 22314

Article Outline

Total hip and knee replacements are widely recognized as being successful and effective for the treatment of degenerative joint disease. The patterns of treatment with primary hip and knee arthroplasty have previously been studied as a function of gender, race, hospital volume, or geography1-6. However, in each of these previous studies, the time frame of the analysis of primary procedures was limited to one to four years. Furthermore, many previous population-based studies of revision patterns in the United States have been limited to hip arthroplasty and have relied on a one or two-year sample of Medicare claims data2,5,7,8. Little information is available to indicate how the incidence of primary hip and knee arthroplasty as well as revision rates are changing over time in the United States.

Longitudinal analysis of administrative databases is useful not only to obtain a better appreciation of historical patterns of treatment with joint arthroplasty, but also to aid in prognostication. The American Academy of Orthopaedic Surgeons (AAOS), for example, has published projections of the rates of primary total hip and knee replacements through 2030 based on a combination of United States Census data and the number of procedures, stratified by age and gender, between 1996 and 1999 reported in the National Hospital Discharge Survey (NHDS)9. However, in the Academy's analysis, the prevalence of joint arthroplasty was assumed to be constant, with the average rate calculated from the four years of NHDS data. The projected growth in the number of hip and knee replacements in the Academy model was fueled solely by the anticipated growth of the United States population. Furthermore, revision hip and knee procedures were not examined in the Academy's analysis.

The purpose of this study was to quantify the revision burden in the United States and to evaluate whether the burden is changing over time. A revision burden can be considered in terms of procedural volume and economic burden. Defined as the ratio of revisions to the sum of revision and primary procedures, the revision burden has been calculated in European countries in which joint arthroplasty registries have been established10. We hypothesized that not only the number of procedures, but also the procedural rates, stratified by gender and age, have increased over time. To test these hypotheses, we analyzed NHDS data between 1990 and 2002 to determine the frequency and prevalence of primary and revision joint arthroplasties during those thirteen years. We sought to determine whether the historical increase in the annual number of both primary and revision arthroplasties can be explained on the basis of an increase in selected age or gender-based segments of the United States population.

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Materials and Methods

Data Sources

Data from the NHDS (described below) were used to assess the prevalence of total hip and knee arthroplasty in the United States. Since 1979, the Ninth Revision of the International Classification of Diseases (ICD-9) has been used in the United States to classify diagnoses and procedures. The procedure codes used for identification of primary total hip and knee arthroplasty (81.51 and 81.54) and revision total hip and knee arthroplasty (81.53 and 81.55) were introduced in the third edition of the ICD-9, which became effective on October 1, 1989. Primary hip hemiarthroplasties, coded separately as partial hip replacements (81.52), were excluded from the present analysis.

We validated our method of determining the frequency of surgery by comparing our values with those presented by Frankowski and Watkins-Castillo9 in their analysis of trends in arthroplasties. Using NHDS data from 1996 through 1999, we calculated the number of hip and knee arthroplasty procedures and discharges according to the same age categories (less than forty-five, forty-five to sixty-four, sixty-five to seventy-four, seventy-five to eighty-four, and eighty-five or more years old) as were used in the AAOS study9. We replicated exactly the results presented in Appendix A of the AAOS report.

NHDS data from 1990 through 2002 were employed for the present study. Although limited data on total hip and knee arthroplasty are available in the NHDS as far back as 1979, we started our analysis with data from 1990 because it is the first full year during which diagnostic codes for revisions were implemented in the NHDS. Thus, we could determine trends in procedural rates for both primary and revision arthroplasties beginning with that year.

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National Hospital Discharge Survey

The NHDS is an annual survey conducted by the National Center for Health Statistics. Started in 1965, this survey program has continuously compiled a representative sample of hospitalizations at non-federal and non-military short-stay community hospitals throughout the United States. Requirements for inclusion of a hospital in the survey include (1) an average length of stay of less than thirty days for all patients, (2) a general hospital or a children's general hospital, and (3) at least six beds staffed for patient use. Between 1990 and 2002, the number of hospitals included in the survey decreased from approximately 6400 to 6100. During this thirteen-year period, the number of hospitals sampled per year ranged from 500 to 540, and the number that responded to the survey ranged from 430 to 490, with approximately 300,000 discharge records sampled per year.

