Influence of Ankle Position and Radiographic Projection Angle on Measurement of Supramalleolar Alignment on the Anteroposterior and Hindfoot Alignment Views


Barg, A.; Amendola, R.L.; Henninger, H.B.; Kapron, A.L.; Saltzman, C.L.; Anderson, A.E.

Foot and Ankle International 36(11): 1352-1361

2016


Using digitally reconstructed radiographs (DRRs), we determined how changes in the x-ray beam projection angle from the horizon, tibiotalar joint angle, and axial rotation of the foot influenced measurements of the medial distal tibial angle (MDTA) on the anteroposterior (AP) and hindfoot alignment views (HAV). Seven cadaver foot-ankle specimens were scanned by computed tomography (CT) at fixed tibiotalar joint positions, ranging from 15 degrees of dorsiflexion to 25 degrees of plantarflexion. DRRs were created from each CT scan to simulate alterations in the horizontal projection angle (0 to 25 degrees) and foot axial rotation (-30 to 30 degrees). The MDTA was measured on each DRR and compared with that quantified on the baseline HAV and AP view. Altering the horizontal projection angle by ≥5 degrees and >10 degrees significantly altered the MDTA for the AP view and the HAV, respectively. Shifting dorsiflexion and plantarflexion caused minor changes in the MDTA that were only statistically significant for the HAV. Axial rotation significantly changed the MDTA on both views, but deviations were more pronounced for the HAV. Compared with the HAV, the MDTA on the AP view was less sensitive to changes in foot-ankle position. However, increasing the tilt of the x-ray beam from the horizon altered the MDTA on the AP view substantially. To avoid misinterpretation of the MDTA, we recommend using the AP view to quantify supramalleolar alignment as it is less sensitive to changes in positioning of the foot-ankle. When acquiring an AP film, the x-ray beam should be directed along the horizon to ensure consistent assessment of the MDTA across patients.

AMERICAN
ORTHOPAEDIC
Article
FOOT
&
ANKLE
SOCIETY.
Influence
of
Ankle
Position
and
Radiographic
Projection
Angle
on
Measurement
of
Supramalleolar
Alignment
on
the
Anteroposterior
and
Hindfoot
Alignment
Views
Foot
&
Ankle
International®
1-10
©
The
Author(s)
2015
Reprints
and
permissions:
sagepub.com/journalsPermissions.nav
DOI:
10.1177/1071100715591091
fai.sagepub.com
Alexej
Barg,
MD
1
'
2
,
Richard
L.
Amendola
l
'
2
,
Heath
B.
Henninger,
PhD
1
'
2
,
Ashley
L.
Kapron,
PhD
1
,
Charles
L.
Saltzman,
MD
1
,
and
Andrew
E.
Anderson,
PhD
1
'
2
'
3
'
4
'
5
Abstract
Background:
Using
digitally
reconstructed
radiographs
(DRRs),
we
determined
how
changes
in
the
x-ray
beam
projection
angle
from
the
horizon,
tibiotalar
joint
angle,
and
axial
rotation
of
the
foot
influenced
measurements
of
the
medial
distal
tibial
angle
(MDTA)
on
the
anteroposterior
(AP)
and
hindfoot
alignment
views
(HAV).
Methods:
Seven
cadaver
foot-ankle
specimens
were
scanned
by
computed
tomography
(CT)
at
fixed
tibiotalar
joint
positions,
ranging
from
15
degrees
of
dorsiflexion
to
25
degrees
of
plantarflexion.
DRRs
were
created
from
each
CT
scan
to
simulate
alterations
in
the
horizontal
projection
angle
(0
to
25
degrees)
and
foot
axial
rotation
(-30
to
30
degrees).
The
MDTA
was
measured
on
each
DRR
and
compared
with
that
quantified
on
the
baseline
HAV
and
AP
view.
Results:
Altering
the
horizontal
projection
angle
by
degrees
and
>
I
0
degrees
significantly
altered
the
MDTA
for
the
AP
view
and
the
HAV,
respectively.
Shifting
dorsiflexion
and
plantarflexion
caused
minor
changes
in
the
MDTA
that
were
only
statistically
significant
for
the
HAV.
Axial
rotation
significantly
changed
the
MDTA
on
both
views,
but
deviations
were
more
pronounced
for
the
HAV.
Conclusions:
Compared
with
the
HAV,
the
MDTA
on
the
AP
view
was
less
sensitive
to
changes
in
foot-ankle
position.
However,
increasing
the
tilt
of
the
x-ray
beam
from
the
horizon
altered
the
MDTA
on
the
AP
view
substantially.
Clinical
Significance:
To
avoid
misinterpretation
of
the
MDTA,
we
recommend
using
the
AP
view
to
quantify
supramalleolar
alignment
as
it
is
less
sensitive
to
changes
in
positioning
of
the
foot-ankle.
When
acquiring
an
AP
film,
the
x-ray
beam
should
be
directed
along
the
horizon
to
ensure
consistent
assessment
of
the
MDTA
across
patients.
Keywords:
ankle,
alignment,
medial
distal
tibia
angle,
radiographs,
digitally
reconstructed
radiographs
Osteoarthritis
(OA)
is
a
common
problem
in
the
ankle
joint
46'52'56
Concomitant
varus
or
valgus
deformities
fre-
quently
accompany
cartilage damage
in
osteoarthritic
ankles.
19
'
20
Surgical
strategies
to
treat
OA
address
both
car-
tilage
damage
and
possible
malalignment.
Treatment
can
be
divided
into
joint-preserving
(eg,
supramalleolar
realign-
ment
osteotomy
5
'
17
'
28
'
37A8
)
and
nonpreserving
(eg,
ankle
,
arthrodesis,
12,14,18,40,41
total
ankle
replacement
15,2324,31A2,45)
procedures.
Preoperative
planning
is
critical
to
the
success
of
ankle
surgery.
Weight-bearing
radiographs,
in
particular,
are
essential
to
evaluate
concomitant
foot
and
ankle
deformi-
tieS.
11
'
32
'
50
'
51
For
this
purpose,
the
medial
distal
tibial
angle
(MDTA)
serves
to
quantify
supramalleolar
ankle
align-
ment
in
the
coronal
plane.
4
'
49
The
MDTA
can
be
measured
on
the
anteroposterior
(AP)
view
as
well
as
the
hindfoot
alignment
view
(HAV).
4
'
27
'
49
In
the
AP
view,
the
MDTA
has
been
measured
as
92.4
±
3.1
degrees
(range,
88.0-100.0
degrees)
in
a
cohort
of
93
asymptomatic
control
subjects
27
and
93.3
±
3.2
degrees
(range,
88.0-100.0
degrees)
in
a
cadaver
study.
22
'Department
of
Orthopaedics,
University
of
Utah,
Salt
Lake
City,
UT,
USA
2
Harold
K.
Dunn
Orthopaedic
Research
Laboratory,
University
of
Utah,
Salt
Lake
City,
UT,
USA
3
Department
of
Bioengineering,
University
of
Utah,
Salt
Lake
City,
UT,
USA
'Department
of
Physical
Therapy,
University
of
Utah,
Salt
Lake
City,
UT,
USA
5
Scientific
Computing
and
Imaging
Institute,
University
of
Utah,
Salt
Lake
City,
UT,
USA
Corresponding
Author:
Andrew
E.
