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Medial opening wedge high tibial osteotomyA prospective cohort study of gait radiographic and patient-reported outcomes.

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Arthritis & Rheumatism (Arthritis Care & Research)
Vol. 61, No. 5, May 15, 2009, pp 648 – 657
DOI 10.1002/art.24466
© 2009, American College of Rheumatology
ORIGINAL ARTICLE
Medial Opening Wedge High Tibial Osteotomy:
A Prospective Cohort Study of Gait, Radiographic,
and Patient-Reported Outcomes
TREVOR B. BIRMINGHAM, J. ROBERT GIFFIN, BERT M. CHESWORTH, DIANNE M. BRYANT,
ROBERT B. LITCHFIELD, KEVIN WILLITS, THOMAS R. JENKYN, AND PETER J. FOWLER
Objective. To evaluate the effect of medial opening wedge high tibial osteotomy on gait, radiographic, and patientreported outcomes over a 2-year postoperative period in patients with varus alignment and medial compartment knee
osteoarthritis, and to identify significant predictors of outcome.
Methods. We used an observational cohort study design and prospectively administered 3-dimensional quantitative gait
analysis, hip to ankle weight-bearing radiographs, and patient-reported outcomes preoperatively and 6, 12, 18, and 24
months postoperatively. Observed changes with 95% confidence intervals (95% CIs) were calculated. Multivariate linear
regression and cluster analysis were used to evaluate associations between patient characteristics and 2-year outcomes
in dynamic knee joint load (external knee adduction moment during gait) and Knee Injury and Osteoarthritis Outcome
Scores (KOOS).
Results. A total of 126 patients (mean age 47.48 years) were included in the study. Mean changes suggested clinically
important improvements in malalignment (change in mechanical axis angle 8.04° [95% CI 7.16°, 8.93°]), medial compartment load during gait (change in knee adduction moment ⴚ1.38 [95% CI ⴚ1.53, ⴚ1.22] percentage body weight ⴛ
height), and all KOOS domain scores (change in pain 23.19 [95% CI 19.49, 26.89] KOOS points). A small (13%) increase
in knee adduction moment was observed from 6 to 24 months postoperatively. Few preoperative clinical and/or gait
characteristics assessed at baseline were significantly associated with 2-year outcomes.
Conclusion. A medial opening wedge high tibial osteotomy with correction to approximately neutral alignment produces
substantial and clinically important changes in dynamic knee joint load and patient-reported measures of pain, function,
and quality of life 2 years postoperatively. Changes in knee adduction moment observed in the first 2 years postoperatively should be explored as potential predictors of longer-term success and subgroups of patients with poor outcomes.
INTRODUCTION
Knee osteoarthritis (OA) ranks among the most common
disabling and costly health conditions in adults, and its
prevalence is projected to increase sharply over the next 2
decades (1–3). There are currently no interventions proven
to alter the course of knee OA, leaving total joint replaceSupported by the Canadian Institutes of Health Research
and Arthrex, Inc. Dr. Birmingham’s work was supported by
the Canada Research Chairs Program.
Trevor B. Birmingham, PhD, J. Robert Giffin, MD, Bert M.
Chesworth, PhD, Dianne M. Bryant, PhD, Robert B. Litchfield, MD, Kevin Willits, MD, Thomas R. Jenkyn, PhD, Peter
J. Fowler, MD: The Wolf Orthopaedic Biomechanics Laboratory and Fowler Kennedy Sport Medicine Clinic, University of Western Ontario, London, Ontario, Canada.
Address correspondence to Trevor B. Birmingham, PhD,
Elborn College, University of Western Ontario, London, Ontario, Canada N6G 1H1. E-mail: tbirming@uwo.ca.
Submitted for publication April 22, 2008; accepted in
revised form February 3, 2009.
648
ment for end-stage disease the most widely accepted surgical treatment option (4). The substantial burden of knee
OA provides strong support for implementing treatment
strategies earlier in the disease process. High tibial osteotomy (HTO) is a biomechanically focused surgical intervention intended to improve pain and function and delay joint
degeneration in patients with OA located primarily in a
single knee compartment (5). Studies suggest that HTO
procedures performed using a variety of techniques can
result in large improvements in self-administered, kneespecific questionnaires (5–7). For example, when converted to overall scores out of 100, patient-reported status
can increase from a mean of approximately 30 points preoperatively to a mean of approximately 70 points postoperatively (6,7). However, concerns regarding variability in
outcomes among patients and the duration of these improvements exist, and administrative databases suggest reluctance in North America to perform these procedures (8).
HTO alters knee alignment in an attempt to redistribute
the weight-bearing load on the tibiofemoral joint. Classi-
Outcomes of Medial Opening Wedge High Tibial Osteotomy
cally, a valgus-producing osteotomy for varus gonathrosis
was performed by removing a lateral wedge of bone from
the proximal tibia to correct coronal malalignment (9).
