Abstract
Background: Several studies have shown that patients who undergo
total hip replacement live longer than control subjects, but the reason for
this apparent protective effect is not clear. The purpose of our investigation
was to assess whether the association had the characteristics of a causal
relationship or whether it might appear to be due to comorbidity or other
factors.
Methods: We compared survival over a six-year period for 28,469
Medicare enrollees who had elective primary total hip replacement in 1996 and
a control group from the general Medicare population, matched 5:1 on year of
birth, sex, race, and whether the Medicaid program paid the Medicare premium
(a proxy for low income). For both the patients treated with total hip
replacement and the control subjects, we used Medicare hospital claims to
ascertain comorbidity, that is, whether the patient had had any of sixteen
serious discharge diagnoses in the year prior to the total hip replacement (or
an equivalent date for the controls). The survival patterns for the total hip
arthroplasty and control cohorts were compared with use of proportional
hazards regression for three follow-up periods: the first ninety days after
surgery (or an equivalent date for the controls), ninety days to five years
after surgery, and more than five years after surgery.
Results: The total hip replacement cohort had less comorbidity than
the controls, with an approximately 30% lower prevalence for most serious
diseases. Sex, age, Medicaid eligibility, and earlier serious comorbid
diagnoses were important predictors of survival, but, even in analyses that
adjusted for these variables, the total hip replacement cohort had better
long-term survival than the controls. The patients who had a total hip
replacement had a higher rate of mortality than the controls immediately after
surgery, but, by three months postoperatively, the mortality rate for the
patients was well below that for the controls. The overall rate of survival
during the first ninety days was comparable for the two groups. From three
months to five years after surgery, the mortality rate for the patients who
had a total hip replacement was only two-thirds of that for the controls.
After five years, the mortality rates for the two groups appeared to be
converging.
Conclusions: Total hip replacement recipients survive longer than do
matched controls in the Medicare population. The very rapid emergence of the
lower mortality rate suggests that it is due to the selection of low-risk
patients for elective surgery. However, our admittedly crude adjustment for
comorbidity did not diminish the protective effect, so some effect of the
procedure itself cannot be ruled out.
Level of Evidence: Therapeutic Level III. See
Instructions to Authors for a complete description of levels of evidence.
Total hip replacement is a paradigmatic quality-of-life
intervention. It is not intended to increase the quantity of life
(survival), but it does dramatically improve the quality of life.
Nonetheless, Scandinavian researchers have found that the long-term survival
of patients who have had primary total hip replacement is better than that for
the general
population1-6,
and they have speculated that the reason may be patient selection for the
total hip replacement, the use of anti-inflammatory drugs among patients with
arthritis, a more active lifestyle after surgery, or some other factor. A few
studies from the United States have described similar
findings7-9,
but those studies had short-term
follow-up7,8
or the patients were from a single surgical
center9. Few of the
studies had an internal control group, and none that we are aware of tried to
take into account the comorbidity status of both the patients and the
controls.
In this report, we compared the six-year survival rate in a cohort of
Medicare recipients who had total hip replacement in 1996 with that of a
matched cohort of enrollees from the general Medicare population. We
hypothesized that patients undergoing total hip replacement would have less
comorbidity than controls and would also have better long-term mortality. We
aimed to assess whether the association between total hip replacement and
survival had the characteristics of a causal relationship or whether it might
appear to be due to comorbidity or other factors.
Total Hip Replacement Cohort
We defined the total hip replacement cohort by using Medicare claims
submitted by United States hospitals and surgeons for procedures performed in
the six months from January 1 to June 30, 1996. A hospital claim with an
ICD-9-CM (International Classification of Diseases, Ninth Revision, Clinical
Modification) procedure code of 81.51 (total hip replacement) was required,
together with a surgeon's claim in the same admission with a CPT (Common
Procedural Terminology) code of 27130 (arthroplasty, acetabular and proximal
femoral prosthetic replacement). Patients undergoing total hip replacement for
fracture of the hip or other femoral fracture were omitted, as we wished to
focus on elective procedures. These femoral fractures were identified by a
hospital ICD-9-CM discharge diagnosis code 820-821.11; a hospital ICD-9-CM
procedure code 79.05, 79.15, 79.25, or 79.35; or a surgeon's claim with a CPT
code 27230-27248 or 27500-27507. We excluded patients enrolled in health
maintenance organizations, those not enrolled in both Part A and Part B of
Medicare, and patients who were not residents of the United States, as claims
for these patients are often incomplete. We also excluded beneficiaries who
were more than ninety-nine years old, since very few centenarian enrollees
receive total hip replacement. To ensure that we had at least a year of
Medicare claims before the total hip replacement, we omitted patients who were
in the first year of Medicare eligibility (that is, the age of sixty-five).