Anderson,
PhD,
Department
of
Orthopaedics,
Harold
K.
Dunn
Orthopaedic
Research
Laboratory,
University
of
Utah,
590
Wakara
Way,
Salt
Lake
City,
UT
84108,
USA.
Email:
2
Foot
&
Ankle
International
The
x-ray
beam
projection
angle
is
inherently
different
between
the
HAV
and
AP
view,
which
likely
explains
why
the
MDTA
differs
between
the
two
views
(up
to
12
degrees
as
reported
by
Barg
et
a1
4
).
However,
within
a
given
view,
there
may
be
subtle
changes
in
the
manner
in
which
the
x-ray
beam
is
aligned
across
patients.
There
may
also
be
variation
in
how
the
patient's
foot
and
ankle
are
positioned
during
the
acquisition
of
the
radiograph.
Since
these
mea-
sures
guide
the
magnitude
of
realignment
osteotomies,
it
is
critical
to
understand
how
these
spatial
factors
may
influ-
ence
the
clinical
interpretation
of
supramalleolar
alignment.
Ideally,
the
MDTA
on
the
AP
view
and
the
HAV
could
be
measured
following
known,
incremental
changes
in
the
projection
angle
and
degrees
of
ankle
dorsiflexion
and
plan-
tarflexion
as
well
as
axial
rotation
of
the
foot.
In
practice,
it
may
be
difficult
to
implement
an
experiment,
with
cadavers
or
live
subjects,
capable
of
achieving
highly
controlled,
repeatable
orientations
of
the
x-ray
equipment
and
foot-
ankle.
To
fully
understand
how
incremental
changes
in
spa-
tial
alignment
and
projection
influence
the
MDTA
could
require
hundreds
of
radiographs,
which
is
not
practical
for
live
subjects.
Alternatively,
using
digitally
reconstructed
radiographs
(DRRs)
generated
from
computed
tomography
(CT)
data
sets,
one
can
create
an
equivalent
radiograph
for
any
projec-
tion
by
applying
computer-coordinated
and
exact
move-
ments
of
the
object
of
interest.
7
'
36
In
the
context
of
scientific
research,
the
primary
advantage
of
using
DRRs
is
that
they
are
generated
from
exact,
user-defined
perspectives.
In
the-
ory,
an
analyst
can
generate
an
infinite
number
of
simulated
radiographs
from
a
single
CT
scan,
which
could
reduce
the
burden
to
x-ray
equipment
and
the
time
required
to
generate
a
large
number
of
films.
As
DRRs
require
a
CT
scan,
they
are
not
a
practical
alternative
to
conventional
radiographs
for
clinical
care.
However,
DRRs
have
served
as
a
valuable
scientific
tool
to
study
the
hip,
16
'
21
'
53
knee,
3
'
47
'
54
'"
and
ankle
29
The
suitability
of
DRRs
as
surrogates
for
standard
radiographs
was
dem-
onstrated
in
two
studies.
16
'
21
To
the
best
of
our
knowledge,
DRRs
have
not
been
used
to
elucidate
how
measurements
of
the
MDTA
may
be
influenced
by
projection
angle
and
patient
positioning
in
the
AP
view
and
the
HAV.
Furthermore,
the
use
of
DRRs
as
a
surrogate
to
conventional
radiographs
for
measurement
of
the
MDTA
has
not
been
demonstrated.
We
undertook
the
present
study
to
better
understand
whether
measurements
of
the
MDTA
are
influenced
by
radiographic
technique.
First,
we
set
out
to
validate
the
use
of
DRRs
as
a
surrogate
for
standard
radiographs
by
compar-
ing
measurements
of
the
MDTA
between
DRRs
and
con-
ventional
radiographs.
Second,
we
wanted
to
understand
whether
MDTA
measurements
of
the
AP
view
and
the
HAV
were
influenced
by
the
x-ray
projection
angle
from
the
hori-
zon
and
degree
of
foot-ankle
axial
rotation
as
well
as
dorsi-
flexion
and
plantarflexion
of
the
tibiotalar
joint.
We
hypothesized
that
the
projection
angle
and
position
of
the
foot-ankle
specimen
would
alter
the
MDTA
relative
to
the
baseline
values
in
both
the
AP
view
and
the
HAV.
Materials
and
Methods
Specimens
and
Multidetector
CT
Imaging
To
avoid
exposure
to
ionizing
radiation
to
patients
or
research
subjects,
we
chose
to
execute
this
research
as
a
combined
in
vitro
and
computational
simulation
study.
Seven
fresh-frozen,
cadaveric,
male
foot-ankle
specimens
were
acquired
with
a
mean
age
of
65.0
±
16.5
years
(range,
52-89
years).
Specimens
were
stored
at
—20°C
and
thawed
for
24
hours
prior
to
use.
On
radiographic
examination,
as
assessed
by
an
orthopedic
surgeon
(A.B.),
all
ankles
were
absent
of
gross
degenerative
changes
and
joint
space
nar-
rowing
that
would
be
indicative
of
OA.
A
custom
radiolu-
cent
acrylic
fixation
device
was
built
to
hold
the
foot
and
ankle
in
a
static
position
at
known
dorsiflexion
and
plan-
tarflexion
angles
(Figure
1).
All
CT
scans
were
performed
without
external
loads
applied
to
the
cadaver.
Axial
CT
images
were
acquired
with
a
Siemens
Sensation
(Siemens
Medical,
Malvern,
PA)
16
CT
scanner.
The
CT
settings
were
the
same
for
all
specimens
at
100
kV,
35
mAS,
512
x
512
acquisition
matrix,
0.6-mm
slice
thickness,
0.5-mm
incre-
ments,
and
0.6
pitch.
Scans
were
performed
on
all
speci-
mens
in
neutral
position
and
at
increments
ranging
from
15
degrees
of
dorsiflexion
through
25
degrees
of
plantarflexion.
CT
scans
were
stored
in
DICOM
(Digital
Imaging
and
Communications
in
Medicine)
format
for
later
processing.
Digitally
Reconstructed
Radiographs
DRRs
of
the
foot-ankle
specimens
were
created
from
CT
data
using
Amira
(version
5.3,
Visage
Imaging,
San
Diego,
CA).
High-resolution
DRR
alpha
scale
transparency
and
gamma
values
were
set
to
0.0194805
and
1.0,
respectively,
in
Amira.
AP
views
for
the
DRRs
were
generated
from
an
orthographic
coronal
view.
Hindfoot
alignment
view
DRRs
were
created
by
axially
rotating
the
CT
images
180
degrees
to
a
coronal
posteroanterior
view.
For
each
ankle,
a
focal
landmark
in
which
to
rotate
the
tibiotalar
joint
cen-
ter
(Figure
2A)
was
required.
The
focal
landmark
was
manually
determined
(Figure
2B)
by
two
independent
observers
(A.B.,
R.L.A.)
trained
in
image
processing.
CT
image
stacks
were
then
rotated
axially
about
the
Cartesian
coordinates
of
the
focal
landmark
at
10-degree
increments
from
30
degrees
of
external
rotation
to
30
degrees
of
inter-
nal
rotation
(to
assess
the
influence
of
axial
rotation)
and
sagittally
from
0
degrees
to
25
degrees
at
5-degree
incre-
ments
(to
assess
the
influence
of
projection
angle
from
the
horizon).
