Evaluation of the effect of distal femoral elevation on radiographic measurement of the anatomic lateral distal femoral angle


Jackson, G.M.; Wendelburg, K.L.

Veterinary Surgery 41(8): 994-1001

2013


To determine the effect of elevation of the distal femur on radiographic determination of the anatomic lateral distal femoral angle (aLDFA) in an in vitro canine model. In vitro study. Cadaveric canine femora (n = 7 pairs). Dissected femora were positioned in a custom-mounting frame. The distal end of the frame was elevated in 5° increments from 0° to 45°, with craniocaudal (CrCd) radiographs obtained at each position. The aLDFA was measured from the radiograph of each femur at each elevation. Statistical comparison of measured values was performed and radiographs were evaluated for radiographic indicators of positioning. There was significant increase in measured aLDFA at all elevations >5° when compared to 0° elevation. The mean value for aLDFA increased from 92.3° at 0° elevation to 95.0° at 45° elevation. The femoral trochlear ridges and walls of the intercondylar fossa were identified as the most useful radiographic landmarks. The fabellae, though extrafemoral and inconsistently retained in the current study, may also be beneficial. The lesser trochanter and nutrient foramen were less useful landmarks because of anatomic variability. Elevation of the distal femur had a significant effect on measured aLDFA at all elevations greater than 5°.

Evaluation
of
the
Effect
of
Distal
Femoral
Elevation
on
Radiographic
Measurement
of
the
Anatomic
Lateral
Distal
Femoral
Angle
Gregory
M.
Jackson,
DVM
Diplomate
ACVS
and
Kirk
L.
Wendelburg,
DVM
Diplomate
ACVS
Animal
Specialty
Group,
Inc.,
Los
Angeles,
CA
Objective:
To
determine
the
effect
of
elevation
of
the
distal
femur
on
radiographic
determination
of
the
anatomic
lateral
distal
femoral
angle
(aLDFA)
in
an
in
vitro
canine
model.
Study
Design:
In
vitro
study.
Sample
Population:
Cadaveric
canine
femora
(n
=
7
pairs).
Methods:
Dissected
femora
were
positioned
in
a
custom-mounting
frame.
The
distal
end
of
the
frame
was
elevated
in
increments
from
to
45°,
with
cranio-
caudal
(CrCd)
radiographs
obtained
at
each
position.
The
aLDFA
was
measured
from
the
radiograph
of
each
femur
at
each
elevation.
Statistical
comparison
of
measured
values
was
performed
and
radiographs
were
evaluated
for
radiographic
indicators
of
positioning.
Results:
There
was
significant
increase
in
measured
aLDFA
at
all
elevations
>5°
when
compared
to
elevation.
The
mean
value
for
aLDFA
increased
from
92.3°
at
elevation
to
95.0°
at
45°
elevation.
The
femoral
trochlear
ridges
and
walls
of
the
intercondylar
fossa
were
identified
as
the
most
useful
radiographic
landmarks.
The
fabellae,
though
extrafemoral
and
inconsistently
retained
in
the
current
study,
may
also
be
beneficial.
The
lesser
trochanter
and
nutrient
foramen
were
less
useful
landmarks
because
of
anatomic
variability.
Conclusion:
Elevation
of
the
distal
femur
had
a
significant
effect
on
measured
aLDFA
at
all
elevations
greater
than
5°.
Corresponding
Author
Gregory
Jackson,
DVM,
Dipl,
ACVS-SA,
VCA—Animal
Specialty
Group,
5610
Kearny
Mesa
Road,
Suite
B,
San
Diego,
CA
92111
E-mail:
Submitted
October
2010
Accepted
July
2011
D01:10.11111j.1532-950X.2012.01059.x
Medial
patellar
luxation
(MPL)
is
a
common
orthopedic
abnormality
in
the
dog.
1-3
Anatomic
malalignment
of
the
extensor
mechanism
(consisting
of
the
quadriceps
mus-
culature,
patellar
tendon,
patella,
trochlear
groove,
patel-
lar
ligament,
and
tibial
tuberosity)
contributes
to
patellar
luxation.