There were 28,469 primary total hip replacements performed in patients between
the ages of sixty-six and ninety-nine years in the final cohort.
Comparison Cohort
To select a comparison cohort of patients from the general United States
Medicare population, we randomly allocated to all enrollees in the standard 5%
Medicare sample the procedure date of one of the total hip replacement cohort
members. If the population member was alive on the day before his or her
allocated index date and was between sixty-six and ninety-nine years old, a
resident of the United States, enrolled in both parts of Medicare, and not
enrolled in a health maintenance organization, then he or she was retained.
This procedure identified 1.28 million Medicare enrollees as potential
controls. From this pool, the control cohort was randomly chosen and
frequency-matched to the patients who had a total hip replacement in a 5:1
ratio by sex, year of birth, race (black, white, or other and/or unknown), and
whether the state Medicaid program paid the Medicare premiums for the patient
(a marker of poverty). The comparison cohort thus comprised 142,345 matched
controls for the 28,469 patients in the total hip replacement cohort
(Table I).
Follow-up
The annual Medicare denominator files contain the date of death of all
enrollees who died during the year in question. All total hip replacement
patients and controls were followed with use of these denominator files from
the total hip replacement (or control index) date until death or December 31,
2001. Thus, all surviving enrollees were followed for a minimum of five and a
half years and a maximum of six years.
Comorbidity Adjustment
We examined the discharge diagnoses from the Medicare hospital claims files
for the 365 days before the date of the total hip replacement or the control
index date and ascertained for each subject whether each of sixteen serious
hospital diagnoses had been made. These diagnoses are listed in the Appendix,
and they are the constituent elements of the Charlson comorbidity
index10,11.
This index is a single number formed by a weighted sum of the diagnoses, with
the set of weights calculated according to their ability to form a single
summary index that predicts death within a year in the Charlson datasets.
Statistical Analyses
The numbers and percentages of patients managed with a total hip
replacement and of controls who had had each of the hospital diagnoses were
counted, and odds ratios with 95% confidence intervals were constructed. We
used actuarial life-table methods to compare the mortality experience of the
patients managed with total hip replacement and the controls over time. Two
functions of survival were calculated. The familiar "survival
curve" plots the cumulative probability of survival as a function of an
increasing duration of follow-up. By definition, this probability cannot
increase. The monthly hazard function, on the other hand, is an estimate of
the death rate within a specified month of follow-up, for those who have
survived to the beginning of that month. Unlike the cumulative survival curve,
this death rate may fluctuate up and down over time. The overall survival
experience of the two groups was compared with use of the log-rank test. The
actuarial life-table technique, survival functions, and log-rank test are
discussed in standard statistical
texts12,13.
A proportional hazards model was created for each of the sixteen comorbid
diagnoses, to assess the separate effect of each diagnosis on mortality. Each
of these models contained the variables total hip replacement versus control,
age, sex, race (black or white subjects only), and Medicaid status as well as
the diagnosis in question.
We then constructed two proportional hazards models to examine whether
total hip replacement had an independent effect on time to death. The first
analysis included adjustment for sex, race (black or white subjects only),
age, and Medicaid eligibility. In the second model, the Charlson comorbidity
index was added. In both models, we included an interaction between the cohort
type (total hip replacement or control) and the time after the total hip
replacement date (or control index date). We estimated the relative risk of
death for the two groups within three time-intervals: less than three months
after surgery, three months to five years after surgery, and more than five
years after surgery. Such interaction is a time-dependent
covariate13,14.
All analyses were carried out with use of the SAS statistical software,
release 8.2 (SAS Institute, Cary, North Carolina).
Cohort Descriptions
Table I summarizes the demographic characteristics of the total hip
replacement and comparison cohorts. Nearly two-thirds of each cohort (18,495
total hip replacement patients and 92,475 control subjects) were women, 95%
(27,018 total hip replacement patients and 135,090 controls) were white, and
6% (1683 total hip replacement patients and 8415 controls) had their premiums
paid by state Medicaid programs. The mean age (and standard deviation) at the
time of the total hip replacement or at the index date was 74.8 ± 5.8
years for both cohorts.
As shown in the Appendix, virtually identical percentages—3490
(12.3%) of the total hip replacement patients compared with 17,944 (12.6%) of
the controls—had had a hospitalization in the year before the index date
with one or more of the comorbid discharge diagnoses listed in the Charlson
index. Nevertheless, there was clear evidence that the control beneficiaries
had worse health than the total hip replacement recipients. The controls were
much more likely than the patients managed with total hip replacement to have
multiple diagnoses; they were substantially more likely to have twelve of the
sixteen diagnoses. Only peptic ulcers (odds ratio, 1.69; 95% confidence
interval, 1.49 to 1.91) and rheumatologic disease (odds ratio, 2.67; 95%
confidence interval, 2.34 to 3.04) were more common in the total hip
replacement cohort.