DRRs
were
extracted
after
each
rotation
or
com-
bination
of
rotations.
Forty-two
DRRs
were
generated
to
study
each
view
(AP
or
HAV)
of
each
specimen
(6
for
dorsiflexion
and
plantarflexion,
7
for
axial
rotation).
Barg
et
al
3
ee
t‘
l
4r
i
i.
a
A
Figure
I.
Sagittal
(left)
and
anterior
(right)
views
of
custom
acrylic
device
that
allowed
for
fixed,
user-defined
dorsiflexion
and
plantar
flexion
angles.
The
device
was
calibrated
using
a
digital
goniometer,
with
tick
marks
added
to
indicate
5-degree
increments.
Medial
Distal
Tibial
Angle
Measurement
The
MDTA
was
measured
using
a
semiautomatic,
com-
puter-assisted
approach
as
described
previously.
4
Briefly,
a
circle
was
automatically
drawn
and
positioned
to
fit
between
the
medial
and
lateral
cortex
at
the
most
proximal
location
of
the
tibial
shaft.
The
center
of
this
circle
was
the
most
proximal
point
of
the
longitudinal
axis
of
the
tibia.
Next,
another
circle
was
drawn
over
the
distal
tibia
and
positioned
to
fit
between
three
cortices:
the
medial,
lateral,
and
tibial
plafond.
The
center
of
this
circle
was
placed
on
the
longitudinal
axis
of
the
tibia,
where
it
passed
through
the
center
of
the
talus.
The
joint
orientation
line
was
drawn
across
the
flat
subchondral
line
of
the
tibial
plafond.
38
The
MDTA
was
then
calculated
automatically
as
the
angle
between
the
longitudinal
axis
of
the
tibia
and
the
joint
ori-
entation
line.
4
All
measurements
of
two
specimens
were
performed
by
two
observers
(A.B.,
R.L.A.)
with
different
levels
of
training
to
assess
interobserver
reliability.
The
MDTA
was
measured
again
six
weeks
after
the
initial
reading
to
assess
intraob-
server
reliability.
In
two
different
specimens,
a
radiological
control
study
was
performed
to
assess
the
ability
of
DRRs
to
serve
as
a
surrogate
for
conventional
radiographs.
Specifically,
the
AP
view
and
the
HAV
were
acquired
at
10-degree
increments
from
30
degrees
of
external
rotation
to
30
degrees
of
internal
rotation
with
the
foot-ankle
in
neutral
dorsiflexion
(Figure
3),
and
the
MDTA
was
measured
as
described
above.
The
film
focus
distance
was
held
constant
at
40
inches
for
the
AP
view.
The
beam
(55
kV,
6
mAS)
was
centered
on
the
tibiotalar
joint.
For
the
HAV,
the
film
focus
distance
was
constant
at
40
inches.
The
beam
(62
kV,
6
mAS)
was
centered
on
the
ankle
with
the
beam
angle
of
20
degrees
to
the
horizontal,
with
a
field
of
view
that
included
the
midshaft
of
the
tibia
to
just
below
the
calcaneus."
Statistical
Analysis
Bland-Altman
plots
were
generated
to
quantify
agreement
between
the
DRR
and
conventional
radiographic
methods
to
measure the
MDTA.
2
'
8
'
9
The
Bland
Altman
plots
included
the
bias
(mean
of
the
differences
in
MDTA
between
DRR
and
conventional
films)
as
well
as
the
upper
and
lower
95%
limits
of
agreement
(estimated
as
1.96
times
the
standard
deviation
of
the
differences).
The
intraclass
correlation
coefficients
(ICCs)
and
associated
95%
confidence
inter-
vals
(CIs)
of
the
ICCs
quantified
inter-
and
intraobserver
repeatability.
ICC
values
were
interpreted
as
follows:
ICC
=
1,
perfect
agreement;
0.81
to
0.99,
excellent
agreement;
and
0.61
to
0.80,
substantial
agreement.
13
The
Shapiro-Wilk
test
was
performed
to
verify
whether
DRR
and
conventional
film
measurements
of
the
MDTA
were
normally
distributed.
With
normality
confirmed,
MDTA
measurements
of
the
DRRs
from
the
first
set
of
observations
of
the
primary
assessor
were
compared
using
paired
t
tests.
Here,
paired
t
test,
with
significance
set
at
P
<
.05,
compared
the
MDTA
measured
on
the
neutral
AP
view
DRR
(0-degree
x-ray
beam
angle
from
the
horizon,
0
degrees
of
axial
rotation,
0
degrees
of
tibiotalar
joint
dorsi-
flexion)
separately
with
the
MDTA
measured
on
the
AP
4
Foot
&
Ankle
International
Coronal
Sagittal
Axial
Figure
2.
Method
used
to
verify
placement
of
focal
landmark.
The
focal
landmark
(yellow
dot)
was
created
manually
by
estimating
the
midpoint
of
the
joint
about
the
3
computed
tomography
(CT)
planes
(top:
shown
for
a
single
specimen).
Soft
tissue
structures
were
then
visualized
as
semitransparent
media
relative
to
the
osseous
anatomy
to
help
the
observer
determine
the
final
position
of
the
focal
point
(bottom:
shown
for
a
single
specimen).
view
DRR
after
it
was
altered
by
an
incremental
change
in
x-ray
beam
projection
angle,
tibiotalar
joint
position,
and
foot-ankle
axial
rotation.
The
same
approach
was
used
to
compare
the
MDTA
measured
on
the
baseline
neutral
HAV
DRR
(20-degree
x-ray
beam
angle
from
the
horizon,
0
degrees
of
axial
rotation,
0
degrees
of
tibiotalar
joint
dorsi-
flexion).
With
such
a
statistical
approach,
multiple
paired
t
tests
were
required.
When
multiplicity
is
present,
the
usual
approach
is
to
adjust
P
values
to
control
for
type
I
error.
However,
in
this
study,
we
sought
to
compare
one
endpoint,
a
single
incremental
change
in
projection
angle/joint
posi-
tion,
to
the
baseline
MDTA
of
the
AP
view
and
the
HAV
to
better
understand
what
level
of
change
induces
statistically
significant
alterations
to
the
MDTA.
If
P
values
were
adjusted,
it
could
lead
to
a
loss
of
significance
for
a
single
change
in
position
or
projection
angle,
which
could
be
oth-
erwise
clinically
important
to
highlight.
Therefore,
despite
having
multiple
t
tests
present,
the
P
values
were
not
adjusted.
43
All
data
were
analyzed
using
IBM
SPSS
Statistics
version
22.0
(IBM,
Armonk,
NY)
and
SigmaPlot
version
12.0
(Systat
Software
Inc,
San
Jose,
CA).
Results
For
the
AP
view,
the
Bland-Altman
plot
indicated
strong
agreement
between
the
MDTA
measured
on
the
DRR
to
that
from
the
conventional
radiograph,
with
no
obvious
directional
preference
(Figure
4,
left).
Specifically,
the
MDTA
could
be
measured
to
a
bias
of
0.16
degrees.