During the past decade, interest in a medial opening
wedge valgus-producing osteotomy technique has increased, owing to claims that it offers improvements over
the lateral closing technique (10). Potential detrimental
effects of HTO include delayed union or nonunion and
inadvertent changes in the posterior tibial slope and patellar height (11). Optimal indications for surgery, particularly age and extent of disease, and the amount of correction in the alignment necessary to adequately unload the
medial compartment are controversial. Despite the proposed benefits of medial opening wedge HTO, a very limited number of prospective cohort studies (12) evaluating
this newer technique exist (6).
Recent studies evaluating mechanical risk factors for
progression of knee OA confirm earlier hypotheses regarding the important role of malalignment and distribution of
tibiofemoral loads (13–17). In particular, the magnitude of
the external knee adduction moment during walking, measured during quantitative gait analysis, has emerged as a
valid and reliable proxy for the dynamic load on the medial tibiofemoral joint and a potential risk factor for disease progression (18,19). Although decreasing the load on
the medial knee compartment is the primary rationale for
valgus HTO, we are aware of few studies with relatively
small sample sizes evaluating changes in the knee adduction moment postoperatively. Published studies include
5 cohorts ranging in size from 7 to 32 patients, a variety of
HTO techniques, and one postoperative gait analysis,
often performed at variable lengths of followup (20 –25).
Together, these studies provide convincing evidence that
valgus HTO procedures produce statistically significant
decreases in the knee adduction moment, although the
magnitude and stability of these reductions and potential
changes in other gait characteristics known to affect the knee
adduction moment remain unclear (20 –25). Additionally,
previous studies evaluating whether preoperative quantitative gait analysis provides useful predictors of clinical
outcome have produced contrary results (23,24,26,27).
Our objectives were to 1) evaluate the effect of medial
opening wedge HTO on gait, radiographic, and patientreported outcomes over a 2-year postoperative period in
patients with varus alignment and medial compartment
knee OA, and 2) identify significant predictors of outcome.
We hypothesized that medial opening wedge HTO would
result in substantial, clinically important improvements in
dynamic knee joint load and patient-reported pain. We
also hypothesized that preoperative patient characteristics
would be associated with these improvements.
PATIENTS AND METHODS
Study design. We used an observational cohort study
design (12) to evaluate patients undergoing medial opening wedge HTO. Four surgeons (JRG, RBL, KW, PJF) at
a tertiary care center specializing in adult reconstruction and orthopedic sports medicine surgery participated.
We prospectively assessed gait, radiographic, and patient-
649
reported outcomes preoperatively and at 6, 12, 18, and 24
months postoperatively. Data were collected between November 2002 and December 2007. Patients provided informed consent to participate in the study. The study was
approved by the Research Ethics Board for Health Sciences
Research Involving Human Subjects of the University of
Western Ontario.
Patients. We recruited patients presenting to this center
for treatment of unresolved knee pain located primarily in
the medial compartment. Patients were referred from family physicians, rheumatologists, and primary care sports
medicine specialists for consultation with an orthopedic
surgeon regarding treatment options. Inclusion criteria
consisted of mechanical varus alignment and knee OA
according to the American College of Rheumatology classification criteria (28), with the greatest severity in the
medial compartment of the tibiofemoral joint. Patients
with concomitant disease in the lateral compartment were
considered eligible as long as pain and radiographic disease were more severe in the medial compartment. Patients with concomitant chronic anterior cruciate ligament
(ACL) deficiency undergoing simultaneous ACL reconstruction were included. We excluded patients age ⱖ60
years with grade 4 degenerative changes in ⱖ2 knee compartments because they were considered better candidates
for arthroplasty. Other exclusion criteria included inflammatory or infectious arthritis of the knee, end-stage disease
in the patellofemoral joint, prior HTO on the contralateral
extremity, multiligamentous instability, major neurologic
deficit that would affect gait, major medical illness with a
life expectancy ⬍2 years or with an unacceptably high operative risk, pregnancy, inability to speak or read English,
and psychiatric illness that limited informed consent.
Intervention. Using an opening wedge osteotomy system and a nonlocking plate (Arthrex Opening Wedge Osteotomy System; Arthrex, Naples, FL), the surgeons used a
similar operative technique to that previously described
by Fowler et al (29). The desired angle of correction was
calculated preoperatively according to the method described by Dugdale et al (30), with the goal of creating very
slight valgus by moving the weight-bearing line laterally
up to a maximum position of 62.5% of the medial to lateral
width of the tibia, depending on the magnitude of malalignment and status of the articular cartilage in the lateral
tibiofemoral compartment. The osteotomy was performed
using both flexible and rigid osteotomes below a fluoroscopically placed guide pin, then opened slowly to the
predetermined width for placement of the plate that was
fixed proximally and distally with cancellous and cortical
screws, respectively. Cancellous allograft bone was used
in osteotomies ⬎7.5 mm.
