Abstract
Background: Preoperative psychological distress has been reported to
be an important risk factor for poor outcome following lower-extremity
arthroplasty. We determined the independent impact of preoperative
psychological distress on three, twelve, and twenty-four-month WOMAC (Western
Ontario and McMaster Universities Osteoarthritis Index) pain and function
scores and on change scores over those time periods.
Methods: Data were obtained from an international group of 952
patients in thirteen centers participating in the Kinemax Outcomes Study.
Patients completed the WOMAC and Short Form-36 (SF-36) questionnaires. The
mental health (MH) scale of the SF-36 was used to quantify the impact of
psychological distress on WOMAC pain and function scores. We also dichotomized
patients into groups with and without psychological distress on the basis of
evidence-based cut-points. Repeated-measures models were used to derive mean
preoperative and three, twelve, and twenty-four-month WOMAC pain and function
scores and general linear models were used to derive change scores for
patients with and without psychological distress after adjustment for
covariates.
Results: Psychological distress, when examined on a continuous
scale, was found to predict pain and function at all time-points. WOMAC pain
scores for psychologically distressed patients were 3 to 5 points lower,
depending on the time-frame, than the scores for the non-distressed patients,
after adjustment for covariates. WOMAC function scores did not differ
significantly between the two groups following surgery. The changes in the
WOMAC pain and function scores for the psychologically distressed patients
were not significantly different from those for the non-distressed
patients.
Conclusions: Many patients with psychological distress demonstrate a
substantial decrease in that distress following surgery. Patients who are
distressed have slightly worse pain preoperatively and for up to two years
following knee arthroplasty as compared with patients with no psychological
distress. With the exception of preoperative scores, these differences are not
likely to be measurable at the individual patient level. WOMAC pain and
function change scores do not differ between patients with and without
distress after adjustment for covariates.
Level of Evidence: Prognostic Level I. See Instructions
to Authors for a complete description of levels of evidence.
The influence of psychological distress on outcomes of procedures in
patients with musculoskeletal disorders has received increased attention in
recent
years1,2.
Psychological distress is a term used to describe a broad array of
psychological symptoms including depression, poor coping, anxiety, and
somatization3.
Authors of recently published studies have examined the impact of
psychological distress on patients undergoing either total hip or total knee
arthroplasty4-8.
Generally, studies have demonstrated that approximately 25% of patients
undergoing hip or knee arthroplasty have preoperative distress that negatively
impacts preoperative and postoperative physical function and pain. Given that
joint replacement surgery is a common procedure (approximately 780,000 hip and
knee replacements were performed in the United States in
20039), the role and
impact of psychological distress appear to warrant additional study. Ethgen et
al., in a seminal review of arthroplasty outcome studies, argued for more
studies designed to determine the extent to which psychological profiles of
patients influence the outcomes of joint replacement
surgery10.
A variety of methods have been used to identify patients who are
psychologically distressed before a planned hip or knee replacement. To
quantify psychological distress, most investigators have used a surrogate
measure of general distress, either the mental health (MH)
score11 or the
mental component summary (MCS) score of the Short Form-36
(SF-36)12, a
commonly used generic health status measure.
We found only two studies in which the investigators identified patients
who were psychologically distressed prior to knee arthroplasty and then
determined the impact of that distress on physical function after
surgery7,13.
Patients with an SF-36 MCS score of <50 points (the mean derived from a
large sample of the United States
population12) were
considered to have psychological distress, whereas a score of =50 points
indicated no distress. Patients with an MCS score of <50 points had poorer
six and twelve-month outcomes than did patients with an MCS score of =50
points. Generalizability was limited because Ayers et al. examined relatively
small samples of
fifty-two7 and
16513 patients, and
the impact of distress, in WOMAC (Western Ontario and McMaster Universities
Osteoarthritis Index) units, was not quantified with use of a multivariate
approach.
The purpose of our study was to quantify the independent impact of
preoperative psychological distress on WOMAC pain and function scores three,
twelve, and twenty-four months following surgery. We employed two strategies
to accomplish our purpose. First, we used a continuous measure of distress,
the MH score of the SF-36. Second, we dichotomized the MH score, to place
patients into distressed and non-distressed groups, using an evidence-based
approach. We compared the change scores for each time interval and the final
scores at each time interval between the distressed and non-distressed groups.