1
This
malalignment
can
occur
with
genu
varum,
external
rotation
of
the
hip,
relative
femoral
head
retrover-
sion,
coxa
vara,
external
distal
femoral
torsion,
medial
dis-
placement
of
the
tibial
tuberosity,
and
tibial
valgus
among
others."
Distal
femoral
varus
(medial
angulation
of
the
distal
femur)
has
also
been
reported
as
a
contributing
fac-
tor
to
MPL.
5
'
7
Distal
femoral
varus
shifts
the
long
axis
of
the
extensor
mechanism
medially,
rather
than
directly
over-
lying
the
femoral
trochlea.
This
induces
significant
medial
tension
on
the
patella
during
muscular
contraction
and
can
lead
to
subsequent
medial
luxation.
In
the
veterinary
literature,
there
are
limited
reports
of
assessment
of
distal
femoral
varus
in
surgical
planning
for
correction
of
MPL."
Distal
femoral
varus
can
be
evaluated
by
measurement
of
the
anatomic
lateral
distal
femoral
angle
(aLDFA;
Fig
1).
Whereas
excessive
distal
femoral
varus
may
necessitate
surgical
correction,
the
degree
at
which
this
should
be
performed
has
not
been
definitively
determined,
though
recommendations
for
correction
of
femoral
varus
greater
than
10°
above
normal
have
been
reported.
5
Distal
femoral
varus
is
typically
measured
from
a
cran-
iocaudal
(CrCd)
radiograph
of
the
femur.
This
projection
allows
observation
of
the
frontal
plane
of
the
femur.
Nor-
mal
aLDFA
values
from
this
projection
have
been
reported
for
4
breeds
of
dogs.
10
To
obtain
a
true
CrCd
projection,
the
femur
must
not
be
internally
or
externally
rotated.
5,6
The
long
axis
of
the
femur
must
be
parallel
to
the
radiographic
cassette.
5,6
The
radiographic
beam
must
be
perpendicular
to
the
long
axis
of
the
femur
and
thus,
the
radiographic
cassette.
5
'
6
This
position
will
direct
the
radiographic
beam
in
a
true
CrCd
direction.
Radiographic
positioning
for
this
projection
can
be
difficult
and
may
be
affected
by
confor-
mation,
concurrent
orthopedic
disease,
patient
tolerance,
or
the
level
of
sedation.
Concurrent
orthopedic
pathology
of
the
hip
or
stifle
typically
limits
the
ability
to
obtain
full
extension
of
those
joints
5
and
may
be
common
in
dogs
with
increased
distal
femoral
varus.
7,8
Inability
to
obtain
full
ex-
tension
of
these
joints
can
lead
to
elevation
of
the
distal
femur
off
of
the
radiographic
table
and
limit
the
ability
to
obtain
a
true
CrCd
projection
of
the
femur.
To
counter-
act
reluctance
or
inability
to
fully
extend
the
hip
and
stifle,
994
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
Jackson
and
Wendelburg
Effect
of
Femoral
Elevation
on
aLDFA
landmarks
for
correct
positioning
of
a
true
CrCd
radio-
graphic
projection
of
the
femur.
MATERIALS
AND
METHODS
Seven
pairs
of
cadaveric
canine
femora
were
studied.
The
femora
were
obtained
from
medium
to
large
breed
dogs
eu-
thanatized
for
reasons
unrelated
to
this
study.
These
dogs
had
no
history
of
lameness
and
were
determined
to
have
no
degenerative
changes
or
gross
pathology
involving
the
distal
aspect
of
the
femur.
The
femora
were
disarticulated
from
the
coxofemoral
and
stifle
joints
and
dissected
free
of
soft
tissue.
An
attempt
was
made
to
preserve
the
fabel-
lae.
Preservation
of
the
fabellae
was
successful
both
medi-
ally
and
laterally
in
5/14
femora
and
only
laterally
in
5/14
femora.
