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Hip abduction moment and protection against medial tibiofemoral osteoarthritis progression.

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Vol. 52, No. 11, November 2005, pp 3515–3519
DOI 10.1002/art.21406
© 2005, American College of Rheumatology
Hip Abduction Moment and Protection Against
Medial Tibiofemoral Osteoarthritis Progression
Alison Chang,1 Karen Hayes,1 Dorothy Dunlop,1 Jing Song,1 Debra Hurwitz,2
September Cahue,1 and Leena Sharma1
95% confidence interval (95% CI) of 0.32–0.85. This
protective effect persisted after adjustment for age, sex,
walking speed, knee pain severity, physical activity,
varus malalignment severity, hip OA presence, and hip
OA symptom presence, with an adjusted OR of 0.43 a
95% CI of 0.22–0.81.
Conclusion. A greater hip abduction moment
during gait at baseline protected against ipsilateral
medial OA progression from baseline to 18 months. The
likelihood of medial tibiofemoral OA progression was
reduced 50% per 1 unit of hip abduction moment.
Objective. To test the hypothesis that a greater
peak internal hip abduction moment is associated with
a reduced likelihood of ipsilateral medial tibiofemoral
osteoarthritis (OA) progression.
Methods. Fifty-seven persons with knee OA (by
definite osteophyte presence and symptoms) were evaluated. Baseline assessments included kinematic and
kinetic gait parameters, obtained with an optoelectronic
camera system and force platform, with inverse dynamics used to calculate 3-dimensional moments at the
joints; pain, using a separate visual analog scale for
each knee; and alignment, using full-limb radiographs.
Radiographs of the knee in a semiflexed position, with
fluoroscopic confirmation of tibial rim alignment, were
obtained at baseline and 18 months later. Disease progression was defined as worsening of the grade of medial
joint space narrowing. Logistic regression obtained with
generalized estimating equations was used to estimate
odds ratios (ORs) for progression per unit of hip
abduction moment, after excluding knees with the worst
joint space grade at baseline (which could not progress).
Results. The 57 participants (63% women) with
mild to moderate OA had a mean age of 67 years and a
mean body mass index of 29. A greater internal hip
abduction moment during gait was associated with a
reduced likelihood of medial tibiofemoral OA progression, with OR/unit hip abduction moment of 0.52 and a
In knee osteoarthritis (OA), the medial tibiofemoral compartment is the most common site of disease. The susceptibility of the medial compartment to
OA development may relate to greater load distribution
(i.e., 60–80%) to the medial than the lateral compartment, even in healthy knees, during gait. Excessive
medial compartment loading is widely believed to contribute to medial OA progression. Because direct measurement of knee load is invasive, external knee adduction moment during gait, a correlate of medial load, has
been used in knee OA studies (1). In keeping with the
concept that load influences progression, a greater knee
adduction moment predicted a greater likelihood of
medial OA progression (2,3).
In theory, reduction of medial load may have a
beneficial disease-modifying effect on medial knee OA,
i.e., it may slow the rate of OA progression. However, it
is unclear how to achieve this reduction. Altering certain
kinematic or kinetic parameters during gait could, theoretically, reduce medial load. Whether any such parameters protect against knee OA progression has not
previously been examined. Such information would
guide and direct the development of novel rehabilitative
interventions to delay medial knee OA progression.
A potentially protective kinetic parameter is the
internal hip abduction moment. During the single-limb
Supported by the NIH (grants P60-AR-48098, R01-48748,
R01-AR-46225, and RR-00048).
Alison Chang, PT, MS, Karen Hayes, PT, PhD, Dorothy
Dunlop, PhD, Jing Song, MS, September Cahue, BS, Leena Sharma,
MD: Feinberg School of Medicine, Northwestern University, Chicago,
Illinois; 2Debra Hurwitz, PhD: Rush–Presbyterian–St. Luke’s Medical
Center, Chicago, Illinois.
