Role of knee kinematics and kinetics on performance and disability in people with medial compartment knee osteoarthritis

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Role of knee kinematics and kinetics on performance and disability
in people with medial compartment knee osteoarthritis
Monica R.Maly
a,
*
,Patrick A.Costigan
b
,Sandra J.Olney
c
a
Elborn College,School of Physical Therapy,The University of Western Ontario,London,Ont.,Canada N6G 1H1
b
School of Physical Health and Education,Queen’s University,Kingston,Ont.,Canada
c
School of Rehabilitation Therapy,Queen’s University,Kingston,Ont.,Canada
Received 7 April 2006;accepted 27 June 2006
Abstract
Background.Although gait characteristics have been well documented in people with knee osteoarthritis,little is known about the
relationships between gait characteristics and performance or disability.Our purpose was to examine the role of knee kinematics and
kinetics on walking performance and disability in people with knee osteoarthritis.We also examined whether pain mediated the relation-
ship between the knee adduction moment and performance or disability.
Methods.Three-dimensional gait analysis was conducted on 54 people with medial compartment knee osteoarthritis.Performance
was quantified with the Six Minute Walk test and disability was self-reported on the Short Form-36.The pain subscale of the Western
Ontario McMaster Universities Osteoarthritis Index and the functional self-efficacy subscale of the Arthritis Self-Efficacy scale were
completed.
Findings.Astep-wise linear regression demonstrated that the variance in Six Minute Walk test scores was explained by functional self-
efficacy (50%) and the range of knee motion (8%).The variance in Short Form-36 was explained by pain (36%),the peak extension angle
(19%) and the range of knee motion (4%).Pain was unrelated to the knee adduction moment so analyses of pain as a mediator of the
adduction moment on either performance or disability were halted.
Interpretation.Kinematic output fromthe motor control systemis useful in understanding some variance in current performance and
disability in people with knee osteoarthritis.The knee adduction moment was unrelated to these variables and pain did not mediate
between the knee adduction moment and performance or disability.Therefore this moment does not explain current clinical status in
people with knee osteoarthritis based on the measures of performance and disability used in this study.
￿ 2006 Elsevier Ltd.All rights reserved.
Keywords:Arthritis;Gait analysis;Outcome assessment;Mobility;Self-efficacy;Physical therapy
1.Introduction
The prevalence of knee osteoarthritis (OA) makes this
disease the single greatest cause of chronic disability in
community-dwelling adults in the United States (Guccione
et al.,1994).Although definitions vary,disability can be
defined as an inability to fulfill a social role,such as work
or family responsibilities (NIH,1993).To quantify disabil-
ity,researchers use self-report questionnaires that include
items that reflect the impact of disease on social function-
ing.For example,Lingard and colleagues used the Short
Form-36 (SF-36),which includes domains of emotional,
physical and social functioning,as an outcome measure
post-arthroplasty in a 2-year prospective study (Lingard
et al.,2004).Although the SF-36 is frequently used as an
outcome measure of disability,little research has examined
the correlates of disability in people with knee OA.Our
previous work showed that pain explained nearly 40% of
the variance in SF-36 raw scores (Maly et al.,2006);but
little is known about other factors that contribute to dis-
ability measures in people with knee OA.
0268-0033/$ - see front matter ￿ 2006 Elsevier Ltd.All rights reserved.
doi:10.1016/j.clinbiomech.2006.06.010
*
Corresponding author.
E-mail address:mmaly@uwo.ca (M.R.Maly).
www.elsevier.com/locate/clinbiomech
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Because disability in people with knee OA has been
linked to limitations in lower extremity mobility perfor-
mance (Guccione et al.,1994),some research has attempted
to uncover the determinants of physical performance.
