Assessing recovery and establishing prognosis following total knee arthroplasty

Research Report

Assessing Recovery and Establishing
DM Kennedy, BScPT, MSc, is Man-
Prognosis Following Total Knee
ager of Program Development, Hol-
land Orthopaedic & Arthritic Cen-
Arthroplasty
tre, Sunnybrook Health Sciences
Centre, 43 Wellesley St E, Toronto,
Deborah M Kennedy, Paul W Stratford, Daniel L Riddle, Steven E Hanna,
Ontario, Canada M4Y 1H1. She also Jeffrey D Gollish
is Instructor, Department of Physi-
cal Therapy, University of Toronto,
and Part-time Assistant Clinical Pro- Background and Purpose
fessor, School of Rehabilitation Sci- Information about expected rate of change after arthroplasty is critical for making
ence, McMaster University, Hamil-
ton, Ontario, Canada. Address all
prognostic decisions related to rehabilitation. The goals of this study were: (1) to
correspondence to Ms Kennedy at: describe the pattern of change in lower-extremity functional status of patients over
d.kennedy@utoronto.ca. a 1-year period after total knee arthroplasty (TKA) and (2) to describe the effect of
PW Stratford, PT, MSc, is Profes-
preoperative functional status on change over time.
sor, School of Rehabilitation Sci-
ence, and Associate Member, De- Subjects
partment of Clinical Epidemiology Eighty-four patients (44 female, 40 male) with osteoarthritis, mean age of 66 years
and Biostatistics, McMaster Uni-
versity, and Scientific Affiliate, De-
(SD 9), participated.
partment of Surgery, Sunnybrook
Health Sciences Centre. Methods
DL Riddle, PT, PhD, FAPTA, is Otto Repeated measurements for the Lower Extremity Functional Scale (LEFS) and the
D Payton Professor, Department Six-Minute Walk Test (6MWT) were taken over a 1-year period. Data were plotted to
of Physical Therapy, Virginia Com- examine the pattern of change over time. Different models of recovery were ex-
monwealth University, Medical
plored using nonlinear mixed-effects modeling that accounted for preoperative status
College of Virginia Campus, Rich-
mond, Va. and gender.
SE Hanna, PhD, is Associate Pro- Results
fessor, Department of Clinical Ep-
idemiology and Biostatistics and Growth curves were generated that depict the rate and amount of change in LEFS
School of Rehabilitation Science, scores and 6MWT distances up to 1 year following TKA. The curves account for
McMaster University. preoperative status and gender differences across participants.
JD Gollish, BASc, MD, FRCSC, is
Medical Director, Holland Ortho- Discussion and Conclusion
paedic & Arthritic Centre, Sunny- The greatest improvement occurred in the first 12 weeks after TKA. Slower improve-
brook Health Sciences Centre, and
Assistant Professor, Department of
ment continued to occur from 12 weeks to 26 weeks after TKA, and little improve-
Surgery, Faculty of Medicine, Uni- ment occurred beyond 26 weeks after TKA. The findings can be used by physical
versity of Toronto. therapists to make prognostic judgments related to the expected rate of improvement
[Kennedy DM, Stratford PW, Rid-
following TKA and the total amount of improvement that may be expected.
dle DL, et al. Assessing recovery
and establishing prognosis follow-
ing total knee arthroplasty. Phys
Ther. 2008;88:22–32.]

© 2008 American Physical Therapy
Association

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22 f Physical Therapy Volume 88 Number 1 January 2008