Address correspondence and reprint requests to Leena
Sharma, MD, Division of Rheumatology, Feinberg School of Medicine, Northwestern University, 240 East Huron, Suite 2300, Chicago,
IL 60611. E-mail:
Submitted for publication March 25, 2005; accepted in revised
form August 9, 2005.
Figure 1. Mechanism by which stance limb hip abductor weakness can
potentially lead to increased load on the ipsilateral medial tibiofemoral
compartment. Hip abductor weakness in the stance limb leads to
additional pelvic drop in the contralateral swing limb, which shifts the
body’s center of mass (CM) toward the swing limb. This shift in the
center of mass increases forces across the medial compartment cartilage of the stance limb.
stance phase of gait, weakness or decreased torque generation of the hip abductor muscles in the stance limb
causes excessive pelvic drop in the contralateral swing
limb (4). This drop shifts the body’s center of mass toward
the swing limb, thereby increasing forces across the medial
tibiofemoral compartment cartilage of the stance limb
(Figure 1). The magnitude of hip abductor muscle torque
generation during walking, the most common human
weight-bearing activity, can be captured in quantitative
gait analysis as the internal hip abduction moment.
Based on the proposed mechanism, a greater
internal hip abduction moment during gait may prevent
excessive medial compartment loading and potentially
protect against ipsilateral medial OA progression. We
tested the hypothesis that a greater peak internal hip
abduction moment assessed during quantitative gait
analysis is associated with a reduced likelihood of ipsilateral medial tibiofemoral OA progression, and we
examined whether any effect persists after adjusting for
potential confounders.
Participants. Mechanical Factors in Arthritis of the
Knee (MAK) is a natural history study of knee OA at
Northwestern University. As previously described (5), MAK
participants were recruited from several community sources.
Inclusion and exclusion criteria were based on findings of a
National Institute of Arthritis and Musculoskeletal and Skin
Diseases (NIAMS)/National Institute on Aging (NIA) workshop (6). Inclusion criteria were definite osteophytes in one or
both knees and at least a little difficulty (graded using a Likert
scale) with 2 or more items in the Western Ontario and
McMaster Universities Osteoarthritis Index physical function
scale. Exclusion criteria were a corticosteroid injection within
the previous 3 months, avascular necrosis, inflammatory
arthritis, periarticular fracture, Paget’s disease, villonodular
synovitis, joint infection, ochronosis, neuropathic arthropathy,
acromegaly, hemochromatosis, Wilson’s disease, osteochondromatosis, gout, pseudogout, osteopetrosis, and bilateral total
knee replacement or plans for replacement within the next
Additional exclusion criteria for the current study were
replacement of any joint in a lower extremity, lateral tibiofemoral OA bilaterally (based upon the presence of grade 2
definite narrowing, or worse, using the 0–3 scale of the
Osteoarthritis Research Society International [OARSI] atlas).
The sample in this study included all MAK participants who
had quantitative gait analysis performed. All of these participants returned for followup. All participants provided informed consent. Institutional Review Board approval was
Quantitative gait analysis to measure hip joint moments. Fifty-seven MAK participants underwent quantitative
gait analysis (at Rush–Presbyterian–St. Luke’s Medical Center) within 1 month of their MAK evaluation. The Computerized Functional Testing Corporation (Chicago, IL) system was
used, including 4 optoelectronic cameras (Qualisys, Gothenburg, Sweden) with a sampling frequency of 120 Hz and a
single multicomponent force plate (Bertec, Columbus, OH).
Six passive markers were used in the link segment model. They
were placed at the following bony landmarks: the lateral-most
aspect of the superior iliac crest, the greater trochanter, the
lateral joint axis line of the knee, the lateral malleolus, the
lateral aspect of the calcaneus, and the base of the fifth
metatarsal. Inverse dynamics were used to calculate the external moments (in 3 planes) at the hip, knee, and ankle joint
centers, using the 3-dimensional kinematic data acquired with
the cameras along with the ground reaction forces and moments obtained with the force plate.