Walking is the activity most commonly reported as difficult
by those with knee OA(Guccione et al.,1994).Self-efficacy,
a person’s beliefs in their capabilities to organize and exe-
cute actions required to achieve a goal (Bandura,1998),is
an important determinant of walking performance in
people with knee OA (Harrison,2004;Sharma et al.,
2003).In one study,self-efficacy explained 50% of the vari-
ance in the distance walked in 6 min in people with knee
OA;while mechanical variables like strength and body
weight contributed an additional 12% (Maly et al.,2005).
It may be surprising that a psychosocial variable such as
self-efficacy would contribute more to physical performance
than mechanical variables.However,strength and obesity
may not be the only or the most influential of mechanical
variables affecting performance.Gait characteristics could
explain variance in walking performance in knee OA.For
example,altered knee loading during weight-bearing may
result in pain due to intraosseous pressure,effusion and
ischemia (O’Reilly and Doherty,1998),which would inter-
fere with walking performance.Thus,it is possible that gait
mechanics would demonstrate a stronger relationship to
walking performance than other mechanical variables pre-
viously studied in people with knee OA.
Gait analysis has proven useful in differentiating
between people with and without knee OA.Compared to
healthy adults,people with knee OA walk more slowly
due to a shorter stride length or decreased cadence (Stauffer
et al.,1977;Brinkman and Perry,1985;Gok et al.,2002;
Kaufman et al.,2001) and with decreased sagittal plane
knee motion (Stauffer et al.,1977;Brinkman and Perry,
1985;Kaufman et al.,2001;Messier et al.,1992).Most
studies show that the external knee adduction moment is
greater in people with knee OA,even when compared to
age,sex-matched controls (Gok et al.,2002).The adduc-
tion moment correlated with bone density distribution at
the proximal tibia (Hurwitz et al.,1998) suggesting that it
is a reasonable proxy for medial loading.The adduction
moment also relates to disease severity and progression
(Miyazaki et al.,2002;Wada et al.,2001;Sharma et al.,
1998) and change in pain intensity with medication
(Hurwitz et al.,2000).In the presence of pain,the knee
adduction moment is lower during stair-climbing or walk-
ing (Schnitzer et al.,1993;Hurwitz et al.,2000;Shrader
et al.,2004).It is possible that this moment will relate
strongly to performance and disability,but these relation-
ships are unclear.Also unclear is whether an elevated knee
adduction moment directly results in pain.It is possible
that pain mediates a relationship between elevated medial
loading and performance or disability (Fig.1).A mediator
is a variable (e.g.,pain) that represents a mechanism
through which an independent variable (adduction
moment) influences the dependent variable of interest (per-
formance) (Baron and Kenny,1986).
Other gait mechanics may also be important to walking
performance,including external rotation (Gok et al.,2002),
flexion (Kaufman et al.,2001) and extension moments
(Hurwitz et al.,2000).These kinetic variables have not
been studied extensively,yet these may be useful in under-
standing performance.For example,people with knee OA
have a higher peak knee flexor moment,which is thought
to improve joint stability during level walking (Schipplein
and Andriacchi,1991).Because our current knowledge
about the role of various gait characteristics on walking
performance in people with knee OA is limited,we aimed
to explore a variety of kinematic and kinetic variables as
potential determinants of performance.
The purpose of this study was to examine the role of
three-dimensional knee angles and moments on walking
performance and self-reported disability.Because self-effi-
cacy is a known determinant of performance and knee pain
is a known determinant of self-reported disability,we con-
trolled for these variables.Second,we investigated whether
pain mediated the relationship between the adduction
moment and both walking performance and disability.
We hypothesized that (1) controlling for self-efficacy,knee
kinematics and kinetics will be significantly related to per-
formance;(2) controlling for pain,knee kinematics and
kinetics will be significantly related to disability and (3)
pain will mediate a relationship between the knee adduc-
tion moment and disability.
2.Methods
2.1.Participants
Fifty-seven participants were involved.Data from three
participants were excluded because radiographs taken dur-
ing the study showed predominantly lateral knee OA.
Intervention studies and theories of pathology suggest that
lateral OA may involve different mechanics from that of
medial (Ogata et al.,1997;Cerejo et al.,2002).