Information collected by the NHDS includes patient demographics (e.g., age and gender), disease diagnosis, type of procedure performed, institutional characteristics, and resource utilization. On the basis of the information collected by the survey and with use of the provided sampling weights, national and regional estimates of characteristics of patients and of surgical and nonsurgical procedures in hospitals with various numbers of beds and types of ownership can be estimated.

Along with the survey records, the sampling variability in the NHDS is provided in the form of an approximate relative standard error, which is used to construct 95% confidence intervals. In the computation of rates, in which the estimate of hospitalization is divided by a denominator (such as the population number provided by the United States Census Bureau), the population is assumed to have little or no variability. Therefore, the standard error of a rate is assumed to be identical to the standard error of the estimate.

In addition to the discharge records, companion data files containing the annual civilian resident population data according to age, gender, and race from 1979 through 2002 were provided by the National Center for Health Statistics. These population data are compiled by the United States Census Bureau on the basis of the 1980, 1990, and 2000 United States Census in conjunction with intra-Census projections and estimates from the Census Bureau's population estimation programs.

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Data and Statistical Analysis

The rate of hip or knee arthroplasty was calculated by dividing the number of NHDS-reported surgical procedures by the corresponding United States civilian resident population for a particular age and gender group and year. We also evaluated the change in procedural rate during the thirteen-year window of interest (1990 through 2002). The number of operations performed in a given year and for a particular age-gender group (e.g., women between the ages of seventy-five and eighty-four years) was assumed to follow a Poisson distribution, which was a suitable discrete distribution for frequency data such as the number of surgical procedures performed. A Poisson regression analysis, similar to the analysis employed by Mahomed et al.5, was used to estimate the magnitude of the annual change in the procedural rate (termed “rate ratio” in this study). The annual change in the procedural rate is represented by the “slope” of the regression derived from the regression analysis, which is equivalent to the “ratio” of two rates from adjacent years (hence “rate ratio”). If there is no change in the procedural rate over time, the overall slope, or rate ratio, will be 1.0. A rate ratio of >1.0 suggests a procedural rate that is increasing over time. Thus, by examining the magnitude of this rate ratio and testing it against the null value of 1.0, we could evaluate whether there were significant changes in procedural rates over the thirteen-year study period. The Poisson regression analysis also allowed other covariates, such as age and gender, to be introduced easily as a parameter for modeling stratified procedural rates.

The effects of gender and age on procedural rates of primary and revision total hip and knee arthroplasties were evaluated. The frequency of the four different arthroplasties from 1990 through 2002 was categorized by gender (male and female) and by age-group (less than forty-five, forty-five to sixty-four, sixty-five to seventy-four, seventy-five to eighty-four, and eighty-five or more years old). The annual procedural rates were subsequently determined from the ratio of the raw procedure counts per 100,000 persons in each gender or age-group. For each arthroplasty procedure, analysis of variance was used to investigate the effect of age-group (forty-five to sixty-four, sixty-five to seventy-four, and seventy-five to eighty-four years old) or gender in the presence of year-to-year variations (JMP 5.1; SAS Institute, Cary, North Carolina). Although data for the age-groups of less than forty-five years old and eighty-five years old or more are included in the NHDS, we did not analyze or further interpret those data because they were limited. Year-matched pairwise comparison was used to further compare the procedural rates between the different gender and age-groups.

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The annual revision burden was derived by calculating the percentage of revision arthroplasties relative to the total number of primary and revision arthroplasties10. This parameter was calculated for both hip and knee arthroplasties and was stratified by both age and gender. Simple linear regression, with the year as the independent variable, was used to evaluate the year-to-year change in the revision burden.

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The installed patient base for overall hip and knee replacements from 1979 through 1990 and from 1979 through 2002 was calculated to determine the cumulative number of prostheses that had been implanted in the population up to that particular time point. The installed base thus refers to the cumulative total number of patients in whom a primary prosthesis had been implanted during a specified time period. The installed base for hip arthroplasty was compared with that for knee arthroplasty.