The external moments were calculated by taking into
account the moment about the joint center (created by the
ground reaction force) and inertial forces. An external moment is equal and opposite to the internal moment created by
muscles, soft tissues, and joint contact forces. The betweensession reliability of hip abduction moment measurement for
this laboratory was studied in 10 persons. The intraclass
correlation coefficient (model 2.1) was 0.83. The examiner and
the investigator (DH) processing the gait data were blinded to
all radiographic data.
Measurement of covariates. Knee pain severity was
measured using 100-mm visual analog scales, with separate
scales for the right and left knees. The scales were anchored at
0 ⫽ “no pain” and at 100 ⫽ “pain as bad as it could be, and”
standardized instructions for assessment were given.
Physical activity was assessed using the Physical Activity Scale for the Elderly (PASE), a self-report measure of
global activity including recreational, occupational, and household activities (higher score indicates greater activity) (7). PASE
was designed to assess activities commonly engaged in by older
persons, and its construct validity and test–retest reliability have
been demonstrated in community-dwelling older adults (7).
Using knee radiographs acquired with the protocol
described below, disease severity was assessed using the Kellgren/
Lawrence (K/L) scale. In the K/L grading system, 0 ⫽ normal, 1 ⫽
possible osteophytes, 2 ⫽ definite osteophytes, possible joint
space narrowing, 3 ⫽ moderate osteophytes, definite narrowing,
some sclerosis, possible attrition, and 4 ⫽ large osteophytes,
marked narrowing, severe sclerosis, definite attrition.
To assess the severity of varus malalignment, an anteroposterior full-limb radiograph was obtained according to a
previously described protocol (5). Alignment was measured by
one reader (LS) as the angle formed by the intersection of the
lines connecting the centers of the femoral head and intercondylar notch with the lines connecting the centers of the ankle
talus surface and tibial spine tips. Our reliability with this
approach is high, as previously reported (5).
Hip symptom presence was defined as pain, aching, or
stiffness lasting at least 1 month during the previous 12
months. Hip OA presence was defined using the American
College of Rheumatology (ACR) clinical criteria (8).
Assessment of OA progression. Bilateral radiographs
of the knees with weight bearing were obtained at baseline and
18 months, following the Buckland-Wright protocol (9). This
protocol meets criteria set by the NIAMS/NIA workshop (6)
and the OARSI (10). Knee position, beam alignment, magnification correction, and measurement landmarks were specified. The semiflexed position superimposed the anterior and
posterior medial tibial margins. Tibial rim alignment and tibial
spine centering in the notch were confirmed fluoroscopically
before radiographs were obtained. Radiographs were obtained
in 1 unit by 2 trained technicians. Foot maps made at baseline
were used at followup.
Medial tibiofemoral OA progression was defined as
any worsening in the radiographic medial joint space grade
between baseline and 18 months. Illustrated OARSI atlas
grades (none, possibly, definitely, or severely narrowed joint
space) (11) were used by 1 reader (LS). Reliability of radiographic grading (of joint space and K/L scoring) by the reader
was very good (␬ ⫽ 0.85–0.86). The knee radiograph reader
was blinded to the gait analysis data.
Statistical analysis. Knees with a tibiofemoral joint
space grade of 3 at baseline were excluded from analysis, since
further progression was not possible. In descriptive analyses,
hip abductor moment means were estimated from ordinary
least squares regression. Since the rationale for our hypothesis
did not support analysis of only 1 knee, we included both
knees, using generalized estimating equations (GEE), which
validly allow data from both knees to be included in the
analysis (12). Logistic regression using GEE was used to assess
the effect of increased hip abduction moment on the odds of
OA progression. Results are presented as odds ratios (ORs)
per unit of hip abduction moment. An OR of ⬍1 represents a
protective effect against OA progression; an associated 95%
confidence interval (95% CI) that excludes 1 denotes a statistically significant effect. Hip moment data were normalized for
body weight and height. Analyses were adjusted for age
(treated as a continuous variable), sex, gait speed (continuous),
knee pain (continuous), disease severity (as reflected by K/L
score coded as indicator variables using K/L ⫽ 0 as the
reference), severity of varus malalignment (continuous, with
varus as a positive value, neutral as 0, and valgus as negative),
physical activity (continuous), hip symptoms (presence/
absence), and hip OA (presence/absence).