The remaining sample of 54 adults were over age 50
(mean =68.3,SD=8.7) with physician-diagnosed medial
knee OA.The diagnosis was made by family physicians
in all but in two cases where the diagnosis was made by
an orthopaedic surgeon.Diagnosis was consistent with
Medial Compartment Loading
(Knee Adduction Moment)
Pain
(WOMAC Pain Scale)
Walking Performance
(Six Minute Walk)
Disability
(Short Form-36)
Fig.1.Hypothesis#3:Proposed mechanism whereby pain intensity
during weight-bearing activities mediates the relationship between medial
joint loading with performance and disability in people with mild-to-
moderate knee OA.WOMAC=Western Ontario McMaster Universities
Osteoarthritis Index.
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the American College of Rheumatology (ACR) criteria,
either based on radiographs or clinical signs and symptoms
(Altman et al.,1986).Radiographs were taken at the begin-
ning of the study to confirm the presence of medial OA.
Participants were recruited by use of a free community
newspaper that is circulated to over 55,000 homes.
Of the 54 participants,32 were women and the left limb
was studied in 29 cases.In cases of bilateral knee OA
(n =26) the more painful knee was tested.As a group,par-
ticipants were highly educated (years of full-time formal
education:mean =14.9,SD=4.3).No participant had
undergone corrective surgery or had had an ipsilateral
hip or ankle condition.All participants were screened for
medical conditions that could be exacerbated by the proto-
col,such as unstable heart disease.Participants had an
average of 2.5 comorbidities,defined as conditions that
required treatment for more than 3 months by a physician.
The most common comorbidities were hand OA,heart dis-
ease,low back pain and hypertension.All participants pro-
vided written informed consent before enrollment.Table 1
summarizes the descriptive data regarding the participants.
2.2.Dependent variables
2.2.1.Physical performance
The Six Minute Walk test (SMW) quantified walking
performance.The SMW yields reliable (intraclass correla-
tion coefficient =0.96),valid data (Cahalin et al.,1996)
and is an inexpensive clinical tool that records the distance
that a participant covers at their own pace for 6 min while
walking indoors.The SMW permits participants to use a
mobility aid (e.g.,cane) or pause.The SMW test was
recorded indoors in a well-lit,25 mtiled hallway.The score
was the total distance traveled during 6 min.Participants
were asked to ‘‘walk as quickly and safely as you can for
6 min.’’
2.2.2.Disability
The SF-36 self-report questionnaire quantified disabil-
ity,recognizing that this measure reflects part of but prob-
ably not all aspects of disability.The SF-36,a culmination
of previously used scales in the medical outcomes study
(Ware and Sherbourne,1992),is a 36-item questionnaire
that measures eight parameters:physical and social func-
tioning,role limitations due to emotional or physical prob-
lems,mental health,bodily pain,vitality and general health
perceptions.The reliability (median reliability coefficient
0.85 for all subscales) has been established (Ware and Sher-
bourne,1992;McHorney et al.,1993,1994;Brazier et al.,
1992).The SF-36 has distinguished between elderly people
with and without poor health,suggesting that the instru-
ment yields valid data in this population (Lyons et al.,
1994).Raw scores,out of 800,where a high score is posi-
tive (low disability),are presented.
2.3.Independent variables
2.3.1.Pain
The pain subscale (PAIN) of the Western Ontario
McMaster Osteoarthritis Index (WOMAC) was chosen to
assess pain because it asks about pain intensity during
walking and stair-climbing.The WOMAC is a self-admin-
istered questionnaire,for patients with hip or knee OA,
consisting of 24 questions categorized in subscales of pain,
stiffness and physical function (Bellamy et al.,1988a).
Using a visual analog format,items on the WOMAC
scores can range between 0 and 100 mm,best to worst
scores.The reliability and validity of the WOMAC has
been well established,including the use of the subscales
separately (Bellamy et al.,1988a,b).