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Results

Both the number and the rate of hip and knee arthroplasties (particularly knee arthroplasties) increased steadily between 1990 and 2002 (Figs. 1-A, 1-B, 2-A, 2-B, and Appendix). The rate of primary total hip arthroplasties per 100,000 persons increased by 46% and the corresponding rate of primary total knee arthroplasties almost tripled during the thirteen-year period. The rates of revision total hip and knee arthroplasties increased by 60% and 166%, respectively. The total number of revision hip procedures almost doubled and the total number of revision knee procedures tripled between 1990 and 2002.

The increase in the prevalence of primary total knee arthroplasty in the overall United States population between 1990 and 2002 was about four times the increase in the prevalence of total hip arthroplasty (Fig. 2-A). The number of primary total hip arthroplasties increased from 119,000 in 1990 to 193,000 in 2002, while the number of primary total knee arthroplasties increased from 129,000 in 1990 to 381,000 in 2002. With the population according to the United States Census Bureau taken into account, the overall rate of primary total hip and knee arthroplasties was fifteen and fifty-nine procedures per 100,000 persons per decade, respectively (Fig. 2-A).

A general trend for an increase in both the number and the rate of revision arthroplasties was also observed (Figs. 1-B and 2-B), although the differences between the frequencies of revision hip and knee procedures were not as marked as the differences between the frequencies of primary hip and knee arthroplasties. The number of revision total hip arthroplasties increased from approximately 24,000 in 1990 to 43,000 in 2002, while the number of revision total knee arthroplasties increased from approximately 12,000 in 1990 to 35,000 in 2002. With the population according to the United States Census Bureau taken into account, the overall rate of revision total hip arthroplasties was 3.7 procedures per 100,000 persons per decade compared with a rate of 5.4 procedures per 100,000 persons per decade for revision total knee arthroplasty.

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Analysis of variance showed age (p < 0.0001) and gender (p ≤ 0.033) to be significant in the presence of year-to-year variations in the rates of all four procedures. The rates of all four procedures were significantly higher for women than for men (p < 0.05): the rates of primary total hip and knee arthroplasties were 36% and 67% higher, and those of revision total hip and knee arthroplasties were approximately 30% higher (Figs. 3-A and 3-B and Appendix). The rates of all procedures in the sixty-five to seventy-four-year-old age-group and the seventy-five to eighty-four-year-old age-group were significantly higher than those in the forty-five to sixty-four-year-old age-group (p < 0.01) (Figs. 4-A and 4-B and Appendix). Also, the rates of primary total hip arthroplasty and revision total hip and knee arthroplasties in the seventy-five to eighty-four-year-old age-group were significantly higher than those rates in the sixty-five to seventy-four-year-old age-group (p < 0.01). For example, the rates of primary total hip arthroplasty for people between sixty-five and seventy-four years old and those between seventy-five and eighty-four years old were 3.6 and 4.5 times greater than the rate for people between forty-five and sixty-four years old. The rate of primary total hip arthroplasty for people between seventy-five and eighty-four years old was 24% higher than that for people between sixty-five and seventy-four years old (Fig. 4-A). Significant differences in the rates of primary total knee arthroplasty were found between all age-groups, except between the sixty-five to seventy-four-year-old group and the seventy-five to eighty-four-year-old group (Fig. 4-B). The rates of revision total hip and knee arthroplasties for people between forty-five and sixty-four years of age were 3.0 to 5.5 times lower than those for the remaining age-groups, whereas the rates for the seventy-five to eighty-four-year-old age-group were 66% (revision total hip arthroplasty) and 39% (revision total knee arthroplasty) higher than those in the sixty-five to seventy-four-year-old age-group (see Appendix).

The rate ratios of both hip and knee and primary and revision arthroplasties either increased or remained somewhat constant over time (e.g., ≥1.0), and no rate decreased significantly (see Appendix). In all age and gender groups, the rate of primary total knee arthroplasty increased significantly over time (p < 0.01), with the year-to-year increases ranging from 4% to 11% depending on age and gender. The largest increases, for both genders, were in the less than forty-five and the forty-five to sixty-four-year-old age-groups.