Figure 2. Trajectory of the internal hip abduction moment during the
stance phase of gait in the progressing knee and nonprogressing knee.
Our sample consisted of 57 participants (63%
women) contributing 103 knees at risk for OA progression.
The mean ⫾ SD age was 67 ⫾ 8.7 years and the mean ⫾
SD body mass index was 29 ⫾ 4.1 kg/m2. The majority
(72%) of the 103 knees had mild OA, without definite
joint space narrowing. The remaining 28% of the knees
had definite (but not severe) medial narrowing.
Figure 2 shows the trajectories of the internal hip
abduction moment for a knee that progressed and a
knee that did not progress from baseline to 18 months.
The peak hip abduction moment occurs at the early stance
phase of gait. The nonprogressing knee had a greater peak
hip abduction moment than the progressing knee.
As shown in Table 1, the mean ⫾ SD peak
internal hip abduction moment in all knees was 4.41 ⫾
0.11 (% body weight ⫻ height). The peak internal hip
Table 1. Baseline peak internal hip abduction moment in all knees in
which medial tibiofemoral OA progressed and in knees in which OA
did not progress from baseline to 18 months*
Peak internal hip
abduction moment
at baseline, % body
weight ⫻ height
All knees (n ⫽ 103)
Progressing knees (n ⫽ 17)
Nonprogressing knees (n ⫽ 86)
OR (95% CI) for the difference between
progressing and nonprogressing knees
4.41 ⫾ 0.11
4.01 ⫾ 0.12
4.49 ⫾ 0.12
0.48 (0.16–0.81)
* Values are the mean ⫾ SD. OA ⫽ osteoarthritis; OR ⫽ odds ratio;
95% CI ⫽ 95% confidence interval.
Table 2. Unadjusted and adjusted odds ratios for medial tibiofemoral OA progression per 1 unit of hip abduction moment*
Age, sex, gait speed, knee pain severity, physical
activity, knee OA severity
Age, sex, gait speed, knee pain severity, physical
activity, varus severity
Age, sex, gait speed, knee pain severity, physical
activity, knee OA severity, hip OA, hip
Age, sex, gait speed, knee pain severity, physical
activity, varus severity, hip OA, hip symptoms
OR for medial
OA progression
(95% CI)†
0.52 (0.32–0.85)
0.50 (0.26–0.96)
0.42 (0.22–0.79)
0.48 (0.24–0.95)
0.43 (0.22–0.81)
* See Table 1 for definitions.
† All ORs were statistically significant.
abduction moment was greater in knees that did not
progress than in knees that did progress over 18 months.
Next, we examined the relationship between the
internal hip abduction moment at baseline and the
likelihood of medial OA progression from baseline to 18
months (Table 2). Greater internal hip abduction moment had a protective effect, i.e., lowered the odds of
progression, and this effect persisted after adjusting for
potential confounders, including age, sex, gait speed,
knee pain severity, physical activity, knee OA severity
(or, in alternate models, severity of varus malalignment),
hip symptoms, and hip OA presence. The hip abduction
moment/progression relationship persisted after further
adjustment for the external knee adduction moment
(adjusted OR 0.27, 95% CI 0.11–0.69).
A greater internal hip abduction moment measured during quantitative gait analysis at baseline protected against medial tibiofemoral OA progression during the following 18 months. The odds of medial OA
progression were reduced by 50% with an additional 1
unit of hip abduction moment. This strong protective effect
persisted after adjusting for potential confounders.
In the study of knee OA, frontal-plane knee joint
mechanics, such as varus or valgus alignment and laxity,
as well as varus or valgus forces and moments acting on
the knee, have received attention (5,13–16). However,
the knee joint does not function in isolation from the
rest of the lower limb kinematic chain during weightbearing activities; hip and ankle/foot mechanics may
influence knee joint load during gait.