Change scores for the outcome of interest most commonly reflect improvements
following surgery, and we were interested in comparing functional improvement
and pain relief between patients with distress and those without distress. We
hypothesized that change scores for patients with distress would be less than
those for patients without distress after adjustment for covariates, including
baseline pain and function.
Final scores reflect the status of patients at a particular point in time
following an intervention, and we were interested in knowing whether the MH
scores predicted outcomes at the different time-points in the study. Patients
with better final outcome scores are more satisfied with their outcome than
are patients with lower final outcome
scores14,15.
We hypothesized that the MH score of the SF-36 would negatively impact the
follow-up scores at various time-points, after adjustment for covariates.
Patients with lower baseline functional status have been shown to have worse
outcomes at three, six, and twenty-four months following surgery (in spite of
having greater change
scores)4,16-19.
Data for these analyses were obtained as part of the Kinemax Outcomes
Study, a prospective cohort study of primary total knee arthroplasty for the
treatment of osteoarthritis conducted in thirteen centers: four in the United
States, six in the United Kingdom, two in Australia, and one in
Canada4. Other
papers on the original cohort study have been
published20-26.
The appropriate institutional review board or ethical committee approved
the study at each of the participating centers, and all patients signed an
approved consent form. Patients were recruited from September 1997 to December
1998 in the United Kingdom, the United States, and Australia. In Canada,
recruitment extended to the end of 1999.
Patients
All patients undergoing primary total knee arthroplasty with the Kinemax
prosthesis (Stryker Howmedica, Mahwah, New Jersey) for the treatment of
osteoarthritis were included in the study. A diagnosis of osteoarthritis was
made by the operating surgeon after clinical and radiographic examination.
Patients were excluded if they had a history of knee joint infection or
previous implant surgery on the index knee or if they were unable to complete
the questionnaires because of cognitive or language difficulties. Patients who
had had bilateral total knee arthroplasty with the procedures performed within
twelve months of each other were also excluded from these analyses to ensure
that the follow-up results reflected the outcome of the index operation and
not a subsequent surgical procedure.
Data Collection Procedures
Independent research assistants obtained consent and collected data using a
standardized protocol. Preoperative data were collected within six weeks prior
to the total knee arthroplasty, and follow-up data were collected at three,
twelve, and twenty-four months following the surgery. One author (E.A.L.)
trained all of the research assistants to standardize data collection, and
data were entered into a single database at the coordinating center.
Data Elements
Preoperatively, the patients filled out the Self-Administered Comorbidity
Questionnaire, which has been
validated27 and
includes queries regarding demographic details, socioeconomic data (education,
income, working status, and living arrangements), height, weight, and history
of comorbid conditions. At each evaluation, two health-status scales were
administered: the
WOMAC28,29,
a disease-specific measure of pain, stiffness, and function, and the
SF-3630-32,
a generic health-status measure. With the usual method of scoring the WOMAC,
pain is assigned a score out of 20 points and function is assigned a score out
of 68 points, with a low score indicating better
health18. For ease
of interpretation, we transformed WOMAC pain and function scores to a 0 to
100-point scale for each domain, with a higher score being better and 100
points being the best. The MH scale of the SF-36 also ranged from 0 to 100
points, with 100 points being the
best12.
For this study, we used the MH scale of the SF-36 as a measure of general
psychological distress (Fig.
1). We chose the MH scale of the SF-36 because it consists
entirely of mental health questions relating to a variety of mental health
dimensions and it has been validated as a general measure of
distress12. We did
not choose the MCS of the SF-36 because it is composed of multiple dimensions
of health and can be influenced by a patient's physical
health33.
Research conducted across multiple countries has suggested that SF-36
scores are reasonably equivalent across Australia, the United Kingdom, and the
United
States34-36.
For example, normative data for Australian women were found to be very similar
to data for women in the United
States37. Clarke et
al. found no clinically relevant differences between the MH scores for
Australian men between the ages of eighteen and sixty-five years old and the
scores for men between those ages in the United Kingdom, suggesting reasonable
equivalence for/across the two
countries38.
Statistical Methods
Using the preoperative MH score as a continuous variable, we generated
parameter estimates and p values from general linear models for WOMAC pain and
function scores at each assessment time. Models were adjusted for age, gender,
number of comorbid medical conditions, country, center within country, and
preoperative score.