The
limits
of
agreement
for
the
AP
view
indicated
that
DRR
measurements
would
be
measured
to
within
approximately
±1.5
degrees
of
that
from
the
conventional
film
in
95%
of
the
cases.
Excellent
agreement
was
also
observed
between
the
MDTA
measured
on
the
conventional
HAV
to
that
on
AP
View
Conv
en
t
iona
Conven
t
iona
HAV
Barg
et
al
5
Figure
3.
Photographs
and
radiographs
demonstrating
the
various
views
acquired
for
the
digitally
reconstructed
radiograph
(DRR)
validation
study.
An
anteroposterior
(AP)
view
was
acquired
at
neutral
position
and
30
degrees
of
external
rotation
as
indicated
in
the
approximate
location
of
the
cadaver
specimen
shown
(top).
Conventional
films
and
corresponding
DRRs
of
the
specimen
in
neutral
rotation
are
shown
for
the
AP
view
(lower
left)
and
hindfoot
alignment
view
(HAV)
(lower
right).
the
corresponding
DRR
(Figure
4,
right).
The
DRR
was
found
to
slightly
underestimate
the
MDTA,
with
a
bias
of
—1.20
degrees.
The
limits
of
agreement
for
the
HAV
indi-
cated
that
DRR
measurements
would
be
made
to
within
approximately
±2.0
degrees
of
that
from
the
conventional
film
in
95%
of
the
cases.
The
ICCs
representing
interobserver
and
intraobserver
repeatability
of
the
MDTA
measurements
using
DRRs
were
excellent
at
0.912
(95%
CI,
0.881-0.952)
and
0.882
(95%
CI,
0.792-0.916),
respectively.
Overall,
the
MDTA
decreased
when
the
x-ray
projection
angle
from
the
horizon
was
increased
(Figure
5).
All
changes
in
the
tilt
of
the
x-ray
beam
significantly
altered
MDTA
measurements
for
the
AP
view
(Table
1).
However,
for
the
HAV,
tilt
of
the
x-ray
beam
only
imparted
significant
changes
when
the
disparity
was
greater
than
10
degrees
from
the
baseline
value
of
20
degrees.
There
was
a
clear
increase
of
the
MDTA
on
the
HAV
when
the
foot
and
ankle
went
from
30
degrees
of
external
rotation
to
30
degrees
of
internal
rotation;
only
a
gradual
increase
was
noted
for
the
AP
view
(Figure
6).
More
spe-
cifically,
for
the
HAV,
significant
changes
in
the
MDTA
were
observed
for
all
axial
rotations
analyzed
(Table
2).
Conversely,
for
the
AP
view,
axial
foot-ankle
rotation
only
significantly
altered
MDTA
measurements
at
20
and
30
degrees
of
external
rotation.
Bland-Altman
Plot
(AP
View)
---
95%
Limits
of
Agreement
(-1.499,
1.175)
Mean
(Bias
=
-0.162,
Std
Dev
=
0.682)
-2.0
87
88
89
90
91
92
Mean
MDTA
[(DRR
+
Conventional)/2]
(deg)
4
Bland-Altman
Plot
(HAV)
95%
Limits
of
Agreement
(-0.464,
2.868)
7:5;"'
a)
3
-
Mean
(Bias
=
1.202,
Std
Dev
=
0.850)
0
2-
TT
0
1
-
a
°
0
-
C
0
70
75
80
85
90
95
100
Mean
MDTA
[(DRR
+
Conventional)/2]
(deg)
2
2.0
Difference
(
Conven
t
iona
l
-
DRR)
(
deg
)
1.5
1.0
0.5
0.0
-
0.5
-
1.5
6
Foot
&
Ankle
International
Figure
4.
Bland-Altman
plots
showing
level
of
agreement
between
medial
distal
tibial
angle
(MDTA)
measurements
acquired
from
digitally
reconstructed
radiographs
(DRRs)
to
those
from
conventional
films
(solid
line
represents
the
bias;
dashed
lines
represent
the
upper
and
lower
bounds
of
the
95%
limits
of
agreement).
Left:
For
the
anteroposterior
(AP)
view,
the
DRR
overestimated
the
MDTA
measured
on
the
conventional
film
by
only
0.162
degrees
(solid
line).
The
limits
of
agreement
for
the
AP
view
indicated
that
DRR
measurements
would
be
measured
to
within
approximately
±
1.5
degrees
of
that
from
the
conventional
film
in
95%
of
the
cases.
Right:
For
the
hindfoot
alignment
view
(HAV),
the
DRR
underestimated
the
MDTA
measured
on
the
conventional
film
by
1.202
degrees.
The
limits
of
agreement
for
the
HAV
indicated
that
DRR
measurements
would
be
measured
to
within
approximately
±2.0
degrees
of
that
from
the
conventional
film
in
95%
of
the
cases.
Overall,
the
MDTA
appeared
to
decrease
slightly
when
the
tibiotalar
joint
progressed
from
dorsiflexion
to
plantar-
flexion
for
both
the
HAV
and
the
AP
view.
However,
sig-
nificant
changes
due
to
position
of
the
tibiotalar
joint
were
only
observed
at
25
degrees
of
plantarflexion
for
the
HAV;
at
20
degrees
of
plantarflexion,
the
change
in
MDTA
approached
statistical
significance
(Table
3).
Significant
changes
in
the
MDTA
were
not
found
for
the
AP
view
as
the
tibiotalar
joint
progressed
from
dorsiflexion
to
plantarflex-
ion
(Table
3).
Discussion
In
this
study,
we
first
demonstrated
that
DRRs
could
be
used
as
a
surrogate
to
conventional
films
for
the
measure-
ment
of
the
MDTA.
Next,
using
DRRs,
we
found
that
the
MDTA
as
measured
on
the
AP
view
did
not
change
substan-
tially
due
to
axial
rotation
or
tibiotalar
joint
position.
However,
measurements
of
the
MDTA
on
the
AP
view
were
clearly
sensitive
to
the
tilt
of
the
x-ray
beam
from
the
hori-
zon.
For
the
HAV,
measurements
of
the
MDTA
changed
substantially
as
the
foot-ankle
experienced
axial
rotation.
Conversely,
only
minor
changes
in
the
HAV
MDTA
were
observed
as
the
x-ray
beam
was
altered
from
the
horizon.
Also,
significant
changes
in
the
MDTA
for
the
HAV
only
occurred
at
25
degrees
of
plantarflexion.
Projections
at
computer-controlled
angles
through
volu-
metric
CT
data
have
been
used
to
generate
DRRs
of
the
hi
p,
16,21,53
k
nee,
3,47,54,55
and
anue.29
Use
of
DRRs
can
be
advantageous
as
they
reduce
the
time
required
to
achieve
a
large
number
of
equivalent
radiographs
and
also
limit
equipment
use
and
personnel
costs
(burden
to
x-ray
equip-
ment,
need
for
radiology
technicians,
etc).
Most
important,
DRRs
are
constructed
from
computer-controlled
projec-
tions
about
a
well-defined,
constant
center.
In
this
study,
it
would
have
been
difficult
to
ensure
that
exact
projection
angles
were
obtained
using
standard
x-ray
equipment.