Recovery and Prognosis Following Total Knee Arthroplasty

K
nee osteoarthritis (OA) is one of The expected rate of change in func- WOMAC and LEFS, site of arthroplasty
the most frequent causes of dis- tional status following surgery is of was not a predictor and preoperative
ability.1 For patients with end- significant interest to both research- levels of function were met and ex-
stage OA, which is characterized by ers and clinicians. Researchers can ceeded much earlier (1–3 weeks) than
severe pain and poor functional sta- apply this information to schedule what was observed for the perfor-
tus, total knee arthroplasty (TKA) is optimal outcome assessment points mance measures (6 –9 weeks post-
recognized as a highly beneficial and in a randomized trial, and clinicians operatively). A ceiling effect around
cost-effective treatment.2– 4 Despite can use this knowledge to bench- 9 to 10 weeks was observed with re-
the benefits and the rise in utilization mark progress and to make prognos- spect to the TUG, indicating that this
of this procedure,5 questions remain tic decisions related to rehabilitation measure is not useful for detecting
unanswered, particularly in the area needs. Studies investigating exercise- improvement beyond 3 months. A lim-
of rehabilitation services. The Na- based interventions have often as- itation of these studies was the inabil-
tional Institutes of Health consensus sessed outcome up to 1 year after ity to predict when patients had
statement on total knee replacement arthroplasty.16 –18 Long-term follow-up reached their maximal functional lev-
indicates that the use of rehabilita- is essential for some interventions spe- els as measured via self-report or gait
tion services is one of the most un- cific to arthroplasty to prevent prob- performance. We found no other stud-
derstudied aspects of the periopera- lems such as prosthetic failure. How- ies that determined the specific time
tive management of this population.6 ever, extended follow-up times are point of maximal functional return fol-
likely not necessary for interventions lowing knee arthroplasty.
One issue that clinicians face when that lead to rapid changes in a patient’s
treating patients with TKA is the de- status over a relatively short period of The purpose of this study, therefore,
cision as to which outcome mea- time. In a study examining the first 4 was to build on the existing work
sures to use for assessment of func- months of recovery in patients fol- by profiling the change in lower-
tional recovery. A growing body of lowing hip and knee replacement, extremity functional status of partic-
literature indicates that self-report Kennedy et al12 found that the greatest ipants during the first year following
measures of function provide differ- period of postoperative change oc- primary TKA using the 6MWT and
ent information than physical perfor- curred in the first 9 weeks. the LEFS. Although the WOMAC is
mance measures in people with OA one of the leading outcome mea-
or arthroplasty.7–11 Physical perfor- Numerous studies7,12,19 –26 have ex- sures for people with arthroplasty,
mance and self-report measures may amined recovery patterns after TKA the LEFS has demonstrated cross-
assess different aspects of physical with differing periods of follow-up. sectional and longitudinal validity
function.12 In the arthroplasty litera- Several authors19,21,23 provided graph- equal to or better than that of the
ture, many studies have used only ical representations of recovery for WOMAC physical function sub-
self-report measures, with the Medi- the WOMAC and SF-36 but did not scale.27 Clinicians find the LEFS easy
cal Outcomes Study 36-Item Short- include performance measures. The to administer in busy clinic settings,
Form Health Survey questionnaire study by Mizner et al24 provided recov- and data are published on its score
(SF-36) and the Western Ontario and ery curves for quadriceps femoris mus- interpretation to a greater extent
McMaster Universities Osteoarthritis cle strength (force-generating capac- than for the WOMAC.28 –30 Our
Index (WOMAC) cited most fre- ity), knee range of motion, the TUG, a choice to report LEFS scores also
quently.13 Although performance- timed stair-climbing test, SF-36 sum- was influenced by the growing body
based measures appear to provide mary scores, and the Knee Outcome of evidence indicating that the
more information about actual phys- Survey–Activities of Daily Living Scale WOMAC lacks factorial validity.31–34
ical ability, consensus is still needed at 1, 2, 3, and 6 month postoperative We chose the 6MWT because it is
on what activities should be in- time points. Two studies12,22 exam- recognized as a useful measure of
cluded for patients with hip or knee ined recovery in the first 4 months functional status and exercise capac-
OA.14 Previously, Kennedy et al15 in- after total hip and knee arthroplasty ity in elderly adults.35–38 Speed and
vestigated the measurement proper- using hierarchical linear modeling to distance abilities are both important
ties of the Six-Minute Walk Test illustrate trajectories of change. Signif- considerations for community mobil-
(6MWT), the Timed “Up & Go” Test icant differences in the patterns and ity in older adults. Older adults need
(TUG), a fast self-paced walk test, predictors of recovery were found to be able to walk, on average, 300 m
and a stair performance measure in when comparing the WOMAC and the during the performance of instru-
subjects with arthroplasty. Lower Extremity Functional Scale mental activities of daily living.39
(LEFS) with the TUG, a timed stair test,
and the 6MWT. In the case of the