Using the age and gender-based MH scores that Ware et al. derived from a
study of 1374 patients who were seeing mental health clinicians (see
Appendix)12, we
dichotomized our data into two groups: those with psychological distress
(patients with a score at or below the age and gender-based median for the
patients in the mental health
dataset12) and
those who were not psychologically distressed (those with a score above the
median). We reasoned that if a patient in our dataset had an MH score at or
below the age and gender-based median of those seeing mental health
clinicians, it was likely that the patient had psychological distress.
In addition, because statistical information can be lost when continuous
data are converted to dichotomous
data39, we tested
the sensitivity of these estimates by using MH score cut-points of 50 and 60
points. We also dichotomized the MCS score of the SF-36 using a cut-point of
50 points, a score that has been used in the joint replacement literature to
dichotomize patients into distressed and non-distressed
groups7,13.
The stability of our estimates would be demonstrated to the extent that the
estimates of effect derived with use of these various cut-points were similar
to those derived with use of our original cut-point.
Demographic, socioeconomic, and preoperative health-status data were
compared between the distressed and non-distressed groups (as categorized with
our original cut-point) with use of Wilcoxon two-sample tests and chi-square
analyses where appropriate. We used repeated-measures analysis to report
preoperative and three, twelve, and twenty-four-month WOMAC pain and function
scores for the distressed and non-distressed groups. As the WOMAC scores
represent multiple assessments of the same patient over time, we used
repeated-measures analysis to adjust for the within-patient factors and
interactions between those factors and the between-patient factors such as
distress group.
A total of 181 patients had missing data at twenty-four months for the
following reasons: thirty-five (19%) had died, nineteen (11%) had been unable
to continue with the study because of other medical conditions (such as
stroke), seventeen (9%) had had revision surgery on the index knee, forty-six
(25%) had asked that they be withdrawn from future follow-up, twelve (7%) had
moved and could no longer be followed, forty (22%) had been lost to follow-up
for unknown reasons, and twelve (7%) were unable to return for the
twenty-four-month examination but were willing to stay in the study. When
follow-up data were missing at twenty-four months, we used the
last-observation-carried-forward method to substitute the missing data with
the twelve-month score, if that was available, for patients who to our
knowledge had not died, had not had other medical conditions that made them
unable to continue with the study, or had not undergone revision surgery.
Scores were adjusted for preoperative status, age, gender, number of comorbid
medical conditions, country, and center within country. Differences in WOMAC
scores of 7 to 12 points on a 100-point scale have been shown to be
perceptible to individual patients and clinically
meaningful40-42.
For groups of patients, changes in WOMAC scores of 3 to 6 points on a
100-point scale are considered clinically
important43.
We also analyzed the changes in the WOMAC pain and function scores at each
follow-up time compared with the preoperative scores. We used general linear
models to report adjusted mean change scores for the distressed and
non-distressed groups to determine whether there was a significant difference
between the groups. Scores also were adjusted for age, gender, number of
comorbid medical conditions, country, and center within the country. For a
sensitivity analysis, the various cut-points described earlier were also
examined. Because our method of substitution of missing data may affect the
results, we also report the results for all analyses without substitution of
missing data44.
Statistical analyses were performed with use of SAS version-8.1 statistical
software45.
Preoperative Description of the Sample
A total of 1249 (78.7%) of all eligible patients were recruited. Of the
eligible patients who were not recruited, 128 (8.1%) refused to consent, 197
(12.4%) were missed preoperatively because of the absence of the research
assistant due to sickness or vacation or were not admitted because of
insufficient time to obtain consent and to inform and/or evaluate the patient
prior to surgery, and fourteen (0.9%) were already enrolled in another study
and the institutional review board protocol at that site did not allow
patients to be recruited into more than one study. During the recruitment
period, only 6% of all patients treated with primary total knee arthroplasty
by the participating surgeons did not receive a Kinemax-Plus prosthesis,
frequently because a more constrained prosthesis was required. After exclusion
of 275 patients who had had bilateral total knee arthroplasty with the two
procedures performed within twelve months of each other, 974 patients met the
inclusion criteria. Twenty-two of those patients did not have complete
preoperative MH-score data and were excluded from further analyses. Of the
remaining 952 patients, 172 (18%) had a preoperative MH score at or below the
age and gender-based median of those seeing mental health
clinicians12 and
were included in the distressed group and 780 (82%) had a score above the
median and were included in the non-distressed group. Differences between the
distressed and non-distressed groups with regard to sociodemographic factors
are shown in Table I. The
distressed group was significantly younger and more likely to be separated or
divorced, to be living alone, and to report current smoking than the
non-distressed group. The distressed group also reported a greater number of
comorbid medical conditions and had a higher body mass index than the
non-distressed group.