Therefore,
use
of
DRRs
in
this
study
effectively
eliminated
bias
that
would
otherwise
be
present
if
an
experimental
approach
were
used.
We
found
that
DRRs
served
as
a
strong
surrogate
to
conventional
radiographs
for
measurement
of
the
MDTA,
with
mean
differences
on
the
order
of
1
to
2
degrees.
Although
minor,
two
sources
could
be
responsible
for
the
observed
difference
between
the
two
methods.
First,
although
we
strived
to
obtain
a
radiographic
projection
that
was
nearly
identical
to
a
corresponding
projection,
it
is
pos-
sible
that
experimental
bias
was
introduced.
For
example,
the
goniometer
used
to
align
the
cadaver
in
axial
rotation
is
Barg
et
al
7
92
100
-
95
-
AP
Hew
O
HAV
90
-
AP
f
3)
I
o
88-
Q
o
86-
c
co
84
n
-
63
90
-
<
85-
HAV
0
2
(
T
3
80
-
m
2
75
-
82
-
70
-
80
65
0
5
10
15
20
25
Projection
angle
from
the
horizon
(deg)
-30
-20
-
10
0
10
20
30
Axial
foot/ankle
rotation
(external
->
internal)
(deg)
Figure
5.
Mean
medial
distal
tibial
angle
(MDTA)
as
an
influence
of
the
projection
angle
from
the
horizon
during
neutral
rotation.
As
the
projection
angle
was
increased,
there
were
significant
decreases
in
the
MDTA
(Table
I).
Graphs
denote
the
anteroposterior
(AP)
view
and
hindfoot
alignment
view
(HAV)
at
0
degrees
and
20
degrees,
respectively.
Bars
represent
standard
deviations.
Table
I.
P
Values
to
Indicate
Significant
Findings
When
the
Baseline
Anteroposterior
(AP)
View
and
Hindfoot
Alignment
View
(HAV)
Were
Compared
With
Incremental
Changes
in
X-ray
Beam
Tilt
From
the
Horizon'.
View
X-ray
Beam
Tilt
P
Value
AP
(0
degrees)
vs
5
degrees
.007
vs
10
degrees
.007
vs
15
degrees
.021
vs
20
degrees
(HAV)
.016
vs
25
degrees
.005
HAV
(20
degrees)
vs
0
degrees
(AP)
.016
vs
5
degrees
.027
vs
10
degrees
.083
vs
15
degrees
.286
vs
25
degrees
.084
'Significant
values
indicated
in
boldface.
not
as
accurate
as
the
computer-controlled
rotation
used
to
create
the
corresponding
DRR.
The
small
discrepancy
between
DRRs
and
conventional
films could
also
be
the
result
of
MDTA
measurement
errors,
which
would
be
within
the
narrow
95%
confidence
interval
of
the
ICCs.
Nevertheless,
because
the
discrepancies
were
very
minor,
we
can
conclude
that
DRRs
serve
as
an
excellent
surrogate
to
conventional
films
for
measurement
of
the
MDTA
in
both
the
AP
view
and
HAY.
Previous
studies
demonstrated
that
the
measurement
of
the
MDTA
depended
on
the
radiographic
technique.
4
'
49
Figure
6.
Mean
medial
distal
tibial
angle
(MDTA)
as
an
influence
of
foot-ankle
rotation
about
the
same
x-ray
projection
angle
for
the
anteroposterior
(AP)
view
and
hindfoot
alignment
view
(HAV).
There
were
significant
increases
in
MDTA
for
the
HAV
over
the
entire
range
of
axial
rotation
analyzed.
Changes
in
the
AP
view
were
only
found
for
20
and
30
degrees
of
internal
rotation
(Table
2).
Negative
angles
indicate
external
rotation.
Bars
represent
standard
deviations.
Table
2.
P
Values
to
Indicate
Significant
Findings
When
the
Baseline
Anteroposterior
(AP)
View
and
Hindfoot
Alignment
View
(HAV)
Were
Compared
With
Incremental
Changes
in
Axial
Rotation
of
the
Foot-Ankle
s
.
View
Axial
Rotation
P
Value
AP
(0
degrees)
vs
30
degrees
external
rotation
.023
vs
20
degrees
external
rotation
.134
vs
10
degrees
external
rotation
.912
vs
10
degrees
internal
rotation
.530
vs
20
degrees
internal
rotation
.604
vs
30
degrees
internal
rotation
.081
HAV
(20
degrees)
vs
30
degrees
external
rotation
<.001
vs
20
degrees
external
rotation
<.001
vs
10
degrees
external
rotation
<.001
vs
10
degrees
internal
rotation
<.001
vs
20
degrees
internal
rotation
<.001
vs
30
degrees
internal
rotation
<.001
'Significant
values
indicated
in
boldface.
However,
in
the
work
by
Stufkens
et
a1
49
and
Barg
et
a1,
4
the
source
for
interview
discrepancies
could
not
be
recon-
ciled.
Our
results
suggest
that
both
x-ray
projection
angle
and
position
of
the
foot
and
ankle
during
the
film
play
an
important
role,
but
the
relative
contributions
to
changes
in
each
depend
on
which
view
is
used
to
measure
the
MDTA.
The
gradual
decrease
of
the
MDTA
as
the
angle
from
the
horizon
was
increased
can
be
partially
explained
by
the
inherent
ankle
joint
anatomy.
Specifically,
the
horizontal
96
-
94
-
92
-
-6,
90
-
4)
Zi--
'
8
8
-
.
2
H-
86-
84-
82
-
80
-
78
-
76
-
-15
-10
-5
0
5
10
15
20
25
Tibiotalar
position
(dorsiflexion
is
negative)
(deg)
AP
0
HAV
}
8
Foot
&
Ankle
International
Table
3.
P
Values
to
Indicate
Significant
Findings
When
the
Baseline
Anteroposterior
(AP)
View
and
Hindfoot
Alignment
View
(HAV)
Were
Compared
With
Incremental
Changes
in
Tibiotalar
Joint
Position'.
View
Tibiotalar
Position
P
value
AP
(0
degrees)
vs
15
degrees
dorsiflexion
.067
vs
5
degrees
dorsiflexion
.488
vs
5
degrees
plantar
flexion
.121
vs
15
degrees
plantar
flexion
.320
vs
25
degrees
plantar
flexion
.104
HAV
(20
degrees)
vs
15
degrees
dorsiflexion
.670
vs
5
degrees
dorsiflexion
.243
vs
5
degrees
plantar
flexion
.875
vs
15
degrees
plantar
flexion
.052
vs
25
degrees
plantar
flexion
.016
'Significant
values
indicated
in
boldface.
flank
of
the
MDTA
is
the
flat
subchondral
line
of
the
tibial
plafond.
38
However,
the
tibia
plafond
is
not
flat
but
rather
concave
in
shape,
with
an
average
medial
angle
of
22
±
4
degrees
at
slight
varus
alignment.
"21'25
Also,
the
subchon-
dral
bone
plate
is
the
cortical
endplate
located
in
the
calci-
fied
region
of
the
articular
cartilage.
It
has
been
demonstrated
that
subchondral
mineralization
is
not
homo-
geneously
distributed
across
the
articular
surface
of
the
distal
tibia.