Postoperative management included use of a hinged
postoperative knee brace and feather-touch weight bearing
with crutch use for at least 6 weeks. With clinical and
radiographic evidence of healing, partial weight bearing
was permitted at 6 weeks, and full weight bearing was
permitted at 12 weeks postoperatively. At ⬃3 weeks postoperatively, the patients started exercises in the brace until
650
healing of the osteotomy site had occurred. Goals of these
exercises during this time period of limited weight bearing
were to limit swelling, joint contracture, and disuse muscle atrophy. Range of motion exercises consisted of active
and passive knee and hip movements. Strengthening exercises initially consisted of isometric knee extension/
flexion and hip extension. At ⬃8 weeks postoperatively,
non–weight-bearing concentric exercises using weights
or Thera-Band (Hygienic Corporation, Akron, OH) were
added and progressed until weight bearing was permitted.
At ⬃12 weeks postoperatively, weight-bearing exercises
emphasizing proprioceptive control and balance were then
implemented and progressed until patients demonstrated
a normal walking gait pattern.
Gait analysis. Gait was evaluated using an 8-camera
motion capture system (Eagle EvaRT; Motion Analysis
Corporation, Santa Rosa, CA) synchronized with a floormounted force platform (Advanced Mechanical Technology, Watertown, MA). We used a modified Helen Hayes 22
passive-reflective marker set (31). Additional markers
were placed over the medial knee joint line and medial
malleolus bilaterally while the patient stood on the force
platform to determine body mass, marker orientation, and
positions of joint centers of rotation for the knee and ankle.
These 4 markers were removed prior to gait testing.
Patients walked barefoot across the laboratory while
3-dimensional kinetic (sampled at 1,200 Hz) and kinematic (sampled at 60 Hz) data were recorded during the
middle of several strides for at least 5 trials from each
extremity. We calculated external knee moments from
the kinematic and kinetic data using commercial software
(Orthotrak 6.0, Motion Analysis Corporation), custom
postprocessing, and previously described data reduction
techniques (32,33).
Based on the external knee adduction moment waveform, we identified the peak magnitudes in the first and
second halves of stance and the area under the curve
(impulse), and normalized these values to body weight
and height. We also calculated gait speed, toe-out angle,
and lateral trunk lean because of their influence on the
knee adduction moment in patients with knee OA (33–35).
Walking speed was calculated as the average walking
speed between successive foot contacts of the tested extremity. Toe-out angle (positive angle) was calculated as
the angle between a line drawn between the center of the
ankle and the head of the second metatarsal joint and the
forward progression of the body. Lateral trunk lean over
the stance extremity (positive angle) was calculated as the
angle of a line drawn from the midpoint of the anterior
superior iliac spines to the midpoint of the anterior tips of
the acromion processes with respect to vertical. All gait
variables were calculated by averaging across 5 trials for
each patient. We have previously reported excellent test–
retest reliability of the first peak knee adduction moment
(intraclass correlation coefficient [ICC2,1] ⫽ 0.86) with a
minimum detectable change (95% confidence level) of
1% body weight (BW) ⫻ height (Ht) (36). We have also
previously reported appropriate reliability of gait speed
Birmingham et al
(ICC2,1 ⫽ 0.92), toe-out angle (ICC2,1 ⫽ 0.69), and trunk
lean angle (ICC2,1 ⫽ 0.91) measurements (34).
Radiographic assessment. Long cassette bipedal standing anteroposterior and lateral digital radiographs were
assessed using custom computerized software (37). For
anteroposterior radiographs, the patients stood with the
patellae centered over the femoral condyles and feet
straight ahead to attain a true anteroposterior image and to
control for effects of foot rotation on measures of lower
extremity alignment (15). The x-ray beam was centered on
the knee at a distance of ⬃2.5 meters and beam exposure
was determined based on each patient’s leg mass.
The mechanical axis angle of the lower extremity was
used to quantify alignment in the frontal plane and was
defined as the angle formed between a line drawn from the
center of the hip to the center of the knee and a line drawn
from the center of the ankle to the center of the knee
(38,39). Negative values indicated varus alignment. The
center of the hip was identified as the geometric center of
the femoral head using a circular template, the center of
the knee was identified as the midpoint of the tibial spines
extrapolated inferiorly to the surface of the intercondylar
eminence, and the center of the ankle was defined as the
mid-width of the tibia and fibula at the level of the tibial
plafond. We have previously reported excellent reliability
of mechanical axis angle measurements (ICC2,1 ⫽ 0.97) (37).
Other radiographic measures of knee geometry relevant
to HTO included the femoral anatomic axis angle (the
angle between the anatomic axes of the femur and tibia),
the proximal tibial articular angle (formed by the tibial
anatomic axis and a line tangent to the tibial plateau on the
medial side), the posterior tibial slope (the angle formed by
a line perpendicular to the lateral longitudinal axis of the
tibia and a line parallel to the medial tibial plateau joint
surface), and patellar height (Blackburn-Peel and InsallSalvati ratios) (40). Measures of joint degeneration included the Kellgren/Lawrence (K/L) scale grade (41) in the
medial and lateral compartments and medial and lateral
joint space in millimeters. Although we have not evaluated the reliability of these additional radiographic measures, the same experienced and independent evaluator
measured all radiographic variables at all followup visits.