January 2008 Volume 88 Number 1 Physical Therapy f 23


The specific study goals of this study Mr Smith likely to reach his maxi- participated in a progressive pro-
were: (1) to describe the pattern of mum functional level? gram of range of motion, strengthen-
change in lower-extremity func- ing exercises, proprioceptive exer-
tional status as measured by the LEFS Method cises, and functional training. At the
and 6MWT of participants over a All data were collected as part of a time of this study, the majority of
1-year period after TKA and (2) to larger observational study conducted the patients were transferred from
explore the effect of preoperative at a tertiary care orthopedic facility the acute care floor on the fourth or
functional status on the pattern of in Toronto, Canada, from Novem- fifth postoperative day to the on-site
change. Clinicians need prognostic ber 2001 to February 2004. Desig- short-term rehabilitation unit to con-
evidence to educate their patients nated a Centre of Excellence for tinue the aforementioned program
about expected time to reach their hip and knee replacement, the facil- for a maximum length of stay of 7
maximal recovery. Having this knowl- ity is one of the largest-volume ar- days. All patients were discharged
edge allows patients and their family throplasty sites in the country. Pa- with a home exercise program, and
members to judge progress over time tients were recruited prospectively some patients received additional
and have realistic expectations.40 We either at point of consultation with physical therapy treatment in the
provide a brief illustration using a hy- the orthopedic surgeon or at the community.
pothetical clinical vignette to illustrate preadmission visit prior to surgery.
how the study results can be applied Only those patients with follow-up Subjects
to assist clinicians in making prognos- for the first year postoperatively Preoperatively, 88 patients con-
tic decisions when treating patients were eligible for this study. During sented to participate in the study;
following TKA. the larger study, there were periods however, only 84 patients contrib-
of interruption of recruitment and uted LEFS and 6MWT data follow-
Clinical Practice Vignette tracking, such as with the outbreak ing arthroplasty. Table 1 provides a
Mr Smith, a 67-year-old with a long- of severe acute respiratory syndrome summary of the participants’ char-
standing history of OA of the right in Toronto from April to June 2003. acteristics. Female participants had
knee, is referred for rehabilitation 2 At the height of the outbreak, thou- a greater body mass index
weeks after a right TKA. As part of sands of people were quarantined, and (t82 2.05, P2 .042); male partici-
the initial assessment, you adminis- there were significant restrictions on pants had higher LEFS scores
ter the LEFS and the 6MWT and ob- patient-related activities in hospitals (t82 3.02, P2 .003) and walked
tain values of 28 LEFS points and for several months. None of the pa- greater distances in 6 minutes
261 m, respectively. These values are tients took part in other interventional (t82 5.28, P2 .001).
substantially lower than Mr Smith’s studies. However, the current sample
preoperative values of 40 points overlaps samples described in earlier Design
for the LEFS and 507 m of the 6MWT. publications, which used data from We applied a prospective study de-
Mr Smith mentions that he has a va- the same observational study.10,12,15,22 sign with repeated measurements
cation cruise scheduled in 8 weeks over a period of approximately 1
and asks what his function is likely to Participant eligibility criteria in- year following arthroplasty. To pro-
be at that time. He also wonders cluded the following: diagnosis of vide an accurate model of change
what his maximum functional status OA, scheduled for primary TKA; suf- over time, participants’ follow-up
is likely to be and when he will reach ficient language skills to communi- measurements were not standard-
this level of functioning. Questions cate in written and spoken English; ized to be at the same time points
arising from the assessment include and absence of neurological, cardiac, during the first 4 postoperative
the following: (1) How much change or psychiatric disorders or other months, the period of greatest
is required in these measures to be medical conditions that would signif- change.7,12 When measurements
reasonably certain that a true change icantly compromise physical func- take place at the same spaced time
has occurred? (2) What factors tion. Ethics approval for the study points, the shape of the curve is dic-
should be considered in scheduling was received from the institution’s tated by the choice of time points.
the next assessment, and when research ethics review board, and all Three assessments were planned
should it occur? (3) What is Mr participating patients provided writ- during this time frame, and subse-
Smith’s lower-extremity functional ten informed consent. Patients re- quently participants were assessed
status likely to be in 8 weeks? (4) ceived standardized inpatient treat- at points corresponding to the next
What is Mr Smith’s maximum func- ment following a primary total knee surgeon follow-up appointments,
tional status likely to be? (5) When is care pathway. All patients were per- which typically might fall at 6 or 9
mitted to be full weight bearing and months and then 12 months postop-