33-35
Rather,
dense
subchondral
mineralization
is
predominantly
found
in
the
ventromedial
and
central
area
of
the
articular
surface
(eg,
Figure
2
in
Milhlhofer
et
81
33
).
33
'
34
The
bicentric
distribution
of
dense
bone
is
com-
mon
33
'
34
and
likely
explains
why
different
MDTA
values
were
observed
as
the
angle
of
the
x-ray
beam
was
altered
relative
to
the
horizon.
The
source
of
changes
in
the
MDTA
between
views
as
axial
rotation
was
altered
is
less
clear.
However,
differences
may
again
be
explained
when
considering
the
shape
and
distribution
of
bone
density
in
the
tibial
plafond.
First,
although
the
distribution
of
dense
bone
located
at
the
ven-
tromedial
and
central
area
of
the
articular
surface
is
bicen-
tric,
areas
of
high
mineralization
are
predominantly
confined
to
the
roof.
When
passing
a
plane
to
create
a
DRR
with
a
horizontal
projection
of
0
degrees
(ie,
AP
view),
one
is
less
likely
to
induce
changes
in
the
2-dimensional
appear-
ance
of
this
roof
(ie,
the
reference
line
used
to
measure the
MDTA)
because
the
roof
itself
is
more
or
less
aligned
with
the
horizon.
However,
when
tilting
the
initial
projection
to
20
degrees,
more
subtle
differences
in
the
location
of
the
dense
bone with
respect
to
the
roof
(and
therefore
horizon)
would
be
discerned.
Although
we
cannot
provide
a
direct
explanation
of
the
directional
preference
of
the
MDTA
(lower
values
at
external
rotation
to
higher
values
at
internal
rotation),
the
change
appeared
linear,
making
it
possible
to
apply
some
level
of
correction
factor
if
the
axial
rotation
angle
was
readily
available.
Figure
7.
Mean
medial
distal
tibial
angle
(MDTA)
as
an
influence
of
tibiotalar
joint
dorsiflexion
and
plantar
flexion
for
the
anteroposterior
(AP)
view
and
the
hindfoot
alignment
view
(HAV).
There
were
no
significant
changes
in
MDTA
for
the
AP
view;
significant
changes
were
observed
only
at
25
degrees
of
plantar
flexion
for
the
HAV
(Table
3).
Negative
angles
indicate
dorsiflexion.
Bars
represent
standard
deviations.
For
both
the
AP
view
and
the
HAV,
MDTA
angles
decreased
slightly
as
the
ankle
went
from
dorsiflexion
to
plantarflexion,
but
these
changes
were
only
significant
for
the
HAV
at
substantial
plantarflexion
(25
degrees).
The
exact
mechanism
responsible
for
the
modest
changes
observed
in
both
views
is
unknown.
However,
it
is
impor-
tant
to
note
that
the
foot
and
heel
were
affixed
to
the
acrylic
device
with
an
axial
rotation
of
0
degrees
as
the
tibiotalar
joint
was
rotated
from
dorsiflexion
to
plantarflexion.
With
these
constraints
in
mind,
it
is
reasonable
to
conclude
that
the
prescribed
rotations
would
occur
almost
exclusively
about
the
sagittal
plane.
Because
both
the
AP
view
and
the
HAV
were
acquired
from
the
posterior
viewing
plane,
one
would
expect
that
the
projected
line
of
the
highly
mineral-
ized
areas
of
tibial
plafond
would
translate
distal-proximal
in
this
projected
plane
but
would
not
become
skewed
with
respect
to
the
horizontal
axis.
Therefore,
our
fmdings,
which
indicated
that
the
MDTA
did
not
change
substantially
as
dorsiflexion
and
plantarflexion
were
altered,
seem
rea-
sonable.
Still,
the
larger
intersubject
variation
in
MDTA
angles
observed
for
the
HAV
as
dorsiflexion
and
plan-
tarflexion
were
altered
(see
standard
deviation
bars
in
Figure
7
for
the
HAV)
suggest
that
the
HAV
may
have
increased
discriminatory
capacity
to
detect
subject-specific
differences
in
the
density
and
location
of
mineralized
bone
in
the
tibial
plafond.
A
possible
limitation
of
the
study
was
that
all
MDTA
measurements
were
performed
on
DRRs
rather
than
con-
ventional
radiographs.
DRRs
have
a
smaller
image
resolu-
tion
than
conventional
radiographs
and
therefore
do
not
Barg
et
al
9
qualitatively
appear
as
crisp
and
detailed.
However,
for
mea-
surement
of
the
MDTA,
DRRs
were
found
to
be
an
excellent
surrogate
for
conventional
films.
Another
possible
limitation
is
that
the
cadavers
used
in
this
study
were
not
pathologic,
and
CT
scans
were
not
performed
in
a
weight-bearing
posi-
tion.
As
such,
our
results
should
be
interpreted
with
caution
as
they may
not
represent
the
same
changes
in
MDTA
mea-
surements
that
would
be
observed
in
the
clinic.
Finally,
although
many
of
our
comparisons
were
statistically
signifi-
cant,
it
was
beyond
the
scope
of
this
study
to
establish
what
level
of
change
in
MDTA
is
clinically
important.
Clinical
Significance
In
the
last
two
decades,
realignment
surgery
has
been
increasingly
performed
in
younger
patients
with
asymmet-
ric
ankle
osteoarthritis.
5,6,26,28
Th
e
MDTA
is
widely
used
to
determine
the
amount
and
orientation
of
these
supramal-
leolar
4,10,22,27,38,39,49,56
deformities.
Incorrect
assessment
of
the
MDTA
may
result
in
under-
or
overcorrection
that
is
associated
with
reduced
outcomes
following
surgery.
5
'
6
Our
study
did
not
establish
superiority
of
a
single
view
to
assess
supramalleolar
alignment.
However,
our
findings
demonstrate
that
the
MDTA,
as
measured
on
the
AP
view,
was
less
sensitive
to
changes
in
foot-ankle
positioning
com-
pared
with
the
HAV.
In
clinical
practice,
the
MDTA
is
most
often
measured
from
an
AP
projection
with
20
degrees
of
internal
rotation
of
the
foot
(ie,
mortise
view).
4'27,38,49,56
we
advocate
for
continued
use
of
the
mortise
view,
but
given
the
finding
that
the
MDTA
on
the
AP
view
was
sensitive
to
tilt
of
the
x-ray
beam,
we
would
also
recommend
that
tech-
nicians
and
clinicians
adopt
strategies
to
ensure
that
the
beam
is
aligned
directly
with
the
horizon.
This
could
be
accomplished
by
resting
the
x-ray
equipment
on
a
flat,
hori-
zontal
surface
with
the
foot
elevated
at
the
correct
height.
Also,
although
not
as
critical
as
tilt
of
the
beam,
a
goniom-
eter
should
be
used
to
ensure
that
the
foot
is
at
20
degrees
of
internal
rotation
when
acquiring
a
mortise
film.
One
interesting
finding
from
our
study
was
that
mea-
surements
of
the
MDTA
had
larger
standard
deviations
on
the
HAV
compared
with
the
MDTA
on
the
AP
view
for
the
same
cadavers.
Clinically,
this
could
suggest
that
the
HAV
has
increased
capacity
to
detect
intersubject
variability
in
supramalleolar
alignment.