Patient-reported outcomes. We used the Knee Injury
and Osteoarthritis Outcome Score (KOOS) to assess patient-reported outcomes (42,43). The KOOS is a 42-item
knee-specific questionnaire with 5 separately reported domains of pain (9 items), other symptoms (7 items), function in daily living (17 items), function in sports/recreation (5 items), and knee-related quality of life (4 items).
Each item has 5 response options. Domain scores represent
the average of all items in the domain standardized to a
score of 0 –100 (where 0 ⫽ worst and 100 ⫽ best). This
instrument has face validity and has demonstrated construct validity, excellent test–retest reliability for each domain (range 0.75– 0.93), and has been shown to be responsive to change (7,42,43). Other patient-reported outcomes
included the Lower Extremity Functional Scale (44), a
region-specific measure of lower extremity function that
Outcomes of Medial Opening Wedge High Tibial Osteotomy
provides a single value for function that varies from 0 to 80
(where 0 ⫽ worst and 80 ⫽ best), and the generic physical
function and mental health domains of the Short Form 36
health survey (45), standardized to scores from 0 to 100
(where 0 ⫽ worst and 100 ⫽ best).
Statistical analysis. We first calculated the mean ⫾ SD
for all variables assessed preoperatively and 2 years postoperatively. We then calculated means and 95% confidence intervals (95% CIs) for changes after 2 years and
evaluated statistical significance using the dependent ttest for continuous variables. We also plotted the mean
and 95% CI for the mechanical axis angle, the first peak
knee adduction moment on both extremities, and each of
the KOOS domain scores preoperatively and 6, 12, 18, and
24 months postoperatively.
We defined 2 primary outcome measures (dependent
variables) observed at the final followup: the first peak
knee adduction moment and the KOOS pain score. Linear
regression was used to quantify the magnitude of the association between predictors and outcomes. Potential predictors (independent variables) were chosen based on previous reports and consisted of factors related to disease
severity and knee joint loading (6,23,24,26,27), including
gait characteristics demonstrated to affect the knee adduction moment (33–35). Independent variables included preoperative scores for knee adduction moment, KOOS pain
domain, age, sex, body mass index, mechanical axis angle,
medial compartment K/L scale grade and lateral compartment K/L scale grade (both entered as binary variables;
grade 1 or 2 versus grade 3 or 4), gait speed, toe-out angle,
and trunk lean angle. To specifically investigate the effect
of the postoperative alignment achieved with surgery, we
also included the 6-month postoperative mechanical axis
angle as an independent variable. We also repeated the
analyses, replacing the first peak knee adduction moment
with the area under the curve and replacing the change in
the KOOS pain domain with the change in the KOOS
function in daily living domain. We then repeated analyses, excluding patients who underwent simultaneous ACL
reconstruction.
We used exploratory cluster analysis to identify groups
of patients who were similar with respect to the 2-year
change in the first peak knee adduction moment and the
2-year change in the KOOS pain score. We used a 2-step
hierarchical clustering model with the unstandardized Euclidean distance algorithm as the similarity index,
Akaike’s information criterion as the measure of goodness
of fit, and the auto-clustering feature of the software
(TwoStep method, Statistical Package for the Social Sciences, version 16; SPSS, Chicago, IL). We repeated the
initial analysis twice after randomizing the order of cases.
Between-cluster comparisons were then made with the
independent t-test, Wilcoxon’s rank sum test, and the chisquare test as required. The cluster analysis method used
screened-out patients who could not be assigned to a cluster because they were not similar with respect to the variables of interest. Because of this dissimilarity, these outliers were not evaluated further.
651
Table 1. Baseline demographics and clinical
characteristics (n ⴝ 128)*
Characteristic
Sex, no. (%)
Male
Female
Age, years
Height, meters
Weight, kg
BMI, kg/m2
Mechanical axis angle, degrees
Femoral anatomic axis angle, degrees
Medial compartment K/L scale grade,
no. (%)†
1
2
3
4
Lateral compartment K/L scale grade,
no. (%)
0
1
2
3
4
Value
102 (79.7)
26 (20.3)
47.48 ⫾ 9.53
1.75 ⫾ 0.09
90.54 ⫾ 16.60
29.50 ⫾ 4.82
⫺7.50 ⫾ 4.10
⫺2.20 ⫾ 4.50
4 (3.13)
32 (25.00)
41 (32.03)
51 (39.84)
8 (6.25)
44 (34.38)
63 (49.22)
11 (8.59)
2 (1.56)
* Values are the mean ⫾ SD unless otherwise indicated. BMI ⫽
body mass index.