Overall Survival of the Total Hip Replacement and Comparison
Cohorts
Figure 1 shows the survival
curves for the patients who had primary total hip replacement and the control
subjects. Immediately after surgery, the total hip replacement cohort
experienced greater mortality than did the controls; however, by ninety days,
309 patients who had a total hip replacement and 1603 controls (1% of each
cohort) had died, and the survival curves crossed. The total hip replacement
cohort had considerably better survival than did the controls thereafter. By
five years of follow-up, another 32,022 (22%) of the controls had died
compared with 4547 (16%) of the patients who had total hip replacement. In the
sixth year after surgery, just over 3% (924 patients who had total hip
replacement and 4938 control subjects) in each group died, showing that very
little of the advantage of the total hip replacement patients remained.
Altogether, after six years, the probability of survival was 79% for the total
hip replacement cohort compared with 72% for the controls (log-rank, p <
0.0001).
Figure 2, the hazard
function, shows the changes in mortality rates over time. It plots the monthly
probability of death among the subjects in each cohort who have survived to
the beginning of that month and is an estimate of the death rate within that
particular month. For the total hip replacement cohort, the probability of
dying in the first month after surgery was much higher than it was for the
comparison cohort, but it fell abruptly and markedly thereafter. Within three
months after surgery, it had dropped to well below the level of the controls.
In subsequent months, the mortality of the total hip replacement cohort
increased fairly steadily and approached that of the controls after
approximately five years. For the comparison cohort, the probability of death
showed monthly fluctuations around a pattern of increasing mortality with
time, an expected pattern for deaths in an aging population.
Multivariate Analyses
Demographic factors had strong effects on survival (see Appendix). Overall,
female subjects were much less likely to die than were male subjects (hazard
ratio, 0.63; 95% confidence interval, 0.62 to 0.64). Subjects who were
Medicaid-eligible had a considerably poorer survival rate than did those with
higher incomes (hazard ratio, 1.87; 95% confidence interval, 1.80 to 1.93).
Elderly subjects were much more likely to die than were younger Medicare
recipients. As expected, each of the comorbid diagnoses also had a strong
effect on mortality. Earlier hospital diagnoses of metastatic cancer, renal
failure, liver disease, and leukemia and/or lymphoma in particular carried
hazard ratios of 4.
Table II shows the mortality
ratios for the total hip replacement patients relative to the controls,
calculated with and without adjustment for comorbidity. After adjustment for
age, sex, race, and Medicaid eligibility, the total hip replacement cohort had
a better survival rate than the controls. For the initial three-month period,
the risks of death for the two groups were comparable (hazard ratio, 0.95; 95%
confidence interval, 0.84 to 1.07), suggesting that by ninety days after total
hip replacement, the early deaths related to surgery were already
counterbalanced by the favorable survival rate of the total hip replacement
group. For the period of ninety days to five years after surgery, the hazard
ratio was 0.65 (95% confidence interval, 0.63 to 0.67), indicating much better
survival for the total hip replacement cohort. In the final year of follow-up,
the hazards had begun converging (hazard ratio, 0.82; 95% confidence interval,
0.76 to 0.87).
Even though the comorbid diagnoses were important predictors of mortality,
adding the Charlson comorbidity index to the model did not explain the
survival difference between the two cohorts
(Table II). In the first three
months following surgery, the mortality risks for the total hip replacement
patients and the controls were still comparable (hazard ratio, 0.97; 95%
confidence interval, 0.86 to 1.10). The estimated hazard ratio for the period
of three months to five years after surgery remained at 0.65 after comorbidity
adjustment, and the estimated hazard ratio in the last year was 0.81 with
comorbidity adjustment compared with 0.82 without adjustment. Similarly,
adding all of the component diagnoses of the Charlson index as separate
variables, instead of the index itself, had little effect on the hazard ratio
(data not shown).
We compared the survival of a cohort of 28,469 United States
Medicare beneficiaries who had elective total hip replacement with 142,345
controls from the general United States Medicare population. We found that the
total hip replacement cohort had less major comorbidity than did the matched
controls, with an approximately 30% lower prevalence for most serious
diseases, as reflected in hospital discharge diagnoses in the year before
surgery.