eratively. As noted earlier, this sched- Table 1.
uling of assessments facilitated ob- Preoperative Descriptive Statistics Expressed as Quartile Values (25th, 50th, 75th)
taining good estimates of the rate of Measure Female Male
change as well as the limit values or Participants Participants
point of maximal return. (n 44) (n 40)
Age (y) 60, 64, 71 61, 67, 74
Measures 2
Body mass index (kg/m ) 29, 32, 34 27, 29, 32
Previous work7,8,10 has suggested
that self-report and performance- Prearthroplasty Lower Extremity 21, 27, 38 26, 38, 45
Functional Scale score
based measures capture different,
but related, aspects of lower- Prearthroplasty Six-Minute Walk 312, 353, 416 397, 501, 552
extremity functional status. Accord- Test distance (m)
ingly, we chose 2 measures of lower-
extremity functional status—one a
self-report measure and the other a ized 6MWT has become a popular if a patient following arthroplasty
performance-based measure. measure of lower-extremity func- missed an appointment due to a
tional limitation for patients with OA change in his or her schedule; how-
LEFS. Conceived by Binkley and col- of the lower extremity and those ever, it would be a problem if the
leagues,28 the LEFS is a 20-item self- progressing to arthroplasty.7,8,15,47,48 patient missed the appointment be-
report measure of lower-extremity Participants were instructed to cover cause of poor functioning due to an
functional status. It includes items as much distance as possible during increase in pain. Therefore, we also
that assess the disablement concepts the 6-minute time frame. The test examined the pattern of missing data
of functional limitation (activity lim- was conducted on a measured 46-m across the time points.
itation) and disability (participation uncarpeted rectangular indoor cir-
restriction).41,42 Each item is scored cuit. The course was marked off in Based on the plotted data, we devel-
on a 5-point scale (0 – 4). Accord- meters, and the distance traveled by oped and tested several nonlinear
ingly, total LEFS scores can vary from each participant was measured to models of change that related the
0 to 80 points, with higher scores the nearest meter. Standardized en- dependent variable of functional sta-
being associated with greater levels couragement—“You are doing well, tus— either LEFS scores or 6MWT
of functional status. Considerable keep up the good work”—was pro- distances—to the independent vari-
support for this measure’s reliability, vided at 60-second intervals.49 The able of number of weeks after arthro-
validity, and ability to detect change outcome was the distance walked in plasty.50 The equation for our non-
exists both for general lower- 6 minutes. A previous investigation linear change model (model 1) was:
extremity conditions43– 45 and spe- with a similar group of subjects dem-
cific to patients with OA progressing onstrated the reliability and validity Functional status (LEFS or 6MWT)
to knee or hip arthroplasty.27,29,46 of data for this measure (intraclass
The test-retest reliability estimate correlation coefficient .94 for test- limit (y0 limit)
(intraclass correlation coefficient, retest reliability, SEM 26.3 m, and
e( e(lnchange rate) weeks),
type 2,1) for the LEFS derived from a MDC90 61.34 m).15
sample of patients following arthro-
plasty was .85, the standard error of Data Analysis where the functional status variable
measurement (SEM) was 3.7 LEFS Before beginning the modeling, we is the LEFS or 6MWT value, e is the
points, and the minimal detectable plotted the data to gain an impres- base of natural logarithms (approxi-
change at the 90% confidence level sion of the pattern of change over mately 2.71828), weeks is the num-
(MDC90) was estimated to be 9 LEFS time. Although one of the benefits of ber of weeks after arthroplasty; y0 is
points.28 In patients undergoing using mixed-effects modeling is that the parameter that represents the
knee or hip arthroplasty, the LEFS it does not require the number and y-intercept value; limit is the param-
has been shown to detect change as timing of observations to be the eter that represents the asymptote or
well as or better than the WOMAC same across all participants, missing maximum LEFS or 6MWT value, and
physical function subscale.27,46 data are still important. Bias will re- lnchange rate is the natural log of
sult if the cause of the missing data the change rate (“change rate” re-
6MWT. Originally conceived as an points is related to the outcome fers to the rate of improvement at
outcome measure for people with that would have been observed. For which patients approach their max-
respiratory problems, the standard- example, it would not be a problem imum functional status). We esti-