† Kellgren/Lawrence (K/L) scale grade of osteoarthritis severity.
RESULTS
A total of 258 potential patients were screened for eligibility. Fifty-two patients were ineligible and 22 patients
elected to not undergo HTO. Of the remaining 184 patients
entered in the study, 36 were lost to followup, 3 died
before completing the study, and 2 withdrew from the
study. Fifteen patients elected to undergo HTO on their
opposite extremity within the 2-year followup, and 2 patients required revision surgery for nonunion and loss of
correction. These 17 patients were excluded from the
present analysis, although we continue to follow them. A
total of 126 patients with 2-year postoperative data remained. Six of these patients had some missing data and
were excluded from the regression analyses (no data imputation methods were used). All 126 patients were included in the cluster analysis.
Three patients had delayed union, which resulted in
missing the 6-month gait test only. Four patients had infections within 2 weeks of surgery that were treated with
irrigation and debridement and a short course of intravenous antibiotics. Three patients had delayed infections at
their tibial incision ⬃1 year postoperatively (likely following a bacteremia) that required surgical removal of hardware and intravenous antibiotics. One patient had a hematoma 2 weeks postoperatively that required surgical
washout. These complications did not require patients to
miss followup tests.
Table 1 shows the preoperative demographics and clinical characteristics of this sample. The patients were predominantly male and, on average, relatively young and
overweight. They had substantial varus alignment and tib-
652
Birmingham et al
Table 2. Gait, radiographic, and patient-reported outcome measures (n ⴝ 120)*
Outcome measure
Gait
Knee adduction moment
First peak, %BW ⫻ Ht
Second peak, %BW ⫻ Ht
Area under the curve, %BW ⫻ Ht ⫻ seconds
Speed, meters/second
Toe-out angle, degrees
Lateral trunk lean, degrees
Radiographic
Mechanical axis angle, degrees
Femoral anatomic axis angle, degrees
Proximal tibial articular angle, degrees
Medial joint space width, mm
Lateral joint space width, mm
Posterior tibial slope, degrees
Blackburn-Peel ratio
Insall-Salvati ratio
Patient reported
KOOS (range 0–100)
Pain
Other symptoms
Function in daily living
Function in sports/recreation
Quality of life
SF-36 physical function (range 0–100)
SF-36 mental health (range 0–100)
LEFS (range 0–80)
Baseline,
mean ⴞ SD
24 months,
mean ⴞ SD
Change, mean
(95% CI)
2.99 ⫾ 0.92
2.37 ⫾ 1.08
1.45 ⫾ 0.49
1.10 ⫾ 0.17
12.00 ⫾ 5.77
3.45 ⫾ 2.97
1.62 ⫾ 0.69
1.30 ⫾ 0.78
0.77 ⫾ 0.36
1.16 ⫾ 0.16
13.20 ⫾ 5.40
1.96 ⫾ 2.07
⫺1.38 (⫺1.53, ⫺1.22)†
⫺1.07 (⫺1.26, ⫺0.88)†
⫺0.68 (⫺0.76, ⫺0.60)†
0.06 (0.08, 0.04)†
1.23 (0.54, 1.92)†
⫺1.49 (⫺2.00, ⫺0.99)†
⫺8.00 ⫾ 4.09
⫺2.15 ⫾ 4.45
81.44 ⫾ 4.51
2.86 ⫾ 1.62
6.30 ⫾ 1.82
5.15 ⫾ 4.66
0.76 ⫾ 0.14
1.05 ⫾ 0.18
⫺0.05 ⫾ 3.05
5.11 ⫾ 3.44
88.47 ⫾ 3.76
3.30 ⫾ 1.48
5.91 ⫾ 1.81
6.37 ⫾ 4.22
0.71 ⫾ 0.14
1.05 ⫾ 0.18
8.04 (7.16, 8.93)†
7.26 (6.24, 8.28)†
7.02 (7.90, 0.44)†
0.44 (0.19, 0.68)†
⫺0.39 (⫺0.71, ⫺0.07)†
1.22 (0.24, 2.20)†
⫺0.05 (⫺0.08, ⫺0.01)†
0.00 (⫺0.03, 0.03)
51.71 ⫾ 17.25
52.62 ⫾ 17.25
60.60 ⫾ 18.64
25.88 ⫾ 20.44
25.59 ⫾ 16.86
46.99 ⫾ 24.18
75.83 ⫾ 14.94
42.62 ⫾ 13.50
74.90 ⫾ 20.25
70.76 ⫾ 19.92
82.26 ⫾ 19.32
54.04 ⫾ 29.87
53.70 ⫾ 24.21
72.20 ⫾ 24.17
82.99 ⫾ 14.69
59.64 ⫾ 16.01
23.19 (19.49, 26.89)†
18.14 (14.65, 21.63)†
21.66 (18.13, 25.20)†
28.16 (22.84, 33.49)†
28.12 (23.96, 32.27)†
25.22 (20.25, 30.18)†
7.16 (3.72, 10.61)†
17.02 (14.43, 19.61)†
* 95% CI ⫽ 95% confidence interval; BW ⫽ body weight; Ht ⫽ height; KOOS ⫽ Knee Injury and Osteoarthritis Outcomes Score; SF-36 ⫽ Short Form
36; LEFS ⫽ Lower Extremity Functional Scale.
