Abstract
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
(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).
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
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