Table 2. All knee prostheses were posterior
Summary of Nonlinear Analysis Without Covariates stabilized, with the majority ce-
mented. At our institution, the peri-
Female Male
Participants Participants operative management and rehabili-
(n 44) (n 40) tation protocols are not influenced
Lower Extremity Functional Scale analysis
by prosthesis selection or method of
fixation. Postoperatively, one partic-
Parameters of average changea
ipant developed a documented deep
Limit (SE) 54.0 (2.3) 60.4 (2.3) vein thrombosis. None of the partic-
Y-intercept (SE) 10.4 (2.6) 19.0 (2.7) ipants required revision surgery
Change rate (SE) 1.7 (0.1) 1.8 (0.1)
within the 1-year follow-up period.
Standard deviation of individual differences
Sixty-seven of the 84 participants
from average
composing the study sample were
Limit 12.1 11.6 assessed within 17 days of arthro-
Y-intercept 7.9 10.2 plasty. Of these 67 participants, 66
Within-patient variation 7.6 6.7 completed the LEFS and 44 per-
formed the 6MWT during this 17-day
6-Minute Walk Test analysis
period. The mean ( SD) preopera-
Parameters of average change tive LEFS score for those participants
Limit (SE) 467.3 (15.4) 577.7 (18.2) who contributed LEFS data within 17
Y-intercept (SE) 154.7 (22.2) 185.7 (22.9) days of arthroplasty was 32.3 points
(SD 12.1) compared with 33.1
Change rate (SE) 2.0 (0.1) 1.7 (0.1)
points (SD 14.4) for those partici-
Standard deviation of individual differences pants who were not assessed within
from average
this period (t82 0.22, P2 .83). Sim-
Limit 84.7 94.6 ilarly, the mean preoperative dis-
Y-intercept 84.5 68.7 tance for those participants who
Within-patient variation 40.7 48.8 contributed 6MWT data within 17
a
days of arthroplasty was 428.4 m
Parameters of average change fixed effects. Some parameters (ie, change rate) have only fixed
effects, indicating that there are no significant individual differences. SE standard error. (SD 114.8) compared with 393.4 m
(SD 105.8) for participants who did
not contribute 6MWT data within
this period (t82 1.46, P2 .15). Fi-
mated the parameters using the rate coefficients. Finally, because nally, the mean LEFS score assessed
nonlinear mixed-effects modeling previous work has shown that func- within 17 days of arthroplasty was
package in S-Plus.50 A mixed-effects tional status levels differ by gen- 26.0 points (SD 10.7) for the partic-
approach indicates that some param- der,22,51 we created separate models ipants who contributed 6MWT data
eters have both fixed and random for female and male participants. during this period compared with
effects. The fixed effects describe 16.7 points (SD 10.2) for the partic-
the average change in the popula- Results ipants who were assessed during
tion (in this case, the sample of All participants completed baseline this interval but not able to contrib-
participants who underwent TKA), preoperative assessments and had a ute 6MWT data (t64 3.45, P2 .001).
and the random effects describe minimum of 2 visits postoperatively. Post-arthroplasty assessments follow-
the individual differences among To summarize, 31 participants were ing the 3-week mark yielded approx-
participants. We explored dif- assessed 3 times, 18 were assessed 4 imately equal representation of LEFS
ferent models, and our final model times, 9 were assessed 5 times, and 2 and 6MWT data points.
specified the limit, y-intercept, and had 6 visits, with the rest of the sam-
change rate parameters as fixed ef- ple having 2 visits. Supplemental Table 2 reports the fixed-effects pa-
fects and the limit and y-intercept as Figures 1 and 2 (available online rameter values and the variation in
random effects. Next, we examined only at www.ptjournal.org) provide random-effects parameter values for
the effect of including preoperative spaghetti plots showing the data the LEFS and 6MWT obtained from
LEFS scores and 6MWT distances points and change profiles for each model 1. Also reported in Table 2 are
on the limit, y-intercept, and change participant. the standard deviations of individual



differences from the estimated aver- A
age parameter values. For example, 80
the standard deviation of individual
70
differences from the average LEFS
limit value for the female partici-

Predicted LEFS Score
60
pants was 12.1. Accordingly, 68%
of the female participants displayed 50
limit values from 42 to 66 LEFS
40
points. The curves shown in Figure 1
were generated by substituting the 30
Male Participants
parameter estimates reported in Female Participants