† P ⬍ 0.01.
iofemoral degeneration primarily of the medial compartment. Although these characteristics are consistent with
other studies evaluating HTO, it should be noted that
⬃10% of the present sample also had substantial degeneration in the lateral compartment (K/L scale grade ⱖ3).
Table 2 shows the changes in all gait, radiographic, and
patient-reported outcomes at the 2-year end point. With
the exception of the Insall-Salvati ratio for patellar height,
95% CIs excluded the value 0 and were quite narrow.
Changes in mechanical axis angle, first peak knee adduction moment, and KOOS scores at all test sessions are
shown in Figure 1. The mechanical axis angle plot shows
that surgery produced a large reduction in varus alignment
that was maintained throughout the 24 months of followup. The knee adduction moment plot also shows a
large reduction in dynamic knee joint load on the operative extremity 6 months postoperatively. Unlike the mechanical axis angle, however, the knee adduction moment
slightly but consistently increased throughout followup. A
post hoc comparison indicated that the increase from 6 –24
months (mean increase 0.19 [95% CI 0.10, 0.28] %BW ⫻
Ht) was statistically significant (P ⬍ 0.001). The KOOS plot
also shows large changes in all domains that continued to
improve throughout followup.
Table 3 shows that the predictor variables explained
43% of the variation in the 2-year postoperative first peak
knee adduction moment. While adjusting for other vari-
ables, higher preoperative knee adduction moment, lower
lateral trunk lean, higher age, and lower 6-month postoperative mechanical axis angle (i.e., more varus) were significantly associated with higher 2-year knee adduction
moments. Analyses that excluded patients who underwent
simultaneous ACL reconstruction revealed no additional
insight into significant predictors, but slightly reduced the
explained variance. Similarly, there were no meaningful
differences when the value for the first peak was replaced
by the area under the knee adduction moment curve. With
the exception of their corresponding preoperative scores,
all linear regression analyses yielded no other significant
predictors of 2-year KOOS pain or KOOS function in daily
living scores. Variance inflation factor and tolerance levels
for all regression analyses suggested acceptable levels of
multicolinearity.
Figure 2 shows that the cluster analysis identified 2
clusters (Akaike’s information criterion ⫽ 1,368.57) of patients who had similar values for their 2-year changes in
first peak knee adduction moment and KOOS pain score.
The larger patient cluster (n ⫽ 92) shows patients with
substantial decreases in both knee joint load (mean ⫾ SD
change in first peak knee adduction moment 1.33 ⫾ 0.61)
and knee pain (mean ⫾ SD change in KOOS 24.47 ⫾
14.45). The smaller cluster (n ⫽ 10) highlights a group of
patients that leaned toward relatively smaller decreases in
knee joint load (mean ⫾ SD change in first peak knee
Outcomes of Medial Opening Wedge High Tibial Osteotomy
653
Figure 1. Means and 95% confidence intervals for A, mechanical
axis angle, B, first peak external knee adduction moment on both
extremities, and C, Knee Injury and Osteoarthritis Outcomes
Scores (KOOS) assessed at each test session. The figures show
large, statistically significant (P ⬍ 0.001) changes in mechanical
axis angle, knee adduction moment on the operative extremity,
and all KOOS domains 6 months postoperatively. Small increases
in the knee adduction moment from 6 to 24 months were statistically significant (P ⬍ 0.01). BW ⫽ body weight; Ht ⫽ height.
adduction moment 0.66 ⫾ 0.73) and increases in knee pain
(mean ⫾ SD change in KOOS ⫺15.27 ⫾ 14.00). Consistent
results were achieved when the order of cases in the sample was randomized. Consistent with the clustering criteria, 2-year change in first peak knee adduction moment
and 2-year change in KOOS pain scores were significantly
different between these groups (P ⬍ 0.01). However, no
other demographic and clinical characteristics were statistically different. Outliers (n ⫽ 24) were not evaluated.
DISCUSSION
We hypothesized that medial opening wedge HTO would
result in substantial, clinically important improvements in
dynamic knee joint load and patient-reported pain. The
present findings strongly support this hypothesis. Effect
sizes and standardized response means for the first peak
knee adduction moment (1.50 and 1.58, respectively) and
each of the KOOS domains (0.89 –1.38) are above the
threshold value for a large effect (0.8) (46). Additionally,
even the lower ends of the 95% CIs for the 2-year changes
in the KOOS pain, function in daily living, function in
sports/recreation, and knee-related quality of life domains
(Table 2) indicate improvements beyond the minimum to
suggest clinically important changes have occurred in the
majority of patients (43). These confidence intervals are
also consistent with previously reported changes in the
KOOS after HTO procedures (6,7). We believe that the
large improvements in both biomechanical and patientreported outcomes observed 2 years postoperatively underscore the potential benefit of surgically restoring neutral lower extremity alignment.