2.3.2.Self-efficacy
The functional self-efficacy subscale (FSE) of the Arthri-
tis Self-Efficacy scale was used to determine self-efficacy for
physical tasks;that is,a person’s belief that he or she can
perform a physical task like walking (Lorig et al.,1989).
The FSE contains nine questions and uses a visual analog
scale in which a higher score indicates greater self-efficacy,
a positive result.A test–retest reliability coefficient (r) of
0.89 and a Cronbach’s alpha of 0.93 to test internal reliabil-
ity were reported for use of the FSE alone (Lorig et al.,
1989).
Table 1
Descriptive participant characteristics,dependent and independent self-report measures (n =54)
Characteristic Variable Mean SD Range
Participant Age (year) 68.3 8.7 50–87
Education (year) 14.9 4.3 8–30
Weight (kg) 82.2 15.0 52–127
Height (cm) 169.7 10.0 149.2–190.5
Chronic comorbidities (n)
a
2.5 1.3 0–6
Dependent measure Six Minute Walk test (m) 440 123 146–642
Short Form-36 507 128 235–744
Independent measure Functional self-efficacy subscale (%)
b
80.7 13.4 39.7–98.3
Pain
c
(/100) 30.3 18.6 3.2–89.2
a
Comorbidities diagnosed by a physician and requiring ongoing treatment (>3 months) (excluding knee osteoarthritis).
b
Scores range between 0 (poor) and 100 (excellent) on this subscale of the Arthritis Self-Efficacy scale.
c
Pain subscale of the Western Ontario and McMaster Universities Osteoarthritis Index.
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2.3.3.Gait
Eighteen gait variables were measured.Three-dimen-
sional knee angle and moment waveforms were calculated.
From each,three variables were identified:positive and
negative peaks and range between peaks.Gait data were
collected using the gait analysis in three dimensions
(GAIT) system,which has been validated for dynamic knee
assessment (DeLuzio et al.,1993;Costigan et al.,1992).
This system incorporates knee alignment and joint surface
geometry data from standardized radiographs to more
accurately transform the surface marker location into par-
ticipant-specific joint centers.GAIT combines an Optotrak
optoelectric system (Northern Digital,Canada),a force
plate (Advanced Mechanical Technology,USA),precision
radiographs and custom processing and analysis.The
model incorporates the inertial characteristics of the foot
in the shank.Infrared emitting diodes (IREDs) were placed
over the greater trochanter,lateral femoral condyle,fibular
head,lateral malleolus and distally on two projecting
wands attached to the thigh and shank.We did not incor-
porate methods to minimize error due to skin motion,such
as IREDclusters (Alexander and Andriacchi,2001).Walk-
ing trials (force plate and motion detection) were sampled
at 100 Hz.Vertical ground reaction force was used to indi-
cate initial foot contact and center of pressure was used as
the application point for the external ground reaction
force.
Standardized radiographs were taken to calculate partic-
ipant-specific correction vectors for surface IREDs.A cor-
rection vector was defined as a vector from the IRED
location indicated by a lead bead,to joint center;this
method has been detailed elsewhere (Kirkwood et al.,
1999).Anthropometrics including body weight,height,
limb circumferences and widths were measured.
The conventions used for this study are illustrated in
Fig.2.Regression equations developed by Clauser et al.
(1969) were used to identify segment mass and segment
center of mass.The floating axis system developed by
Grood and Suntay was used to calculate relative angles
(Grood and Suntay,1983).Net moments were calculated
using a linked-segment model consistent with Winter
(1990).External knee moments were determined from an
inverse dynamics approach and were expressed per
kilogram of body mass.
2.4.Protocol
Participants attended two visits,one week apart.In all
but three cases,the second visit occurred within one week
of the first.In the three cases where data were collected
more than one week apart,the WOMAC was completed
at both visits to confirm that status had not changed.Dur-
ing visit 1,after providing written informed consent,IRED
locations were identified and marked for radiographs and
gait.Standardized standing radiographs were taken by a
trained radiology technician.Five gait trials were collected.