Table 2 into model 1. This figure 20

shows that most of the change oc-
10
curred in the first 16 weeks after
arthroplasty. 0
0 4 8 12 16 20 24 28 32 36 40 44 48 52

Our exploration of the effect preop- Weeks After Arthroplasty

erative functional status scores had
B
on limit values, y-intercept, and 700
change rate coefficients revealed
that limit values only were signifi- 600
Predicted 6MWT Distance (m)

cantly affected. This finding indi-
cates that preoperative levels of 500

function help to predict the maximal
functional status that patients attain 400

postoperatively. Better preoperative Male Participants
300
scores will be associated with pa- Female Participants
tients attaining higher maximum
200
postoperative levels of function. Ac-
cordingly, our revised model (model 100
2) was as follows:
0
Functional status (LEFS or 6MWT) 0 4 8 12 16 20 24 28 32 36 40 44 48 52
Weeks After Arthroplasty
(limit preoperative function) Figure 1.
(A) Change in Lower Extremity Functional Scale (LEFS) scores over time. (B) Change in
(y0 limit 6-Minute Walk Test (6MWT) distances over time.
preoperative function)
2A represents LEFS scores for male over a 1-year period for patients who
e( e(lnchange rate) weeks), participants with a preoperative underwent TKA and received stan-
LEFS score of 45 points (ie, third dardized inpatient physical therapy
where is the regression coefficient quartile value reported in Tab. 1). care for 1 to 2 weeks (acute and
associated with preoperative func- The 16-week value of 61 points on subacute short-term rehabilitation).
tional status level. Gender- and this curve was obtained by substitut- The subsequent discussion will first
measure-specific coefficients are re- ing the coefficient values reported in provide a synthesis of our findings
ported in Table 3. Figures 2 and 3 Table 3 into model 2 and applying a and then illustrate applications of
display the change curves for the preoperative value of 45 points. this information by referring to the
LEFS and 6MWT adjusted for preop- Again, these figures show that most vignette introduced early in this
erative scores. The 3 curves pre- of the change occurred within the article.
sented in each figure depict the first 16 weeks after arthroplasty.
gender- and measure-specific change To our knowledge, this is the first
curves based on the preoperative Discussion study to sample patients at different
quartile values reported in Table 1. Our goal was to describe the change time points over a 1-year period after
For example, the top curve in Figure in lower-extremity functional status TKA and to apply a nonlinear mixed-



Table 3. 6MWT distances were influenced
Summary of Nonlinear Analysis With Preoperative Score as a Covariate only by their respective preoperative
values. Accordingly, it is important
Female Male
Participants Participants that clinicians take the preoperative
(n 44) (n 40) value into account when making a
Lower Extremity Functional Scale analysis
prognosis concerning a patient’s fi-
nal level of lower-extremity func-
Parameters of average change
tional status.
Limit (SE)a 38.1 (5.3) 42.2 (5.6)
Preoperative ( ) (SE) 0.50 (0.1) 0.50 (0.1) As illustrated in the section on re-
Y-intercept (SE) 10.3 (2.6) 19.0 (2.7)
sponses to the clinical practice vi-
gnette, we believe that graphical rep-
Change rate (SE) 1.7 (0.1) 1.8 (0.1)
resentations of recovery can be very
Standard deviation of individual differences useful in assisting clinicians to
from average benchmark recovery. The graphs
Limit 10.4 9.8 can be used to compare measured
Y-intercept 7.5 10.0 scores obtained on patients with the
predicted scores to monitor progress
Within-patient variation 7.7 6.7
and guide treatment decisions. Nor-
6-Minute Walk Test analysis mative scores for the measures in
Parameters of average change similar populations also are available
Limit (SE) 277.2 (55.0) 326.2 (56.9) to enable further benchmarking.52,53
Preoperative ( ) (SE) 0.6 (0.1) 0.5 (0.1)
The recovery curves in our study
also facilitate determination of the
Y-intercept (SE) 154.7 (22.2) 188.6 (22.8)
critical time points for measuring
Change rate (SE) 2.0 (0.1) 1.7 (0.1) change. For example, if researchers
Standard deviation of individual differences were interested in determining the
from average effect of interventions on improving
Limit 64.0 71.2 the rate of recovery and the maxi-
mum level of function attained, they
Y-intercept 83.7 71.5
could apply these graphs to assist in
Within-patient variation 41.1 48.5 their decision making. More studies
a
SE standard error. are needed to determine the effect
of various postoperative physical
therapy interventions on recovery.
Frequently cited assessment points
are 3, 6, and 12 months after arthro-
effects analysis to model change. we applied a nonlinear mixed-effects plasty13; however, based on the in-
Previously, members of our team analysis. The current study’s results formation from the current study, to
have applied hierarchical linear over the initial 16 weeks after TKA assess the effect of interventions, it
modeling and a second-degree poly- compare favorably with those mod- would be important to assess pa-
nomial to model LEFS scores and eled using a second-degree poly- tients more frequently in the first 3
6MWT distances over the first 16 nomial in patients who similarly re- months. In addition, because most of
weeks after arthroplasty.12,22,30 Par- ceived a mixed-effects model of the recovery has occurred by 6
ticipants’ LEFS scores and 6MWT physical therapy intervention with months, researchers might decide to
distances increased rapidly over unknown parameters including out- not assess individuals beyond this
this period, and a second-degree patient treatment and natural point to avoid unnecessary costs.
polynomial fit the data well within recovery.22,30
this interval. However, a second-
degree polynomial specifies a parab- Our study also explored the effect of
ola that clearly does not represent preoperative LEFS scores and 6-MWT
the change pattern of LEFS scores distances as potential predictors of
or 6MWT distances over a 1-year y-intercept, change rate, and limit
period, and it is for this reason that values. Maximal LEFS scores and