It is important to note that these changes were achieved
with intentional minimal to no overcorrection of alignment. Figure 1 shows that, on average, alignment was
altered to a position of only slight valgus (mechanical axis
angle 1°, anatomic axis angle 5°), which was maintained
throughout the 2-year followup. The absolute magnitude
of change in the mechanical axis angle achieved with
surgery is approximately equal to the amount of preoperative varus (Table 2). This is consistent with our aim of
HTO, which generally attempts to establish neutral align-
654
Birmingham et al
Table 3. Predictors of 2-year first peak knee adduction moment (n ⴝ 120)*
␤ (95% CI)‡
Potential predictor variables†
Demographic
Age, years
Sex (0 ⫽ female, 1 ⫽ male)
BMI, kg/m2
Radiographic
Mechanical axis angle, degrees
Preoperative measure
6-month postoperative measure
OA severity
Medial compartment
Lateral compartment (0 ⫽ less, 1 ⫽ more)§
Gait
First peak knee adduction moment, %BW ⫻ Ht
Speed, meters/second
Toe-out angle, degrees
Lateral trunk lean, degrees
Patient-reported outcome
KOOS pain domain
0.02 (0.00, 0.03)
⫺0.10 (⫺0.39, 0.20)
⫺0.01 (⫺0.04, 0.02)
0.01 (⫺0.03, 0.06)
⫺0.14 (⫺0.18, ⫺0.10)
P
0.03
0.51
0.37
0.56
⬍0.01
⫺0.17 (⫺0.47, 0.12)
0.07 (⫺0.16, 0.29)
0.25
0.57
0.24 (0.08, 0.39)
⫺0.09 (⫺0.79, 0.60)
0.01 (⫺0.01, 0.03)
⫺0.04 (⫺0.07, 0.00)
0.01
0.79
0.45
0.05
0.00 (⫺0.01, 0.01)
0.58
* 95% CI ⫽ 95% confidence interval; BMI ⫽ body mass index; OA ⫽ osteoarthritis; BW ⫽ body weight;
Ht ⫽ height; KOOS ⫽ Knee Injury and Osteoarthritis Outcomes Score.
† All were assessed preoperatively unless stated otherwise.
‡ Effect on 2-year peak knee adduction moment, adjusting for all other predictors in model. Adjusted r2 ⫽
0.43.
§ Less severe ⫽ Kellgren/Lawrence scale grade of OA severity 0, 1, 2; more severe ⫽ 3, 4.
Figure 2. Scatter plot of 2-year changes in knee adduction moment and Knee Injury and Osteoarthritis Outcomes Score (KOOS)
pain domain scores shows results of the cluster analysis. The
larger patient cluster (large open circles, n ⫽ 92) shows patients
with substantial decreases in both knee joint load and knee pain.
The smaller patient cluster (solid circles, n ⫽ 10) shows a group
of patients that leaned toward relatively smaller decreases in knee
joint load and increases in knee pain. Outliers did not fit into
either cluster (small open circles, n ⫽ 24). BW ⫽ body weight;
Ht ⫽ height.
ment in all patients despite the preoperative measure.
Change in alignment was accompanied by a very small
change in tibial slope and patellar height. However, even
the upper ends of the 95% CIs for change in tibial slope
(2.2°) and Blackburn-Peel ratio (⬍0.1) were extremely
small and likely not clinically important (Table 2). Change
in alignment was also accompanied by a very small increase in medial joint space and a decrease in lateral joint
space. Overall, these radiographic findings suggest that
the change in alignment produced a sustained lateral shift
in weight-bearing load without producing detrimental
changes in knee geometry. These findings are consistent
with recent reports of favorable radiographic results with
minimal complications after medial opening wedge HTO
(11,47).
We observed several changes in gait that are also consistent with the biomechanical rationale for this surgery
(Table 2). Despite a slight increase in speed and a decrease
in lateral trunk lean toward the stance extremity (both
reflect a more normal gait pattern and would increase
medial joint load), there was a large reduction in all measures of the external knee adduction moment postoperatively. Interestingly, there was also a small increase in
toe-out angle that would serve to further reduce the knee
adduction moment. It is unclear whether the change in
toe-out angle was a structural change as a result of surgery
or a gait adaptation adopted by patients. Because toe-out
angle has been demonstrated to have a protective effect on
future knee joint degeneration (48), the effect of medial
opening wedge HTO on toe-out angle should be evaluated
further.