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Overall, when we considered all data collected over the thirteen-year period, we found the mean revision burden for total hip arthroplasty (17.5% [annual range, 15.2% to 20.5%]) to be more than twice that for total knee arthroplasty (8.2% [annual range, 7.3% to 9.7%]). The linear regression model of year-to-year change in revision burden (Table I) showed that the revision burden for total hip arthroplasty remained consistent for men, except for those in the forty-five to sixty-four-year-old age-group, in which there was a significant annual decrease of 0.5% (95% confidence interval, -1.00 to -0.05). The revision burden for total hip arthroplasty was consistent over time for all women of all ages except those who were eighty-five years of age or older, who demonstrated a significant annual increase in revision burden of 1%. There was no significant change in the revision burden for total knee arthroplasty over time for men in any age-group, and there was no change for women in any age-group except sixty-five to seventy-four years old (0.5% annual increase). The rate of revision total knee arthroplasty for women was higher than that for men in all but one age-group (forty-five to sixty-four years old) (Table I).

The installed base for hip arthroplasty was 2.17 million from 1979 through 1990 and 5.49 million from 1979 through 2002. Similarly, the installed base for knee arthroplasty was 0.91 million from 1979 through 1990 and 4.15 million from 1979 through 2002.

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Discussion

Our data indicate that between 1990 and 2002, the number and prevalence of primary hip and knee replacement procedures increased substantially in the United States. The trend was considerably more pronounced for primary total knee arthroplasty, the prevalence of which almost tripled during the study period. The explosive growth in both the number and the prevalence of primary total knee arthroplasty for both genders and all age categories can be explained, at least in part, by the increased recognition, by the candidates for surgery and the members of the orthopaedic community, of the effectiveness of the procedure in treating degenerative disease of the knee. Furthermore, the importance of this treatment modality was reflected in the National Institutes of Health (NIH) Consensus Development Conference on Total Knee Replacement in 200311. In addition, there is a strong relationship between obesity and the risk of osteoarthritis, especially in the knee12,13. Recent epidemiological research has indicated that the prevalence of obesity in the United States has increased over the past decade14,15, which may help to explain the increase in total knee arthroplasty rates. Although we could not directly correlate the prevalence of obesity and the prevalence of joint replacement on the basis of the data available in the NHDS, the major increase in the population of obese patients in the United States appears to contribute to the increasing trends in primary joint replacement observed in this study.

The number and prevalence of revision total hip and knee arthroplasties also increased substantially in the United States between 1990 and 2002. The number of primary and revision hip procedures increased by 62% and 79%, respectively, and the number of both primary and revision knee procedures increased by approximately 200%. Thus, the increasing number of revision knee procedures appears to be fueled by the growth in primary procedures.

The discrepancy between overall hip and knee revision burdens in the United States may be partly explained by differences in the sizes of the patient populations treated with each type of joint replacement. In 1990, the patient population in which a hip prosthesis was implanted (2.17 million) was 2.4 times greater than that in which a knee prosthesis was implanted (0.91 million), so there was a greater pool of hip implants from which revisions may result. This partly explains the greater hip revision burden (16.7%) compared with the knee revision burden (8.4%) in 1990. In 2002, the installed base for hip prostheses was only 1.3 times that of the base for knee prostheses. However, the hip revision burden (18.1%) remained more than twice that of the knee revision burden (8.4%), which suggests that this difference may only be partially explained by differences in the patient populations treated with the different implants.

With the exception of total hip arthroplasties performed in Sweden, which will be discussed below, the revision burden in the United States compared favorably with that in several countries with established total joint registries (Table II). Overall, the total hip arthroplasty revision burden of 17.5% in the United States from 1990 through 2002 fell within the range of revision burdens of 15.0% to 18.3% observed in Norway, Finland, and Australia16-18. In Canada, the revision burden for total hip arthroplasty was somewhat lower (13.1% for 2002 to 200319). Similarly, the United States revision burden for total knee arthroplasty of 8.2% was comparable with the revision burdens of 7.2% to 8.0% for Norway, Sweden, and Finland16,18,20. The Canadian knee revision burden was slightly lower (6.1% for 2002 to 2003)19, whereas the Australian knee revision burden was slightly greater (10.8% for 1999 through 2002)17.