As might be expected after any major surgical procedure, the total hip
replacement patients had a higher mortality rate than the typical Medicare
patients in the first month after the procedure. However, the mortality rate
in the total hip replacement group then dropped precipitously, and, by ninety
days, the cumulative survival in the two groups was comparable. Subsequently,
the mortality rate in the total hip replacement group was lower than that in
the control group, although the mortality rates appeared to be converging
after about five and a half years.
The low mortality rate among total hip replacement patients persisted in
our analysis even after adjustment for sex, age, race, Medicaid eligibility,
and major comorbid diagnoses as captured on hospital discharge data. It is
possible that this survival advantage could be due to the surgery. After the
procedure, the use of nonsteroidal anti-inflammatory drugs and narcotic pain
medications typically decreases and patients are able to exercise
again—factors that could improve their health. Also, the fact that they
were once again in control of their own lives could conceivably affect their
life
expectancy15.
The mortality differential could also be due to unmeasured differences
between the total hip replacement and control populations rather than to the
effects of the procedure itself. We were able to use only hospital discharge
diagnoses recorded on Medicare claims for the year before the index date. Only
about 20% of each cohort (5966 total hip replacement patients and 25,132
controls) had a hospitalization in this one-year period, and it is likely that
some of the major medical problems were missed. We also had no information on
smoking status, body mass index, and other factors that have strong effects on
mortality. Access to the patients' medical records, to earlier hospital
claims, or even to patient self-reports would have increased our knowledge of
the comorbid
conditions16-18.
There was some evidence suggesting that the mortality differential we
observed was due to patient-related factors rather than to the effects of the
surgery. The real effects of a procedure such as total hip replacement are
likely to require some time to become evident, whereas we observed a
startlingly rapid improvement, with mortality rates falling substantially
below those of the matched Medicare controls within three months
(Fig. 2). A reasonable
interpretation of this pattern is that, after the (small) perioperative
mortality rate had run its course, the observed low death rates simply
reflected the inherently low mortality rate of the patients selected for
surgery. These differences may be reinforced by the preoperative evaluation
process, as the medical history, physical examination, laboratory tests, and
electrocardiogram serve as a screen to uncover potentially
life-threatening conditions in time to treat them or to suggest that
total hip replacement is not appropriate. Thus, patients who go on to have a
total hip replacement have passed such a selection screen.
Our observations confirm and extend the observations made in the
Scandinavian studies and in the earlier American studies that patients who
have had a primary total hip replacement have a better survival rate than a
matched population after the perioperative
period1-6,8,9.
One previous study also compared mortality in total hip replacement patients
and controls in different time-periods after surgery and found, as we did,
that the differences in mortality rate lessened over time. Among 24,638
Finnish total hip replacement recipients, the mortality rate was 44% lower
than that of the general population for up to five years after total hip
replacement and was 16% lower after five years or
more1. After six
years, approximately 20% of our total hip replacement patients had died, the
same percentage as reported for patients who were sixty-five years old or more
who had elective total hip replacement in
Sweden4.
The major limitation of our study is the use of administrative data.
Medicare inpatient claims data are sparse on clinical details, and comorbidity
is a key potential confounding factor in this study. Our hospital-claims-based
diagnoses were only a crude measure of major comorbidity, in part because only
about 20% of the patients (5966 total hip replacement patients and 25,132
control subjects) had any hospitalization in the year prior to surgery.
Further research should be done with use of data that permit more detailed
comorbidity assessment.
The strengths of the study include the large sample size and virtually
complete follow-up for at least five and a half years for both cohorts. Our
control cohort was taken from the Medicare population rather than from
published life tables, and so we were able to match simultaneously on sex,
year of birth, race, and also on Medicaid status. Enrollees whose state
Medicaid program pays the Medicare premiums are a third less likely to receive
a total hip replacement, and nearly twice as likely to die in the first ninety
days after total hip replacement, than are other
enrollees19, so we
controlled for this important confounder by matching. We were also able to use
a series of comorbidity measures, however imperfect, for both patients and
controls.
We concluded that older persons who have elective total hip replacement
have a substantially higher six-year survival rate compared with controls
matched for age, sex, race, and income status. We found consistent differences
in major comorbidity between total hip replacement recipients and control, but
the improved survival rate for the total hip replacement cohort persisted
despite our admittedly crude adjustment for measured comorbidity. The survival
differences emerged so rapidly after the procedure that they seem to reflect
inherent characteristics of the patients, such as unmeasured comorbidity or
the effects of selection for elective surgery. However, an effect of the
procedure itself cannot be ruled out. Additional studies should attempt to
identify the mechanisms that account for the low mortality rate in total hip
replacement recipients.
Tables presenting details of comorbidities and the proportional hazards
models 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). ?
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