The
remaining
fabellae
were
determined
to
be
loose
or
to
have
shifted
position
after
soft
tissue
dissection,
and
were
thus
not
evaluated.
The
patella
was
not
preserved
in
any
specimen.
Figure
1
The
femoral
anatomic
axis
(FAA,
green)
was
determined
in
the
frontal
plane
by
marking
points
along
the
femoral
length
at
1
/,
1
6,
and
1
4
distances.
The
width
of
the
femur
was
measured
at
each
of
these
points
(white
circles).
The
FAA
was
the
best-fit
line
connecting
the
center
of
the
femoral
width
at
these
points.
The
transcondylar
axis
(TCA,
red)
was
drawn
as
a
line
tangential
to
the
most
distal
aspects
of
both
femoral
condyles.
The
anatomic
lateral
distal
femoral
angle
(aLDFA,
blue)
was
measured.
The
angle
was
formed
between
the
FAA
and
the
TCA.
numerous
alternatives
have
been
reported.
These
include
the
use
of
sedation
or
anesthesia,
horizontal
beam
radiol-
ogy,
angled-beam
projections,
fluoroscopic
verification
of
positioning,
a
"torso
elevated
position,"
or
a
caudocra-
nial
projection,
though
the
ability
for
these
techniques
to
capture
a
true
CrCd
projection
have
not
been
directly
assessed.
5,6,9,
"
Radiographic
landmarks
to
aid
in
the
evaluation
of
femoral
positioning
have
been
described.
5,6,9-12
These
in-
clude
the
patella,
the
fabellae,
the
lesser
trochanter,
the
walls
of
the
intercondylar
fossa,
the
nutrient
foramen,
and
the
femoral
trochlea.
Evaluation
of
reliability
of
these
land-
marks
is
limited
in
the
veterinary
literature.
10-12
Incorrect
positioning
of
a
patient
for
radiographs
may
have
an
effect
on
measurements,
and
thus
on
treatment
recommendations
and
surgical
planning.
We
hypothesized
that
elevation
of
the
distal
femur,
as
is
typical
with
incorrect
positioning,
would
lead
to
an
increased
measurement
of
aLDFA.
Our
objective
was
to
assess
the
effect
of
increasing
dis-
tal
femoral
elevation
on
the
radiographic
measurement
of
distal
femoral
varus,
as
measured
by
the
aLDFA.
Addi-
tionally,
we
evaluated
reliability
of
reported
radiographic
Femoral
Positioning
After
dissection,
each
femur
was
mounted
in
a
customized
orthogonal
frame
to
ensure
consistent
CrCd
positioning
(Fig
2A).
The
distal
and
caudal
portions
of
the
femoral
condyles
were
positioned
flush
against
the
orthogonal
por-
tion
of
one
end
of
the
frame
(Fig
2B).
The
proximal
as-
pect
of
the
femur
was
elevated
using
a
radiolucent
position
screw
until
the
lesser
trochanter
and
lateral
supracondylar
tuberosity
were
of
equal
heights
above
the
frame
(Fig
2C).
Positioning
was
similar
to
a
previous
report.'
Once
posi-
tioned
appropriately,
the
femur
was
not
altered
or
adjusted.
The
distal frame
and
femur
were
elevated
in
increments
from
to
45°
and
a
CrCd
digital
radiograph
(TruDRTM,
Sound
Technologies
Medical
Systems
Inc.,
Carlsbad,
CA)
was
obtained
at
each
increment.
At
each
increment,
foam
wedges
were
used
to
maintain
position
and
a
goniometer
was
used
to
verify
the
angle
of
elevation
(Fig
3).
The
radio-
graphic
beam
was
centered
over
a
point
marked
on
the
distal
third
of
the
femoral
diaphysis
to
maintain
consistency.