The overall revision burden for total hip arthroplasty in the United States was substantially greater than the revision burden of 7.7% to 11% reported for Sweden10,21 (Table II). The differences between the Swedish and United States hip revision burdens may be due to the differences in prosthetic survivorship in the elderly population with hip implants. The revision hip burden (from 1992 through 2000) of 6.4% for the elderly population (more than sixty-five years old) in Sweden was substantially lower than the burden of 11% for the general Swedish population. Because the majority of patients requiring total hip arthroplasty are elderly, the hip revision burden for the overall population will be significantly affected by the revision burden for elderly patients. Conversely, the revision hip burden from 1990 through 2002 for the elderly population in the United States (16.9%) was similar to that for the general population (17.5%).

In the United States, the revision burden for hip arthroplasty was approximately twice that for knee arthroplasty from 1990 through 2002, a finding that is consistent with the differences in hip and knee revision burdens reported in Finland, Norway, Canada, and Australia. Conversely, the Swedish hip-to-knee revision burden ratio was essentially unity. The steady trend in revision burden over the years is especially disconcerting when the installed patient base continues to increase, as was observed in the United States between 1990 and 2002. This is particularly true in the case of total knee replacements.

To our knowledge, this is the first population-based study, based on NHDS data, of the rates of revision of both total hip and knee replacements. The procedural rates of primary and revision hip arthroplasty between 1995 and 1996 (Figs. 2-A and 2-B) were consistent with those previously calculated for the same time period in a Medicare population-based study5. In the present study, year-matched procedural rates for men and women were significantly different for all operations except for revision total knee arthroplasty. The procedure rates for women were generally greater than those for men. The small differences between the rates determined in our study and the rates reported by Mahomed et al.5 are due to differences in the data source, exclusion criteria, and age-group stratification.

The hip and knee revision burdens identified in this study have important implications for health costs in the United States. For example, if the 2002 hip revision burden of 18.1% was reduced by 1% (a decrease of approximately 2844 hip revision procedures), the potential cost savings could range from $42.5 million to $112.6 million, on the basis of recent procedural cost estimates of $15,000 to $40,000 for revision total hip arthroplasty7,8,22,23. Similarly, the potential cost savings for a 1% reduction in the knee revision burden (which was 8.4% in 2002) could range from $53.5 million to $98.4 million, on the basis of a decrease of approximately 4497 knee revision procedures and procedural cost estimates of $11,900 to $21,90022,23. Hence, a 1% decrease in the revision burden for both hip and knee arthroplasty would translate into an overall savings of between $96.0 million and $211.0 million for the United States health care system.

Although NHDS data for the age-groups of less than forty-five years old and of eighty-five years old or more were included in the present analyses, conclusions about age effects should be limited to the remaining age-groups (forty-five to eighty-four years old). The small sample sizes (less than thirty patients) resulted in large data variability within these two age-groups. The use of the National Inpatient Sample, which has a substantially (twenty-five times) larger number of sampled records than does the NHDS, may alleviate the limitations regarding the ability to analyze age effect with use of the NHDS data. However, the AAOS used NHDS data to make projections of total knee arthroplasty and total hip arthroplasty discharges, within the same age-groups and for the same procedures that we studied, so we used NHDS data for consistency and comparative purposes. In addition, since the number of years (ten to twenty) spanned by each age classification is large compared with the total thirteen-year time period evaluated in this study, the study may have had some limitations associated with demographic changes within the age-groups themselves. Perhaps, if we had evaluated smaller age-groups (e.g., spanning five years) among the elderly, we would have been better able to capture those differences.

Another limitation of the study pertains to the system for coding of the type of arthroplasty. Although the number of total knee replacements far exceeds the number of unicondylar knee replacements, the current ICD-9 coding system does not allow detailed classification of knee arthroplasty. As surgical techniques improve and the demand for less invasive surgery and shorter recovery time increases, the number of unicondylar knee procedures may increase substantially. Such an increase would require clearer differentiation between total and unicondylar knee procedures in order to better assess the clinical and economic impacts of the two procedures. Also, primary partial hip replacements (hemiarthroplasties) were excluded from the present study to limit the comparisons to primary total and revision arthroplasties.