Responses to the Clinical Practice A
Vignette 80
How confident can the clinician
70
be in the measured values of 28
points on the LEFS and 261 m 60
on the 6MWT, and how much

change is required in these mea- 50
Preop LEFS 45
sures to be reasonably certain
40 Preop LEFS 38
that a true change has occurred? Preop LEFS 26
To answer these questions, the re- 30
sults from 2 other studies that
20
examined the reliability of data for
the LEFS29 and 6MWT,15 whose 10
estimates are reported in the
Method section, are used. For exam- 0
0 4 8 12 16 20 24 28 32 36 40 44 48 52
ple, the 90% confidence level (ie, 1
SEM 1.65) for an estimate of the
“true score” is 6.1 points for the B
LEFS and 43.4 m for the 6MWT. We 70
can say with 90% confidence that
Mr Smith’s true LEFS score is likely to 60

fall between 21.9 and 34.1 points

50
and that his true 6MWT distance is
likely to lie between 222.9 and 304.4 40
m. To identify the minimal detect- Preop LEFS 38
able change (MDC), the confidence 30 Preop LEFS 27
values reported are multiplied by Preop LEFS 21

the square root of 2. For example, 20
90% of patients who are truly stable
10
will display random fluctuations,
when assessed on multiple occa-
0
sions, of less than 9 points on the
0 4 8 12 16 20 24 28 32 36 40 44 48 52
LEFS and 61.3 m on the 6MWT. Ac-
cordingly, a change of 9 points or
more on the LEFS and of 61.3 m or Figure 2.
more on the 6MWT is interpreted (A) Change in Lower Extremity Functional Scale (LEFS) scores for male participants,
adjusted for preoperative (preop) LEFS scores. (B) Change in LEFS scores for female
as evidence of a true change. Esti- participants, adjusted for preoperative LEFS scores.
mates obtained from this approach
often are referred to as MDC with
the confidence value subscripted
(eg, MDC90).54 a change of 9 points. This informa- week is required for an expected
tion is coupled with the curve for change of 61.3 m.
What factors should the clinician patients with a preoperative LEFS
consider in scheduling the next value of 38 (ie, the curve closest to What is Mr Smith’s lower-
assessment, and when should it Mr Smith’s value). Referring to the extremity functional status likely
occur? Clearly, many factors, in- middle curve presented in Figure 2A, to be in 8 weeks? To answer this
cluding feasibility, influence the it appears that a change of 9 LEFS question, the clinician can inspect
choice of reassessment interval. Two points occurs between 2 and 4 the predicted functional status val-
factors specific to the context of this weeks postoperatively. Accordingly, ues for 10 weeks after TKA (the first
article are MDC and the interval over the interval between assessments for assessment occurred at the 2-week
which a typical patient is likely to this specific instance is approxi- mark). For a person with Mr Smith’s
achieve a change equal to the MDC. mately 2 weeks. Applying the same preoperative values, the expected
Mr Smith’s 2-week postoperative approach to 6MWT distance, it ap- LEFS and 6MWT scores are approxi-
LEFS value was 28, and the MDC90 is pears that a minimum interval of 1 mately 52 points and 520 m, respec-