The
process
was
repeated
for
all
femora
individually.
aLDFA
Measurement
The
aLDFA
was
calculated
from
each
radiographic
pro-
jection,
using
a
digital
measurement
program
(VetPacs
TM
2006
TruPacs©,
Sound
Technologies)
for
all
lines,
angles,
and
measurement.
All
measurements
were
made
a
single
time,
by
a
single
observer
(GMJ).
The
femoral
anatomic
axis
(FAA)
was
created
on
all
femora.
This
axis
was
created
by
first
measuring
the
length
of
the
femur
from
the
junction
of
the
femoral
neck
and
the
intertrochanteric
fossa
proximally
to
the
center
of
the
intercondylar
fossa
distally.
Points
were
marked
along
this
measurement
at
1
/
4
and
1
/
3
(proximally)
and
1
/
2
of
the
length
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
995
Effect
of
Femoral
Elevation
on
aLDFA
Jackson
and
Wendelburg
A
C
Figure
2
(A)
Position
of
a
cadaveric
femur
in
the
custom
frame;
(B)
the
distal
and
caudal
aspects
of
the
femoral
condyles
were
placed
flush
against
the
frame;
and
(C)
the
proximal
femur
was
elevated
using
a
radiolucent
screw
until
the
lesser
trochanter
and
the
lateral
supracondylar
tuberosity
were
equal
heights
above
the
frame
(red
arrows).
Figure
3
Positioning
of
the
femur
and
custom
frame
with
20°
distal
femoral
elevation.
Foam
wedges
were
used
to
maintain
elevation
and
a
goniometer
was
used
to
verify
degree
of
elevation.
Inset:
Close
up
of
goniometric
verification
of
20°
elevation.
of
the
femur.
All
3
marks
were
located
between
the
lesser
trochanter
and
the
mid-femoral
diaphysis.
Femoral
width,
from
the
outer
lateral
cortex
to
the
outer
medial
cortex,
was
measured
at
each
of
these
points
using
a
circular
measur-
ing
tool,
and
the
center
of
the
femoral
width
was
marked.
The
FAA
was
the
best-fit
line
connecting
the
3
mid-width
points
and
was
extended
proximally
and
distally
beyond
the
femur.
The
distal
femoral
joint
line
or
transcondylar
axis
(TCA)
was
determined
on
each
projection
by
creat-
ing
a
line
tangential
to
the
distal
aspect
of
both
femoral
condyles.
The
aLDFA
was
measured
as
the
proximolateral
angle
of
intersection
between
the
FAA
and
the
TCA
(Fig
1).
The
aLDFA
was
recorded
for
each
femur,
at
each
elevation
point.
The
mean
aLDFA
value
±
standard
deviation
(SD)
at
each
elevation
was
calculated
for
all
femora.
Radiographic
Landmark
Evaluation
Radiographic
femoral
landmarks,
including
the
lesser
trochanter,
nutrient
foramen,
fabellae,
the
walls
of
the
inter-
condylar
fossa,
and
the
proximal
and
distal
portions
of
the
trochlear
ridges
were
evaluated
for
consistent
positioning
at
each
level
of
femoral
elevation.
Specifically
each
landmark
was
evaluated
for
changes
in
proximodistal
and
sagittal
po-
sition
during
increasing
elevation.
Statistical
Analysis
Descriptive
statistics
were
generated
to
summarize
outcome
measures.
Specifically,
outcome
measures
were
summarized
in
means
±
SD
of
measured
aLDFA
for
each
elevation
996
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
5
35
9.11
aLDFA
Idtarees)
.9?
91.
-*-M
ran
aLDFA
S551
11e1
113
Is
....a
15
la
30.
.11
Jackson
and
Wendelburg
Effect
of
Femoral
Elevation
on
aLDFA
Table
1
Summary
Statistics
for
Overall
aLDFA
(Average
Between
Left
and
Right
Femora)
for
Each
Elevation.