As noted in previous studies5,24, analysis of total joint replacements with the use of data derived from administrative claims may be limited by random coding inaccuracies as well as by the more serious problem of systematic bias (e.g., underreporting), which are inherent in the design and implementation of claims databases. In the present study, we focused on joint replacement procedures, not the underlying diagnoses of the patients receiving them. Furthermore, coding inaccuracies may be considered as random uncertainty, which was accounted for in our statistical analysis of the entire thirteen-year NHDS data set.

We validated our procedure counts by using the four-year NHDS data (for 1996 through 1999) that had been previously reported by the AAOS9, and we confirmed that we were accurately estimating the national total-joint-replacement rates on the basis of the available data. The accuracy of NHDS-based national projections of joint arthroplasty procedure counts has been verified by the Department of Health and Human Services with use of the independent National Inpatient Sample (NIS) database25. The discrepancies between national estimates of hip and knee arthroplasty rates based on 1995 NHDS and NIS data were 9% and 6%, respectively26. Consequently, the systematic bias in population-based estimates of total hip and knee arthroplasty rates based on NHDS data was judged to fall within an acceptable margin of uncertainty (≤10%) for the type of analyses performed in this study.

Previous investigators have raised concerns about the implantation of joint replacements in younger patients27. Our results showed that the rates of primary hip and knee procedures were highest in people between the ages of sixty-five and eighty-four years. Similar trends were observed for revision hip and knee replacements, with the greatest prevalences in people between the ages of sixty-five and eighty-four years. Although we found little evidence to suggest that the increases in the prevalence of joint replacements between 1990 and 2002 were due to a disproportionate increase in the prevalence of patients younger than sixty-five years old, an evaluation of the raw procedure counts suggested otherwise. In the population of patients treated with primary total hip arthroplasty between the ages of forty-five and eighty-four years, the relative proportion who were between forty-five and sixty-four years old increased from 28% to 40% between 1990 and 2002. In the population of patients treated with primary total knee arthroplasty between the ages of forty-five and eighty-four years, the relative proportion who were between forty-five and sixty-four years old increased from 26% to 36% between 1990 and 2002. However, a similar relative increase was not observed in the relative proportion of patients between the ages of forty-five and sixty-four years undergoing revision total hip arthroplasty (36% in 1990 and 32% in 2002) or revision total knee arthroplasty (49% in 1990 and 25% in 2002). The discrepancy between the relative proportion and the prevalence (procedure rate) of primary total hip and knee arthroplasties among the younger patients is masked by the 42% increase in the forty-five to sixty-four-year-old patient population from 1990 to 2002 compared with the 1% and 26% increases in the sixty-five to seventy-four-year-old and seventy-five to eighty-four-year-old patient populations. Hence, the increasing number of procedures performed on younger patients is not reflected by the population-normalized prevalence (procedure rate). However, the use of procedural prevalence is relevant to account for year-to-year variation in patient populations. Future studies will be done to examine age and gender-stratified procedure counts in more detail.

The prevalence trends reported in the current study have important ramifications with regard to the number of hip and knee replacements expected to be performed in the future. The projected increase in the number of hip and knee replacements in a previous AAOS model9 was fueled solely by the anticipated growth of the United States population and was based on a constant prevalence of arthroplasty for a four-year window of data reported in the NHDS. However, our study has demonstrated that the mean procedural rates from 1996 through 1999 underestimate the future total hip and knee arthroplasty rates, as evidenced by the rapidly increasing rates of both primary and revision hip and knee arthroplasties between 1990 and 2002. Consequently, the projections of future primary joint replacement rates reported by the AAOS should be interpreted as a conservative lower boundary if the prevalence trends observed during the thirteen-year time-period that we studied continue forward in time indefinitely. Because the revision burden has been relatively constant between 1990 and 2002, we can expect that a greater number of primary replacements will, in turn, result in a greater number of revisions unless some limiting mechanism can be implemented to reduce the future revision burden.

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Appendix

Tables representing the overall procedure counts and rates and the annual changes in rate ratios based on age and gender as well as figures depicting procedural rates of revision arthroplasties stratified by gender and by age-group are available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).

Investigation performed at Exponent Inc., Philadelphia, Pennsylvania

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

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