There was no significant difference in the overall proportion of distressed
patients among the four countries, but there was considerable variation among
the centers. Proportions ranged from 13% to 32%, excluding one clinic in which
the proportion of distressed patients was only 3%. For this reason, in all of
our multivariate models we adjusted for country and center nested within
country.
Longitudinal Analyses
A total of 682 patients (72%) had complete data over the two-year period of
the study. Patients with missing data were more likely to have lower
preoperative MH scores and lower WOMAC function scores (p < 0.05). No other
significant differences were found between those with and those without
missing data.
When the preoperative MH score was examined as a continuous variable, it
was found to have a significant impact on WOMAC pain and function scores at
all time periods (see Appendix) after adjustment for covariates. For the
preoperative WOMAC pain score, the parameter estimate for the MH score was
0.136, indicating that for each point increase in the MH score, the WOMAC pain
score increases by 0.136 point—that is, patients with higher MH scores
(indicating better mental health) report significantly higher WOMAC pain
scores (indicating less pain). The R2 values ranged from 0.13 to
0.29 for the models reported, indicating that the included variables explained
13% to 19% of the variation in WOMAC pain scores and 19% to 29% of the
variation in WOMAC function scores.
After the preoperative MH score was dichotomized, the unadjusted WOMAC pain
and function scores were found to be consistently higher for the
non-distressed group, indicating less pain and functional limitation at each
assessment time. Seven (4.1%) of the 169 patients with distress had a WOMAC
pain score of >75 points, indicating only mild to no pain, compared with
twenty-two (2.8%) of the 777 non-distressed patients. Conversely, almost a
quarter (187; 24.1%) of the 777 non-distressed patients reported a WOMAC pain
score of <25 points, indicating severe-to-extreme pain, compared with
almost half (seventy-nine; 46.7%) of the 169 distressed patients.
Despite the distressed group having much lower preoperative WOMAC scores,
they had a dramatic improvement in these scores in the first three months and
this improvement was maintained at the twelve and twenty-four-month reviews.
The distressed group also demonstrated substantial improvements in their MH
scores, but these scores were approximately 20 points lower than those in the
non-distressed group at all follow-up time-points. There was little change in
the MH scores in the non-distressed group throughout all assessment times
(Table I).
The preoperative and three, twelve, and twenty-four-month outcomes
according to the dichotomized MH score are presented in
Table II. The least square
means for WOMAC pain and function scores based on multivariate models were
adjusted for preoperative score, age, gender, number of comorbid medical
conditions, country, and center within country.
Table II includes the analyses
of the complete data as well as the data with substitution of missing values
as outlined in the Statistical Methods section. Preoperatively, both WOMAC
pain and WOMAC function scores for the distressed group were significantly
worse than those for the non-distressed group (p < 0.0003). Both groups had
significantly better scores at each follow-up visit compared with the baseline
values (p < 0.0001). At the twelve-month and twenty-four-month assessment
times, the WOMAC pain scores in the distressed group were significantly lower
(p < 0.05) than those in the non-distressed group. Differences in the WOMAC
pain scores between the two groups were on the order of 3 to 5 points, with
the distressed group having worse pain. Differences in the WOMAC function
scores were on the order of 1 to 3 points and were not significant at any time
period. These findings remained consistent with and without substitution of
missing data. In the three sensitivity analyses, approximately 85% of the mean
differences between the two groups were within 2 points of the estimates
reported with use of the original cut-point (see Appendix for data derived
with use of an MH-score cut-point of 60 points; data for other analyses not
shown).
The least square means for changes in WOMAC pain and function scores from
the preoperative time-point to the three, twelve, and twenty-four-month
time-points are summarized in Table
III. There were no significant differences between groups with
regard to the amounts of change between the preoperative assessment and any of
the follow-up assessments analyzed. This finding was consistent between the
analyses performed with and those performed without substitution of missing
values. Sensitivity analyses indicated that mean difference estimates derived
with use of the three additional cut-points (MH scores of 50 and 60 points and
an MCS score of 50 points) were not significant (see Appendix for data derived
with use of an MH-score cut-point of 60 points; data for other analyses not
shown).