A What is Mr Smith’s maximum
700 functional status level likely to
be? Recalling that a patient’s pre-

600 operative level of function is a deter-
minant of his or her postoperative
500
maximal function level, Figures 2A
400 and 3A are referenced to answer this
Preop 552 m question. Because Mr Smith had a
300 Preop 501 m
preoperative score of 40 points on
Preop 397 m
the LEFS, the middle curve is se-
200
lected, and this would lead to a pre-
100 diction that Mr Smith would have a
terminal LEFS score of just over 60
0 points. In terms of the 6MWT, using
0 4 8 12 16 20 24 28 32 36 40 44 48 52 a similar approach, the maximal dis-
Weeks After Arthroplasty tance that Mr Smith would be able to
cover would be around 600 m.
B
600
When is Mr Smith likely to reach
his maximum functional level?

500 In both the case of the LEFS and
6MWT, Mr Smith would reach his
400 maximum functional level sometime
between 6 and 7 months.
Preop 416 m
300
Preop 353 m
Preop 312 m
Study Limitations
200 One limitation is that all participants
in the change study were able to
100
complete the LEFS and 6MWT pre-
operatively. Accordingly, the gener-
alizability of our findings are re-
0
0 4 8 12 16 20 24 28 32 36 40 44 48 52
stricted to patients who are able to
complete these tests preoperatively
and who have preoperative charac-
Figure 3. teristics similar to those reported in
(A) Change in 6-Minute Walk Test (6MWT) distances for male participants, adjusted for Table 1. A second limitation is that
preoperative (preop) 6MWT distances. (B) Change in 6MWT distances for female
fewer participants provided 6MWT
participants, adjusted for preoperative (preop) 6MWT distances.
data than LEFS data within a few
weeks of arthroplasty. Our analysis
showed that participants who were
tively. Although the meaning of ing up or down 10 stairs or lifting an assessed within 17 days of arthro-
520 m is straightforward, the inter- object such as a bag of groceries plasty and who did not contribute
pretation of an LEFS score of 52 from the floor; and (3) “no difficulty” 6MWT data during this period had
points is not intuitively obvious, and sitting for 1 hour, putting on shoes significantly lower LEFS scores at this
the clinician will need to translate or socks, or walking short distances. time point. A consequence of this
this number into a narrative. Based These data will likely assist the clini- missing value pattern is that the pre-
on information provided in a previ- cian in advising Mr Smith on what he dicted 6MWT distances over the first
ous article,30 a person with an LEFS can expect regarding his mobility, several weeks after arthroplasty are
score of approximately 52 points which will likely assist Mr Smith in not applicable to the entire sample,
will have: (1) “moderate difficulty” deciding whether he should con- but rather are restricted to those par-
with heavy activities around the sider rescheduling his trip. ticipants who were capable of per-
house, recreational activities, walk- forming the 6MWT within this time
ing a mile, or standing for 1 hour; frame. This could have resulted in
(2) “a little bit of difficulty” with go- overestimation of the predicted



scores for the 6MWT during this Neil Reid for data collection, project man- 14 Terwee CB, Mokkink LB, Steultjens MP,
agement, and clerical support. Dekker J. Performance-based methods for
time frame. measuring the physical function of pa-
This article was received February 11, 2007, tients with osteoarthritis of the hip or
and was accepted August 20, 2007. knee: a systematic review of measurement
Although mixed-effects modeling properties. Rheumatology (Oxford). 2006;
will stabilize the estimates of pa- DOI: 10.2522/ptj.20070051 45:890 –902.
tients who have limited data by an- 15 Kennedy DM, Stratford PW, Wessel J, et al.
Assessing stability and change of four per-
choring them to the group average, formance measures: a longitudinal study
it should be noted that 66% of the evaluating outcome following total hip
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