An
additional
study
will
be
nec-
essary
to
identify
the
precise
mechanism
in
which
the
HAV
is
able
to
detect
such
differences.
Thus,
given
our
immedi-
ate
findings,
we
recommend
that
the
AP
view
be
acquired
with
the
x-ray
beam
aligned
with
the
horizon
to
measure the
MDTA.
Declaration
of
Conflicting
Interests
The
author(s)
declared
no
potential
conflicts
of
interest
with
respect
to
the
research,
authorship,
and/or
publication
of
this
article.
Funding
The
author(s)
received
no
fmancial
support
for
the
research,
authorship,
and/or
publication
of
this
article.
References
1.
Ali
AA,
Gregory
JJ,
Ockenden
M,
Hill
SO,
Makwana
NK.
Anatomic
description
of
the
distal
tibia:
implications
for
inter-
nal
fixation.
J
Foot
Ankle
Surg.
2012;51(3):296-298.
2.
Allen
BC,
Peters
CL,
Brown
NA,
Anderson
AE.
Acetabular
cartilage
thickness:
accuracy
of
three-dimensional
reconstruc-
tions
from
multidetector
CT
arthrograms
in
a
cadaver
study.
Radiology.
2010;255(2):544-552.
3.
Anderst
W,
Zauel
R,
Bishop
J,
Demps
E,
Tashman
S.
Validation
of
three-dimensional
model-based
tibio-femoral
tracking
during
running.
Med
Eng
Phys.
2009;31(1):10-16.
4.
Barg
A,
Harris
MD,
Henninger
HB,
et
al.
Medial
distal
tibial
angle:
comparison
between
weightbearing
mortise
view
and
hindfoot
alignment
view.
Foot
Ankle
Int.
2012;33(8):655-
661.
5.
Barg
A,
Pagenstert
GI,
Horisberger
M,
et
al.
Supramalleolar
osteotomies
for
degenerative
joint
disease
of
the
ankle
joint:
indication,
technique
and
results.
Int
Orthop
.
2013;37(9):1683-
1695.
6.
Barg
A,
Saltzman
CL.
Single-stage
supramalleolar
osteotomy
for
coronal
plane
deformity.
Curr
Rev
Musculoskelet
Med.
2014;7(4):277-291.
7.
Bethune
C,
Stewart
M.
Accelerated
computation
of
digitally
reconstructed
radiographs.
Int
Congr
Ser.
2005;1281(1):98-
103.
8.
Bland
JM,
Altman
DG.
Measuring
agreement
in
method
com-
parison
studies.
Stat
Methods
Med
Res.
1999;8(2):135-160.
9.
Bland
JM,
Altman
DG.
Statistical
methods
for
assessing
agreement
between
two
methods
of
clinical
measurement.
Lancet.
1986;1(8476):307-310.
10.
Chao
EY,
Neluheni
EV,
Hsu
RW,
Paley
D.
Biomechanics
of
malalignment.
Orthop
Clin
North
Am.
1994;25(3):379-386.
11.
Ellis
SJ,
Deyer
T,
Williams
BR,
et
al.
Assessment
of
lateral
hindfoot
pain
in
acquired
flatfoot
deformity
using
weightbear-
ing
multiplanar
imaging.
Foot
Ankle
Int.
2010;31(5):361-371.
12.
Ferkel
RD,
Hewitt
M.
Long-term
results
of
arthroscopic
ankle
arthrodesis.
Foot
Ankle
Int.
2005;26(4):275-280.
13.
Fleiss
JL.
Statistical
Methods
for
Rates
and
Proportions.
New
York,
NY:
John
Wiley;
1981.
14.
Guo
C,
Yan
Z,
Barfield
WR,
Hartsock
LA.
Ankle
arthrodesis
using
anatomically
contoured
anterior
plate.
Foot
Ankle
Int.
2010;31(6):492-498.
15.
Guyer
M,
Richardson
G.
Current
concepts
review:
total
ankle
arthroplasty.
Foot
Ankle
Int.
2008;29(2):256-264.
16.
Harris
MD,
Kapron
AL,
Peters
CL,
Anderson
AE.
Correlations
between
the
alpha
angle
and
femoral
head
asphericity:
impli-
cations
and
recommendations
for
the
diagnosis
of
cam
femo-
roacetabular
impingement.
Eur
J
Radiol.
2014;83(5):788-796.
17.
Harstall
R,
Lehmann
0,
Krause
F,
Weber
M.
Supramalleolar
lateral
closing
wedge
osteotomy
for
the
treatment
of
varus
ankle
arthrosis.
Foot
Ankle
Int.
2007;28(5):542-548.
18.
Hendrickx
RP,
Stufkens
SA,
de
Bruijn
EE,
et
al.
Medium-
to
long-term
outcome
of
ankle
arthrodesis.
Foot
Ankle
Int.
2011;32(10):940-947.
10
Foot
&
Ankle
International
19.
Horisberger
M,
Hintermann
B,
Valderrabano
V.
Alterations
of
plantar
pressure
distribution
in
posttraumatic
end-stage
ankle
osteoarthritis.
Clin
Biomech.
2009;24(3):303-307.
20.
Horisberger
M,
Valderrabano
V,
Hintermann
B.
Posttraumatic
ankle
osteoarthritis
after
ankle-related
fractures.
J
Orthop
Trauma.
2009;23(1):60-67.
21.
Imai
N,
Ito
T,
Suda
K,
Miyasaka
D,
Endo
N.
Pelvic
flexion
measurement
from
lateral
projection
radiographs
is
clinically
reliable.
Clin
Orthop
Relat
Res.
2013
;471(4):
1271-1276.
22.
Inman
VT.
The
Joints
of
the
Ankle.
Baltimore,
MD:
Williams
&
Wilkins;
1976.
23.
Jastifer
J,
Coughlin
MJ,
Hirose
C.
Performance
of
total
ankle
arthroplasty
and
ankle
arthrodesis
on
uneven
surfaces,
stairs,
and
inclines:
a
prospective
study.
Foot
Ankle
Int.
2015;36(1):
11-17.
24.
Jastifer
JR,
Coughlin
MJ.
Long-term
follow-up
of
mobile
bearing
total
ankle
arthroplasty
in
the
United
States.
Foot
Ankle
Int.
2015
;36(2):143
-150.
25.
Kelikian
AS,
Sarrafian
SK.
Sarrafian's
Anatomy
of
the
Foot
and
Ankle:
Descriptive,
Topographic,
Functional.
Baltimore,
MD:
Lippincott
Williams
&
Wilkins;
2011.
26.
Knupp
M,
Hintermann
B.
Treatment
of
asymmetric
arthri-
tis
of
the
ankle
joint
with
supramalleolar
osteotomies.
Foot
Ankle
Int.
2012;33(3):250-252.
27.
Knupp
M,
Ledermann
H,
Magerkurth
0,
Hintermann
B.
The
surgical
tibiotalar
angle:
a
radiologic
study.
Foot
Ankle
Int.
2005
;26(9):
713
-716.
28.
Knupp
M,
Stufkens
SA,
Bolliger
L,
Barg
A,
Hintermann
B.
Classification
and
treatment
of
supramalleolar
deformities.