There
are
Significant
Increases
From
Elevation
to
10°,
15°,
20°, 25°,
30°, 35°,
40°,
and
45°
Elevation
Elevation
Mean
aLDFA
SD
P
value*
92.32
2.46
92.88
2.93
0.0525
10°
93.11
2.90
0.0220t
15°
93.34
2.48
0.0202
1
20°
93.56
2.38
0.0065k
25°
93.91
2.49
0.0022t
30°
93.99
2.61
0.0020
1
35°
94.22
2.80
0.0013t
40°
94.47
3.13
0.0021
1
45°
95.04
3.35
0.0026
1
*P
value
for
comparison
to
elevation
0°.
<
.05.
increment.
A
paired
t-test
was
used
to
compare
joint
angle
degrees
between
elevation
degrees
(0°
versus
5°,
10°,
. . .
,
45
0
).
Linear
regression
analysis
was
performed
to
evaluate
the
association
between
increasing
degree
of
elevation
and
measured
aLDFA
across
all
femora.
All
P
values
were
two-sided,
with
P
<
.05
indicating
sig-
nificant
statistical
differences.
Data
analysis
was
performed
using
software
(SAS®
version
9.2
software,
SAS
Corp.,
Cary,
NC).
RESULTS
When
comparing
all
femora
together
(mean
aLDFA),
there
was
a
significant
increase
for
aLDFA
at
all
elevations
greater
than
(Table
1).
Linear
regression
analysis
revealed
a
posi-
tive
association
between
increasing
elevation
and
increasing
aLDFA
measurement
(Fig
4).
Calculated
R
2
was
0.07,
the
slope
parameter
was
0.06,
and
P
=
.03.
The
mean
value
for
aLDFA
increased
from
92.3°
at
elevation
to
95.0°
at
45°
elevation.
Radiographic
landmarks
to
aid
in
the
assessment
of
distal
femoral
elevation
and
femoral
rotation
were
identi-
fied.
The
femoral
trochlear
ridges
and
walls
of
the
inter-
condylar
fossa
appeared
most
useful.
The
fabellae,
though
inconsistently
retained
in
the
current
study,
may
be
benefi-
cial,
when
present,
in
assessing
femoral
rotation.
The
lesser
trochanter
and
nutrient
foramen
were
less
useful
because
of
the
variability
in
radiographic
position.
DISCUSSION
We
confirmed
our
hypothesis
that
distal
elevation
of
the
femur
significantly
altered
radiographic
measurement
of
aLDFA.
A
significant
difference
was
noted
between
mean
aLDFA
value
of
femora
at
elevation
and
all
elevations
greater
than
5°.
Mean
aLDFA
increased
from
92.3
±
2.5°
at
elevation
to
95
±
3.4°
at
45°
elevation.
Elevallon
{degree5.)
Figure
4
Association
between
femoral
elevation
and
measured
aLDFA.
Linear
regression
analysis
was
performed
to
evaluate
whether
there
was
a
linear
trend.
The
R
2
was
0.07
with
a
significant
slope
parameter
of
0.06
(P
=
.03),
ie,
there
was
a
evidence
for
a
positive
association
between
increasing
elevation
and
increasing
measured
aLDFA.
The
clinical
impact
of
these
results
is
unknown.
Distal
femoral
elevation
of
up
to
45°
only
resulted
in
increases
in
mean
aLDFA
measurements
of
3°.
Increased
measure-
ment
of
of
aLDFA
is
not
suspected
to
lead
to
unnec-
essary
surgical
therapy
in
a
dog
and
in
the
current
study
none
of
the
dogs
would
have
been
candidates
for
corrective
surgery.
However,
because
this
study
only
evaluated
normal
femora,
it
is
not
known
what
effect
excessive
femoral
varus,
in
addition
to
malposition
would
have
on
measurement
of
aLDFA.
Clinically,
it
is
important
to
try
and
obtain
as
accu-
rate
a
measurement
of
distal
femoral
angulation
as
possible.
With
the
association
between
increased
femoral
elevation
and
increased
measured
aLDFA
reported
here,
there
is
po-
tential
to
recommend
surgical
therapy
based
on
a
falsely
elevated
aLDFA
measurement.