Patients with psychological distress have noticeably worse preoperative
pain and function as compared with patients who are not distressed. The mental
health of patients with preoperative distress markedly improves following
surgery, but the mental health of patients classified preoperatively as
non-distressed stays essentially the same. These data suggest that
preoperative distress is reversible, at least to some extent, and that this
reversal is probably related to the surgical intervention.
Our analytic strategy was designed to determine if preoperative distress
predicted poor outcome and, if so, to quantify the extent of this impact in a
clinically meaningful way. The MH scale, when examined as a continuous
variable, was a highly significant predictor of pain and function scores at
all time periods. However, this analysis does not provide a description or
context for judging the clinical importance of the prediction. Our approach to
dichotomizing the MH scale allowed us to judge the clinical impact, in WOMAC
scale points, and the sensitivity analysis indicated that our estimates were
reasonably stable.
Our study appears to be the first in which multivariate approaches were
used to quantify the independent impact of preoperative psychological distress
on the change in status following knee replacement surgery and on the outcomes
at various time-points. Distinguishing between change (representing
improvement or worsening) and outcome (representing the final status) appears
to be clinically important because change and outcome are conceptually
different phenomena. Patients who are on the higher-functioning end of the
WOMAC function scale preoperatively, for example, can have only a small amount
of change because of the ceiling
effect46. The score
can only get so high. Alternatively, patients who score very poorly on the
WOMAC function scale prior to surgery can have a dramatic change in their
score but still not be near the ceiling of the scale. We therefore believe
that it is clinically important to examine outcomes from both the perspective
of score changes and the perspective of final scores at various time-points.
Reports of health-care quality (the so-called report
cards47) related to
knee replacement should, in our opinion, address both changes in functional
status and pain as well as final status.
Our data indicate that psychological distress adversely affects outcome but
only when considered at the group level and only in terms of pain.
Preoperative psychological distress does not appear to have a deleterious
effect on function, as measured by the WOMAC, in terms of either change scores
or outcomes at various time-points. Differences in WOMAC pain scores were on
the order of 3 to 5 points during the two-year study, which is below
literature-based estimates of minimal clinically important differences for
individual
patients42,48.
Clinically important differences between groups of patients are estimated to
be on the order of 3 to 6 WOMAC
points49. Change
scores appear to be less affected by psychological distress than are follow-up
scores because we found no significant differences between the change scores
for the two groups during the three follow-up periods
(Table III).
Psychological distress is reversible, as shown in our study, and
interventions designed to reduce distress have been shown to be effective for
elderly patients with
arthritis2,50,51.
However, interventions would be of value for improving the results of knee
replacement only if distress were convincingly shown to have a deleterious
effect on such results. More research is clearly needed before large-scale
trials are conducted because the magnitude of the effect of distress on pain
and function appears to be very small.
Our study was conducted in thirteen centers from four countries, which may
have reduced the generalizability of the results to any one country. Also, the
loss to follow-up was on the order of 28%, which may have influenced our
estimates of effects attributable to psychological distress. We suspect that
this influence is likely given that a higher proportion of patients with
distress and lower WOMAC function scores were lost to follow-up. This loss of
data may have resulted in an underestimate of the effect of distress since a
greater proportion of patients with distress had missing data. In addition,
the great majority (92%) of our patients were white, so the results are not
generalizable to patients of other races.
Finally, and potentially most importantly, we used a general measure of
psychological distress rather than more specific distress measures, so we
cannot determine the potential impact of more specific types of distress. The
distress scores did not tell us if the distress was related to an acute event
or patient state or was a chronic condition more related to a patient trait.
As a result, we cannot make inferences about what may have caused the
distress. Additional research is needed to clarify the potential effect of
more specific forms of psychological distress on outcome.
In summary, patients with preoperative psychological distress have twelve
to twenty-four-month pain outcomes that are slightly worse than those for
patients who are not distressed. Postoperative changes in pain scores do not
differ between patients with and those without distress. Physical function
outcome and change scores also do not differ significantly between distressed
and non-distressed patients.
Tables showing a summary of the median scores for the MH scale from the
study by Ware et
al.12, the
parameter estimates from the logistic regression analyses, the sensitivity
analysis for outcome with use of a cut-point of 60 points for the MH score,
and the sensitivity analysis for change scores with use of a cut-point of 60
points for the MH score 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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