The relationship between hip and knee mechan-
ics in knee OA gait has been examined by McGibbon
and Krebs (17). They focused on sagittal plane alterations in hip, knee, and ankle mechanics that develop
presumably to compensate for ipsilateral knee OA disease. In their study, persons with OA had reduced knee
extension concentric power and increased hip extension
eccentric power when compared with healthy agematched elderly subjects. They proposed that the
changes in knee and hip power might be a compensation
mechanism to avoid using the quadriceps, and thereby
reduce articular loads. Their study illustrates the importance of considering the whole kinematic chain to better
understand knee loads in individuals with knee OA.
Because their focus differed from ours, they did not
describe frontal-plane hip joint mechanics.
The contribution of frontal-plane hip joint mechanics to knee joint loading has not been examined.
Especially since few knee muscles specifically provide
knee frontal-plane stability, hip frontal-plane muscles
may play an important role in regulating medial/lateral
knee load distribution and providing frontal stability. In
addition, hip muscles have a large cross-sectional area
and can generate significant forces to control load.
Fredericson and colleagues (18) found decreased hip
abductor strength (measured with a hand-held dynamometer) in runners with iliotibial band syndrome
(ITBS) when compared with the healthy limb and with
runners without ITBS. After 6 weeks of hip abductor
strengthening, these runners had increased hip abductor
muscle strength and returned to pain-free running. It is
plausible that decreased hip abductor activity might lead
to excessive tensile stress on the lateral knee structures,
such as the iliotibial band, and increased load on the
medial compartment. The results of our study provide
some longitudinal evidence for this theoretical framework. In order to capture the magnitude of the hip
abduction muscle torque generated during a common
weight-bearing activity, we focused on the hip abduction
To our knowledge, this is the first longitudinal
study on the effect of the hip abductor moment during
gait on the course of medial knee OA. While surface
electromyogram (EMG) can be performed during quantitative gait analysis, needle EMG is a superior, albeit
more invasive, approach. However, EMG is classically
used to assess motor unit recruitment and the temporal
characteristics of muscle contraction, which are not well
correlated with torque generation.
The major source of hip abduction moment
magnitude is hip muscle strength. The hip joint ligaments and capsule also make a small contribution; there
is no established way to account for this contribution.
Hip symptoms could inhibit hip abductor muscle torque
generation during gait (19) and could reflect an OA
process in the hip that in and of itself contributes to knee
OA progression. Only 5 participants met the ACR criteria
for hip OA. We adjusted for both the presence of hip OA
and the presence of hip pain without any impact on the
protective effect of the hip abduction moment magnitude.
We also considered other potential confounders.
Slower walking speed is associated with lower hip and
knee joint moments (20,21) and may accompany progressive knee OA. Greater physical activity could theoretically be associated with greater hip torques and
protection against OA progression. Greater varus malalignment or knee adduction moment each increases the
likelihood of medial knee OA progression, and, in theory,
could be linked to a reduction in hip abductor muscle
torque. A strong protective effect of internal hip abduction
moment persisted after adjusting for these factors.
As is almost always the case in knee OA cohorts
in the US, the study sample was, on average, overweight.
Because quantitative gait analysis uses skin markers, skin
movement is an inherent limitation in any gait study of
knee OA. One way to address this concern, placement of
markers in bone, is of course not feasible. Measurement
muddiness introduced by skin movement, if anything,
would reduce the likelihood of identifying a relationship.
Despite this, we were able to detect a protective effect.
The magnitude of increase in hip abduction
moment with hip exercise is not known, and would be an
important focus of an interventional study. Our results
suggest the need for future studies to examine the
therapeutic effect of interventions targeting hip abductors. Such interventions may be disease modifying, either
on their own or as an adjunct to pharmacologic therapy.
In conclusion, a greater hip abduction moment
during gait at baseline protected against ipsilateral medial OA progression from baseline to 18 months. The
likelihood of medial tibiofemoral OA progression was
reduced 50% per 1 unit of hip abduction moment.
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