Additionally,
repeatability
is
important,
as
inconsistent
position
during
postoperative
or
follow-up
radiographs
could
affect
the
precision
of
mea-
surements.
We
recommend
close
evaluation
of
radiographic
position,
in addition
to
clinical
evaluation,
in
dogs
with
in-
creased
measurements
of
aLDFA,
to
avoid
unnecessary
sur-
gical
procedures.
Additionally,
follow-up
evaluation
should
be
performed
with
consistent
radiographic
positioning
and
technique
to
reduce
variation.
In
this
study,
a
single
observer
(GMJ)
made
all
mea-
surements
and
measurements
were
only
obtained
once.
Based
on
consistent
positioning
of
defined
landmarks,
we
felt
a
single
measurement
was
adequate.
Similar
measure-
ment
methods
have
been
reported.
'
11
Repeatability
and
reproducibility
of
measurements
of
distal
femoral
varus
have
also
been
assessed,
with
acceptable
results.'
As
such,
we
feel
it
is
unlikely
that
repeated
measurements
or
multi-
ple
measurers
would
have
significantly
affected
the
results
of
the
current
study.
We
measured
aLDFA
in
this
report
to
maintain
con-
sistency
with
previous
reports,
in
both
human
and
veteri-
nary
literature,
for
the
angulation
of
the
distal
femur.
"c"
,14
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
997
A
DFAA
TCA
9C
°
90
°
aLDFA
Effect
of
Femoral
Elevation
on
aLDFA
Jackson
and
Wendelburg
This
system
was
developed
to
allow
ease
in
naming
specific
joint
angles,
based
on
their
location
and
has
become
the
recognized
nomenclature
within
the
human
field
of
limb
deformity
correction.
13
Normal
aLDFA
has
been
reported
for
4
dog
breeds."
In
these
breeds,
the
mean
aLDFA
was
greater
than
90°
which
may
indicate
that
the
typical
conformation
of
the
canine
distal
femur
has
a
slight
degree
of
varus.
This
is
in
contrast
to
the
human
femur
that
typically
has
distal
valgus."'
16
The
mean
aLDFA
values
in
the
current
study
were
below
the
previously
reported
canine
means,
but
the
values
reported
here
fell
within
the
95%
confidence
inter-
val
for
individual
values
in
the
previous
report.
In
that
report,
projections
from
a
radiographic
database
were
eval-
uated.
Acceptable
positioning
required
full
femoral
exten-
sion,
though
distal
femoral
elevation
was
not
objectively
assessed.
This
may
explain
the
difference
in
measurement
of
the
aLDFA
between
the
studies.
If
distal
femoral
eleva-
tion
was
present
in
the
radiographic
positioning,
increased
measurements
of
aLDFA
could
have
been
induced.
Addi-
tionally,
differences
in
calculated
aLDFA
may
be
because
of
smaller
numbers
in
the
present
study,
breed
differences,
possible
outliers from
the
normal
population,
or
differences
in
position
of
the
radiographic
beam.
Early
veterinary
reports
measured
the
femoral
varus
angle
(FVA)
rather
than
the
aLDFA
to
assess
distal
femoral
varus.'
8
'
11
'
12
The
FVA
is
the
angle
between
the
FAA
and
the
distal
FAA,
a
line
perpendicular
to
the
TCA
through
the
center
of
the
intercondylar
fossa
(Fig
5).
Using
stan-
dard
geometry,
the
aLDFA
and
FVA
measurements
can
be
related.
The
aLDFA
equals
90°
+
FVA
(Fig
5).
All
long
bones
have
an
anatomic
and
mechanical
axis.
The
anatomic
axis
is
defined
as
a
mid-diaphyseal
line
through
the
long
axis
of
the
bone.
13
The
mechanical
axis
is
defined
as
a
straight
line
connecting
the
centers
points
of
the
proximal
and
distal
joints
of
a
bone.