Importantly, unlike alignment, we observed an unex-
Outcomes of Medial Opening Wedge High Tibial Osteotomy
pected increase in the knee adduction moment from 6 to
24 months postoperatively (Figure 1). The increase was
not observed on the opposite extremity, and therefore is
not likely an overall response to the small increase in gait
speed observed over the same postoperative time period.
The magnitude of the increase in postoperative knee adduction moments from 6 to 24 months was small (13%),
and its value at 2 years postoperatively was still far less
than those observed preoperatively or on the opposite
extremity (55% and 65%, respectively). However, we believe that these relatively early postoperative increases in
the knee adduction moment are of potential concern and
may be a precursor to poorer longer-term outcomes.
Changes in the knee adduction moment without concomitant changes in mechanical axis angle are consistent with
previous reports of the low to moderate correlation between these measures, and further emphasize the difference between dynamic measures of knee joint load and
static measures of alignment (48 –50). Further research
evaluating changes in knee joint load observed within 2
years postoperatively, including potential underlying
mechanisms, is warranted. Most importantly, the potential
for these changes to affect long-term outcomes needs to be
evaluated.
We also hypothesized that preoperative patient characteristics would be associated with 2-year outcomes, which
was only partly supported. Linear regression analyses
yielded no significant predictors of 2-year KOOS pain or
KOOS function in daily living scores other than their
preoperative values. Only preoperative knee adduction
moment, trunk lean, age, and mechanical axis angle at the
6-month followup (i.e., alignment achieved with surgery)
explained substantial variance in 2-year knee adduction
moments. It is possible that other factors not evaluated in
the present study, including other clinical characteristics
or more elaborate measures of neuromuscular function
and motor control, contributed to changes. However, we
believe that the relatively little variance in all outcomes 2
years postoperatively is the primary reason for our findings and we continue to evaluate this objective with longer-term followup.
Owing to our 2 primary goals of HTO, decreasing the
load on the medial tibiofemoral compartment and improving patient-reported outcomes, we performed a cluster analysis to identify patients who experienced similar changes
in these outcomes. A large cluster of 92 patients experienced substantial improvements in both knee adduction
moment and KOOS pain domain scores, and reflect the
general success of this procedure. However, a cluster of 10
patients who had poor results in both outcomes was also
identified. Given the small sample size, we could not
identify any significant differences in patient characteristics between these clusters. Similarly, no trends for increased prevalence of patient characteristics, including
disease severity in the lateral compartment, age, sex, or
correction angle, were evident. The exploratory nature of
cluster analysis should be acknowledged. Given the goals
of surgery, however, we believe that this small cluster
represents a subgroup of patients with particularly poor
results that require further investigation.
Inclusion and exclusion criteria for the present study are
655
also somewhat different from previous reports and should
be highlighted. We recruited patients of all ages with varus
gonarthrosis and greatest radiographic severity and pain in
the medial compartment, but did not exclude those with
concomitant lateral compartment disease. Typically, these
were young patients with substantial varus deformities
who were not candidates for total knee arthroplasty. We do
not advocate HTO as a widespread treatment option for
patients with substantial bicompartmental tibiofemoral
disease. However, 2 patients (ages 43 and 50 years) had
lateral compartment OA that also met the K/L scale criteria
for grade 4 severity (although medial compartment degeneration was greater) and underwent the procedure. Both
patients experienced results similar to many in the sample, with reductions in the first peak knee adduction moment of approximately 1% BW ⫻ Ht and improvements in
the KOOS pain score of approximately 30 points 2 years
postoperatively. We continue to closely follow patients
meeting these criteria.
Strengths of our study include its prospective design
that adhered to published guidelines for conducting observational studies (12), its relatively large sample size,
and the spectrum of validated outcome measures relevant
to HTO. A limitation in this design is the inability to
compare the effects of HTO with other surgical and nonsurgical treatment options. Similarly, although the meaning and expected outcome patients attribute to treatment is
likely to influence outcomes (51), comparison of HTO with
sham surgery is not realistic. However, the importance of
comparing other less invasive surgical procedures for knee
OA with medical and physical therapies (52) and even
sham treatment (53) has been established. Clearly, future
research comparing HTO with different treatment strategies is required. Other limitations in the present study
include the relatively small number of women in our sample and the inability to generalize the results beyond 2
years postoperatively. Overall, the present findings suggest
that improvements in malalignment achieved by medial
opening wedge HTO produce substantial and clinically
important changes in knee joint loading during gait and
patient-reported measures of pain, function, and quality of
life 2 years postoperatively. Changes in knee adduction
moment observed within the first 2 years postoperatively
should be explored as potential predictors of long-term
success and subgroups of patients with poor outcomes.
AUTHOR CONTRIBUTIONS
All authors were involved in contributions to study conception
and design, acquisition of data, or analysis and interpretation of
data, and drafting the article or revising it critically for important
intellectual content, and all authors approved the final version to
be submitted for publication. Dr. Birmingham had full access to
all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
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