The starting point of the walking trials was adjusted to
ensure that the participants did not aim for the force plate.
Trials where the force profiles appeared unreasonable or
there were missing motion data (>2 consecutive points at
100 Hz) were excluded and repeated.The PAIN and SF-
36 questionnaires were completed.During visit 2,the
SMW and FSE were completed.
2.5.Statistical analysis
Descriptive statistics were calculated on the participant
characteristics,dependent variables (SMW,SF-36) and
self-report questionnaires (FSE,PAIN).For the knee
angles and moments,the positive and negative peaks were
identified in three dimensions and averaged across five tri-
als.Also,maximum range values were averaged across five
trials.Therefore,for each gait waveform three variables
were identified:two peaks and a range.
2.5.1.Correlations
Pearson Correlation Coefficients were calculated.Bon-
ferroni correction for 36 correlations at an alpha level of
0.05 in a two-tailed test requires a P-value < 0.001 for
significance.
2.5.2.Regression
Multivariate analyses were used to investigate which
factors explained performance.A two-block step-wise lin-
ear regression was performed using the SMW as a depen-
dent measure.Block 1 included FSE,PAIN and block 2
included all gait variables significantly correlated with
SMW.The same procedure was repeated using the SF-36
as the dependent variable.In the regressions,an F value
of 0.05 or greater was necessary to be included and an F
DP
PA
LM
Adduction/Abduction
Internal Rotation/External Rotation
Flexion/Extension
Fig.2.Conventions used in QGAIT system.Adduction and abduction
occur about a posterior-to-anterior (PA) axis;flexion and extension occur
about a lateral-to-medial (LM) axis;internal rotation and external
rotation occur about a distal-to-proximal (DP) axis.The right-hand rule,
applied to the right limb,defined the positive direction.Only external
moments are reported.
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value of 0.10 or less was necessary to be removed from the
model.Tolerance values were examined for multi-
collinearity.
2.5.3.Mediation
We elaborated on the three criteria proposed by Baron
and Kenny to identify whether pain mediated the relation-
ship between the knee adduction moment with each of
performance and disability (Fig.1) (Baron and Kenny,
1986).First,a relationship,explored with a linear regres-
sion,must exist between pain and the knee adduction
moment.Second,a relationship must exist between the
SMW and knee adduction moment to ensure that joint
loading relates to performance.Finally,a step-wise regres-
sion must show that controlling for the mediator (pain)
reduces the effect of the independent variable (knee adduc-
tion moment) on the dependent (SMW).The procedure
was repeated with the SF-36.All analyses were completed
using SPSS v12 (SPSS Inc.,Chicago).
3.Results
Descriptive statistics are presented in Table 1.Partici-
pants were overweight,with a body mass index of
28.6 kg/m
2
.The 18 gait variables are presented in Table 2.
3.1.Correlations
Correlation coefficients (r) between the gait variables
and the SMW ranged between 0.01 and 0.65.Significant
relationships existed between the SMWand peak extension
angle and range of flexion/extension angle.The significant
correlations between SF-36 and gait variables involved the
same kinematic variables (Table 3).Range of flexion/exten-
sion angle was related to the peak extension angle
(r =￿0.58).