13
Additionally,
in
all
joints,
orientation
lines
can
be
determined
based
on
anatomic
points
along
the
articular
surface.
The
intersec-
tion
of
the
joint
orientation
line
and
either
the anatomic
or
mechanical
axis
is
used
to
determine
the
angles
of
a
particular
joint.
We
chose
FAA
as
the
reference
axis,
rather
than
the
femoral
mechanical
axis.
Clinically,
the
FAA
is
typically
used
for
evaluation
of
candidacy
for
surgical
correction.'
The
canine
femur
is
relatively
straight
in
the
frontal
plane;
therefore,
the
anatomic
axis
is
a
straight
line
in
this
plane.
In
the
sagittal
plane,
the
canine
femur
has
some
degree
of
procurvatum.
Therefore,
in
the
sagittal
plane,
the
FAA
fol-
lows
a
curved
mid-diaphyseal
line
rather
than
a
straight
line.
13
The
amount
of
femoral
procurvatum
varies
between
individuals.
Because
of
this
we
chose
to
standardize
sagittal
positioning
in
this
study
using
anatomic
landmarks,
bring-
ing
the
proximal
and
distal
portions
of
the
femoral
diaph-
ysis
to
same
level
of
elevation
off
of
the
radiographic
plate.
Similar
positioning
has
been
reported
in
the
evaluation
of
distal
femoral
varus."
The
FAA
was
determined
using
3
points
in
the
prox-
imal
half
of
the
diaphysis
similar
to
previous
reports.
11,12
Figure
5
Relationship
between
anatomic
lateral
distal
femoral
angle
(aLDFA,
blue)
and
femoral
varus
angle
(FVA,
red)
measured
on
a
femoral
radiograph.
Using
standard
geometry
aLDFA
equals
90°
+
FVA.
The
femoral
anatomic
axis
(FAA),
distal
femoral
anatomic
axis
(DFAA),
and
transcondylar
axis
(TCA)
are
identified,
all
in
green.
A
parallel
line
to
DFAA
is
identified
by
the
dashed
line.
Varying
numbers
and
locations
of
axis
points
have
been
used
to
determine
the
FAA
10-13
;
however,
the
overall
method
for
determination
of
the
FAA
was
similar.
Loca-
tion
of
these
points,
rather
than
the
number
of
points
used,
is
likely
the
most
important
aspect
and
it
is
recommended
that
all
points
are
between
the
lesser
trochanter
and
the
mid-diaphysis
of
the
femur,
as
most
angular
deformities
occur
in
the
distal
1
/
3
of
the
diaphysis.
Location
of
the
points
may
need
to
be
adjusted
if
there
is
obvious
anatomic
abnormality
of
the
femur.
Centering
the
radiographic
beam
at
or
near
the
level
of
the
joint
of
focus
has
been
recommended
in
people."
We
centered
the
radiographic
beam
over
the
distal
third
of
the
femur.
In
our
experience,
radiographic
evaluation
of
the
fe-
mur
in
dogs
is
typically
performed
with
the
beam
centered
at
the
mid-diaphysis
of
the
femur.
Radiographic
evaluation
with
the
beam
centered
over
the
joint
of
interest
may
be
limited
by
the
size
of
available
radiographic
cassettes
in
vet-
erinary
facilities.
Smaller
cassettes
may
limit
the
ability
to
998
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
Jackson
and
Wendelburg
Effect
of
Femoral
Elevation
on
aLDFA
center
the
beam
over
the
stifle
and
still
capture
a
projection
of
the
entire
femur.
We
chose
to
center
the
beam
over
distal
third
of
the
femoral
diaphysis
as
we
felt
this
was
a
clinically
applicable
situation,
allowing
both
centering
of
the
beam
closer
to
the
joint
of
interest,
and
ensuring
the
entire
femur
was
projected
onto
the
cassette.
Position
of
the
radiographic
beam
in
this
area
may
reduce
radiographic
parallax
artifact
of
the
distal
femur.