Foot
Ankle
Int.
2011
;32(11):
1023
-1031.
29.
Kuo
CC,
Lu
HL,
Lu
TW,
et
al.
Effects
of
positioning
on
radio-
graphic
measurements
of
ankle
morphology:
a
computerized
tomography-based
simulation
study.
Biomed
Eng
Online.
2013;12:131.
30.
Madry
H,
van
Dijk
CN,
Mueller-Gerbl
M.
The
basic
sci-
ence
of
the
subchondral
bone.
Knee
Surg
Sports
Traumatol
Arthrosc.
2010;18(4):419-433.
31.
Mann
JA,
Mann
RA,
Horton
E.
STAR
ankle:
long-term
results.
Foot
Ankle
Int.
2011
;32(5):473
-484.
32.
Min
W,
Sanders
R.
The
use
of
the
mortise
view
of
the
ankle
to
determine
hindfoot
alignment:
technique
tip.
Foot
Ankle
Int.
2010;31
(9):
823
-827.
33.
Muhlhofer
H,
Ercan
Y,
Drews
S,
et
al.
Mineralisation
and
mechanical
strength
of
the
subchondral
bone
plate
of
the
infe-
rior
tibial
facies.
Surg
Radiol
Anat.
2009;31(4):237-243.
34.
Muller-Gerbl
M.
[Anatomy
and
biomechanics
of
the
upper
ankle
j
oint]
.
Orthopade.
2001
;30(1):3-11.
35.
Muller-Gerbl
M.
The
subchondral
bone
plate.
Adv
Anat
Embryol
Cell
Biol.
1998;
141
:III-XI,
1-134.
36.
Nelson
V,
Deshpande
S,
Gray
A,
Vial
P,
Holloway
L.
Comparison
of
digitally
reconstructed
radiographs
generated
from
axial
and
helical
CT
scanning
modes:
a
phantom
study.
Australas
Phys
Eng
Sci
Med.
2014;37(2):285-290.
37.
Pagenstert
GI,
Hintermann
B,
Barg
A,
Leumann
A,
Valderrabano
V.
Realignment
surgery
as
alternative
treatment
of
varus
and
valgus
ankle
osteoarthritis.
Clin
Orthop
Relat
Res.
2007;462(1):156-168.
38.
Paley
D.
Principles
of
Deformity
Correction.
Berlin,
Germany:
Springer;
2002.
39.
Paley
D,
Herzenberg
JE,
Tetsworth
K,
McKie
J,
Bhave
A.
Deformity
planning
for
frontal
and
sagittal
plane
corrective
osteotomies.
Orthop
Clin
North
Am.
1994;25(3):425-465.
40.
Paremain
GD,
Miller
SD,
Myerson
MS.
Ankle
arthrodesis:
results
after
the
miniarthrotomy
technique.
Foot
Ankle
Int.
1996;17(5):247-252.
41.
Plaass
C,
Knupp
M,
Barg
A,
Hintermann
B.
Anterior
double
plating
for
rigid
fixation
of
isolated
tibiotalar
arthrodesis.
Foot
Ankle
Int.
2009;30(7):631-639.
42.
Queen
RM,
De
Biassio
JC,
Butler
RJ,
et
al.
J.
Leonard
Goldner
Award
2011:
Changes
in
pain,
function,
and
gait
mechan-
ics
two
years
following
total
ankle
arthroplasty
performed
with
two
modern
fixed-bearing
prostheses.
Foot
Ankle
Int.
2012;33(7):535-542.
43.
Rothman
KJ.
No
adjustments
are
needed
for
multiple
com-
parisons.
Epidemiology.
1990;1(1):43-46.
44.
Saltzman
CL,
el-Khoury
GY.
The
hindfoot
alignment
view.
Foot
Ankle
Int.
1995;16(9):572-576.
45.
Saltzman
CL,
Mann
RA,
Ahrens
JE,
et
al.
Prospective
con-
trolled
trial
of
STAR
total
ankle
replacement
versus
ankle
fusion:
initial
results.
Foot
Ankle
Int.
2009;30(7):579-596.
46.
Saltzman
CL,
Salamon
ML,
Blanchard
GM,
et
al.
Epidemiology
of
ankle
arthritis:
report
of
a
consecutive
series
of
639
patients
from
a
tertiary
orthopaedic
center.
Iowa
Orthop
J.
2005;25(1):44-46.
47.
Scarvell
JM,
Pickering
MR,
Smith
PN.
New
registration
algo-
rithm
for
determining
3D
knee
kinematics
using
CT
and
sin-
gle-plane
fluoroscopy
with
improved
out-of-plane
translation
accuracy.
J
Orthop
Res.
2010;28(3):334-340.
48.
Stamatis
ED,
Cooper
PS,
Myerson
MS.
Supramalleolar
oste-
otomy
for
the
treatment
of
distal
tibial
angular
deformities
and
arthritis
of
the
ankle
joint.
Foot
Ankle
Int.
2003;24(10):754-764.
49.
Stufkens
SA,
Barg
A,
Bolliger
L,
et
al.
Measurement
of
the
medial
distal
tibial
angle.
Foot
Ankle
Int.
2011;32(3):288-293.
50.
Tochigi
Y,
Suh
JS,
Amendola
A,
Pedersen
DR,
Saltzman
CL.
Ankle
alignment
on
lateral
radiographs,
Part
1:
sensitivity
of
measures
to
perturbations
of
ankle
positioning.
Foot
Ankle
Int.
2006;27(2):82-87.
51.
Tochigi
Y,
Suh
JS,
Amendola
A,
Saltzman
CL.
Ankle
align-
ment
on
lateral
radiographs,
Part
2:
reliability
and
validity
of
measures.
Foot
Ankle
Int.
2006;27(2):88-92.
52.
Valderrabano
V,
Horisberger
M,
Russell
I,
Dougall
H,
Hintermann
B.
Etiology
of
ankle
osteoarthritis.
Clin
Orthop
Relat
Res.
2009;467(7):1800-1806.
53.
van
der
Bom
MJ,
Groote
ME,
Vincken
KL,
Beek
FJ,
Bartels
LW.
Pelvic
rotation
and
tilt
can
cause
misinterpretation
of
the
acetabular
index
measured
on radiographs.
Clin
Orthop
Relat
Res.
2011;469(6):1743-1749.
54.
van
Eck
CF,
Martins
CA,
Lorenz
SG,
Fu
FH,
Smolinski
P.
Assessment
of
correlation
between
knee
notch
width
index
and
the
three-dimensional
notch
volume.
Knee
Surg
Sports
Traumatol
Arthrosc.
2010;18(9):1239-1244.
55.
van
Eck
CF,
Wong
AK,
Irrgang
JJ,
Fu
FH,
Tashman
S.
The
effects
of
limb
alignment
on
anterior
cruciate
ligament
graft
tunnel
positions
estimated
from
plain
radiographs.
Knee
Surg
Sports
Traumatol
Arthrosc.
2012;20(5):979-985.
56.
Wang
B,
Saltzman
CL,
Chalayon
0,
Barg
A.
Does
the
sub-
talar
joint
compensate
for
ankle
malalignment
in
end-stage
ankle
arthritis?
Clin
Orthop
Relat
Res.
2015;473(1):318-325.