3.2.Role of gait variables on performance and disability
The step-wise linear regression models that explain the
variance in the SMW and SF-36 are shown in Table 4.In
both,tolerance statistics were over 0.85 and the indepen-
Table 2
Descriptive gait variables (n =54)
Knee Axis Variable Mean SD
Angles X Peak adduction angle (￿) 9.4 5.2
Peak abduction angle (￿) 1.6 5.5
Range (￿) 7.8 3.1
Y Peak flexion angle (￿) 64.3 9.2
Peak extension angle (￿) 8.8 7.2
Range (￿) 55.5 10.7
Z Peak internal rotation angle (￿) 3.8 5.2
Peak external rotation angle (￿) ￿8.1 5.1
Range (￿) 11.8 3.1
Moments X Peak adduction moment (Nm/kg) 0.46 0.17
Peak abduction moment (Nm/kg) ￿0.06 0.03
Range (Nm/kg) 0.52 0.16
Y Peak flexion moment (Nm/kg) 0.27 0.16
Peak extension moment (Nm/kg) ￿0.27 0.10
Range (Nm/kg) 0.54 0.18
Z Peak internal rotation moment (Nm/kg) 0.13 0.05
Peak external rotation moment (Nm/kg) ￿0.01 0.01
Range (Nm/kg) 0.14 0.05
Table 3
Pearson correlation coefficients between gait variables,performance and
disability
Knee Axis Gait variable SMW SF-36
Angles X Peak adduction Angle (￿) ￿0.16 ￿0.08
Peak abduction angle (￿) ￿0.13 ￿0.07
Range (￿) ￿0.05 ￿0.01
Y Peak flexion angle (￿) 0.27 0.19
Peak extension angle (￿) ￿0.55
*
￿0.65
*
Range (￿) 0.60
*
0.59
*
Z Peak internal rotation
angle (￿)
￿0.26 ￿0.19
Peak external rotation
angle (￿)
￿0.27 ￿0.27
Range (￿) 0.11 0.11
Moments X Peak adduction moment
(Nm/kg)
￿0.01 0.06
Peak abduction moment
(Nm/kg)
￿0.17 ￿0.10
Range (Nm/kg) 0.34 0.09
Y Peak flexion moment
(Nm/kg)
0.14 0.14
Peak extension moment
(Nm/kg)
￿0.36 ￿0.42
Range (Nm/kg) 0.33 0.36
Z Peak internal rotation moment
(Nm/kg)
0.09 0.12
Peak external rotation moment
(Nm/kg)
￿0.17 ￿0.23
Range (Nm/kg) 0.13 0.18
SMW=Six Minute Walk test,SF-36 =Short Form-36 questionnaire.
*
Correlation is significant at a P-value of <0.001 (two tailed,Bonferroni
correction for 48 comparisons).
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dent variables were entered in two blocks:block 1 included
FSE and PAIN,block 2 included range of flexion/exten-
sion angle and peak extension angle.
For the model of physical performance,the FSE (50%)
and the range of flexion/extension angle (8%) explained
the variance in SMWscores.For the model of self-reported
disability,pain explained 36% of the variance in the SF-36.
The peak extension angle added another 19%and the range
of peak flexion/extension explained another 4% of the
SF-36.
The regression analyses were repeated forcing the knee
adduction moment in the second block of independent
variables.No difference in either regression model resulted.
We also repeated the step-wise linear regression analyses
excluding FSE and PAIN to identify if gait mechanics
would show better relationships with walking performance
and disability.The resultant model for the SMWincluded
the range of flexion/extension angle and the peak extension
angle to explain 41% of the variance.Because this model
explained 17% less variance than the original,it was dis-
carded.The model calculated for the SF-36 also included
these variables,in the opposite order,to explain 48% of
the score variance.This model was also discarded because
it explained 11% less than the original model.
3.3.Pain as a mediator
The linear regression examining whether the knee
adduction moment related to pain was not significant
(unstandardized coefficient =32.9,Adjusted R
2
=0.02,
P < 0.668).The analyses of mediation were not completed
because this first criterion was not met.
4.Discussion
The purpose of this study was to explore the role of
three-dimensional knee angles and moments on walking
performance and self-reported disability.We theorized that
the knee adduction moment,a proxy for medial loading,
would create pain during weight-bearing activities,thereby
limiting walking performance and contributing to disabil-
ity.Self-efficacy and the dynamic range of knee flexion/
extension motion were related to walking performance.
Pain,the peak knee extension angle and range of motion
were statistically related to disability.Therefore,knee
angles helped to explain current levels of performance
and self-reported disability in people with mild-to-moder-
ate knee OA.The knee adduction moment did not relate
to current levels of performance or self-reported disability
and therefore did not explain current clinical status in this
population.
Our first hypothesis that controlling for self-efficacy,
knee kinematic and kinetic variables would significantly
relate to performance was supported.As we anticipated
(Harrison,2004;Sharma et al.,2003),the FSE was strongly
related to walking scores.In this study we found that a
person with an FSE score higher by 10%,indicating a
higher level of confidence for physical tasks,walked an
extra 52 m compared to his/her peers.The minimal detect-
able change in the SMW in this population is 61 m (Ken-
nedy et al.,2005).An education-based intervention
improved self-efficacy by 16% at a four year follow-up
(Lorig et al.,1993) suggesting that improving self-efficacy
alone is possible and could result in a clinically significant
improvement in walking performance.Our findings also
show that over and above the effects of FSE,the sagittal
knee range of motion was a determinant of walking perfor-
mance.Limitations in the dynamic range of sagittal plane
knee motion in people with knee OA are well documented
(Stauffer et al.,1977;Brinkman and Perry,1985;Kaufman
et al.,2001;Messier et al.,1992) and knee range of motion
is related to radiographic disease severity (Ersoz and
Ergun,2003).Our findings suggest a person with 10￿
greater total range of motion during gait walked 38 m fur-
ther on the SMW,a substantial increase froma clinical per-
spective.For some people,improving dynamic range of
motion by 10￿ is possible through exercise but the clinical
relevance of this improvement on walking performance is
questionable.Interestingly,six month outcomes following
arthroplasty demonstrate that total knee range of motion
is decreased by approximately 20￿ (Beaupre et al.,2001)
suggesting that walking performance may be significantly
impaired.Further research is necessary to confirm this
effect of range of motion on walking performance.
Our hypothesis that controlling for pain,knee kinematic
and kinetic variables would significantly relate to self-
reported disability was supported,though the clinical
importance warrants further discussion.The data show
that pain explained 36% of the variance in the SF-36 such
that the greater a person’s self-reported pain intensity,the
Table 4
Models of performance and disability measures (n =54)
a
Domain Dependent variable Model Unstandardized
coefficient
Standardized
coefficient
P Adjusted
R
2
Performance Six Minute Walk test (m) (1) Functional self-efficacy subscale (%) 5.17 0.56 0.001 0.50
(2) Range knee flexion/extension angle (￿) 3.83 0.33 0.002 0.58
Disability Short Form-36 (1) Pain
a
￿0.52 ￿0.38 0.001 0.36
(2) Peak knee extension angle (￿) ￿6.42 ￿0.36 0.001 0.55
(3) Range knee flexion/extension angle (￿) 3.04 0.26 0.021 0.59
a
Pain subscale of the Western Ontario McMaster Universities Osteoarthritis Index.
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greater their self-reported level of disability.The clinical
significance of this effect may be small however,because
a person reporting 10% less pain would only experience
an increase of 5 out of 800 points on the SF-36.The peak
knee extension angle,which occurs at either loading
response or terminal swing in the gait cycle,contributed
to the regression model over and above pain,indicating a
statistically significant role of output from the motor con-
trol system on disability.A reduction in full extension at
terminal swing shortens the stride thereby limiting walking
ability necessary for socially relevant activities.Those
unable to achieve near full extension during loading
response risk loading the extensors eccentrically to a
greater extent.Improving knee extension by 10￿ would
result in an increase in SF-36 raw score of 64 points.
Because the mean peak extension angle in this study was
8.8￿ these finding suggest that clinicians could aim for full
knee extension but this goal may not be practical in all
cases.The third variable explaining disability was the sag-
ittal knee range of motion.Similarly,the clinical signifi-
cance for this finding is unclear.An increase in total
range of knee motion by 10￿ would only increase SF-36
raw scores by 30 out of 800 points.Although gait mechan-
ics are related to both performance and self-reported dis-
ability,the role of gait kinematics on disability appear
more tenuous.This finding is not surprising given the com-
plex nature of disability,which is influenced by many fac-
tors ranging from mechanical to societal (NIH,1993).
This conclusion is consistent with our finding that perfor-
mance and self-report have different determinants (Maly
et al.,2006).
Knee kinetics were not significantly related to either per-
formance or self-reported disability suggesting that kinetics
have limited value to contribute to the understanding of
clinical outcomes,at least in a cross-sectional study of this
nature.The knee adduction moment has particular signifi-
cance in knee OA (Andriacchi et al.,2004).Forcing the
adduction moment into the regression models did not
change the result in our models incorporating this variable.
However,our sample,recruited from the community,had
relatively mild knee OA suggesting that the mean peak
knee adduction moment would be low compared to a more
severe population (Mundermann et al.,2005).Our findings
do not discount the utility of the knee adduction moment
in differentiating people with knee OA from others nor
the possibility that this variable may be useful in tracking
progression over time.Furthermore,we do not suggest that
kinetics in general are unimportant to performance or dis-
ability because joint powers and energies should be
strongly related to walking performance.For example,
other researchers have demonstrated the utility of a power
and energy approach to understanding compensatory gait
mechanics in similar populations (McGibbon and Krebs,
2002).
Our third hypothesis suggested that pain intensity would
mediate the relationship between medial loading and each
of performance and disability.Our findings did not satisfy
the criteria proposed by Baron and Kenny (1986):pain and
the adduction moment were unrelated.Several factors may
explain why these variables were unrelated in this cross-sec-
tional study.First,the WOMAC pain subscale assesses
‘‘recent’’ pain which may not be the same as the pain that
occurred during gait testing.For example,participants
could have been experiencing a relatively ‘‘good’’ day dur-
ing testing,leading to a poor correlation between pain
reported on the WOMAC and the knee adduction moment
recorded in the laboratory.Second,pain may result in
complex compensatory strategies that complicate any rela-
tionship between pain and the adduction moment.For
example,a prospective study demonstrated that after
administering an analgesic,an increased adduction
moment was observed (Schnitzer et al.,1993).Although
these findings suggest that pain mitigated the knee adduc-
tion moment,there was no statistically significant correla-
tion between the adduction moment and change in pain
(Schnitzer et al.,1993).
Some limitations must be considered.Our gait protocol
did not incorporate methods of controlling for skin marker
motion,for example by using marker clusters but these
effects were likely to be negligible given the purpose of this
study.Some participants had bilateral disease which may
have influenced the gait characteristics observed.In terms
of generalizability,this sample appeared to have milder
symptoms of OA than those involved in many studies,
likely because we recruited from the community rather
than surgical waiting lists.For example,these participants
had relatively low levels of pain and the self-efficacy scores
were considerably higher than those obtained in other
studies.
Future research could be directed toward examining the
effect of interventions to improve self-efficacy,pain and
dynamic range of motion on performance and disability
in people with knee OA.The construct of self-efficacy
requires further study to more precisely model its role in
performance in knee OA.It is also possible that we would
have different results with a more severe or variable study
sample.Finally continued research is necessary to develop
more sophisticated models that provide insight into the
meaning of performance and disability in people with knee
OA.
5.Conclusions
This study examined the clinical significance of gait
analysis in people with knee OA by exploring the relation-
ships between knee angles and moments,pain,self-efficacy,
performance and disability.Our findings demonstrated
that kinematic output from the motor control system is
useful in understanding some variance in current levels of
performance and disability in people with knee OA,over
and above self-efficacy and pain respectively.Despite the
theoretical importance of the knee adduction moment,it
was unrelated to performance and disability.Our proposed
model that pain mediated between the knee adduction
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moment and performance or disability was not supported.
Therefore,the adduction moment does not explain current
clinical status of performance and disability in people with
mild-to-moderate knee OA.
Acknowledgements
This work has been supported by Canadian Institutes
for Health Research (Grant#99034),Toronto Rehabilita-
tion Institute,and Natural Sciences and Engineering
Research Council.
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