We
are
not
aware
of
any
current
veteri-
nary
reports
that
have
assessed
the
effect
of
varied
position
of
the
radiographic
beam
on
the
calculation
of
joint
angles;
however,
this
should
be
investigated
as
variation
that
may
lead
to
inaccurate
or
inconsistent
measurements.
Addition-
ally,
position
of
the
radiographic
beam
at
various
locations
may
project
different
radiographic
landmarks
that
may
be
useful
as
indicators
of
position.
Rotation
of
the
femur,
either
because
of
positioning
or
anatomic
torsion,
is
suspected
to
alter
the
measurement
of
femoral
joint
angles.
The
effect
of
rotation
on
calculation
of
the
aLDFA
was
not
assessed
in
this
study,
but
rotation
is
suspected
to
additionally
contribute
to
inaccurate
cal-
culations.
External
rotation
has
been
reported
to
increase
measured
femoral
varus,
11
but
the
extent
to
which
this
oc-
curs
has
not
been
objectively
assessed.
Clinically,
we
find
that
rotation
can
significantly
alter
the
radiographic
assess-
ment
of
distal
femoral
angulations,
though
this
is
only
a
subjective
observation
at
this
time.
The
combined
effects
of
elevation
and
rotation
may
also
be
more
significant
than
malpositioning
in
a
single
plane.
In
our
study,
rotation
was
eliminated
by
placement
of
the
distal
and
caudal
aspects
of
the
femoral
condyles
flush
on
planar
surfaces
in
the
custom
frame,
rather
than
by
the
use
of
femoral
landmarks.
Similar
technique
has
been
reported,
12
though
it
is
limited
to
cadaveric
rather
than
clinical
specimens.
As
such
alternate
methods
of
evaluation
of
positioning
in
a
clinical
setting,
including
radiographic
landmarks,
would
be
beneficial.
Such
landmarks
have
been
reported,5,6,
9-12
but
their
reliability
has
not
been
previously
assessed.
The
patella
has
been
reported
as
a
positional
landmark,
10-12
but
was
not
evaluated
in
the
current
report.
The
patella
has
the
potential
for
variability,
as
it
is
a
freely
moving,
extra-femoral
structure
and
may
luxate
in
patients
with
increased
femoral
varus.
Based
on
this,
we
did
not
evaluate
the
patella
as
a
positional
landmark.
The
use
of
the
fabellae
as
reference
points
has
been
reported,
with
correct
position
showing
transection
of
the
fabellae
by
the
femoral
cortices.
5,6,16-12
In
our
study,
15/28
fabellae
were
evaluated.
The
remaining
fabellae
shifted
po-
sition
or
had
variable
amounts
of
movement
after
initial
soft
tissue
dissection.
Ten
of
the
15
retained
fabellae
were
lateral
fabellae.
The
reason
for
this
is
unknown,
but
could
indicate
that
the
lateral
fabella
has
reduced
positional
vari-
ability,
making
it
a
better
landmark.
Throughout
the
range
of
elevation
in
the
current
study,
the
retained
fabellae
were
consistently
transected
by
the
femoral
cortices
(Fig
6A—D).
This
supports
the
previously
reported
use
of
the
fabellae
as
landmarks
for
assessing
rotation,
but
limits
their
ability
to
be
used
as
landmarks
to
assess
distal
elevation.
However,
the
fabellae
are
still
extra-femoral
landmarks.
Variable
position
and
absence
have
also
been
reported"
and
noted
clinically
by
the
authors,
and
could
limit
their
use
as
landmarks.
Use
of
landmarks
confined
to
the
femur
would
be
ideal
to
aid
in
consistency
and
reproducibility
of
radiographic
po-
sitioning.
The
corticocancellous
tip
of
the
lesser
trochanter
has
been
reported
to
be
only
partially
visible
medially
when
the
femur
is
not
rotated.
5,6,16-12
In
this
report,
radiographic
projection
of
the
lesser
trochanter
was
variable.
Minimal
or
no
portion
of
the
lesser
trochanter
was
visible
radiographi-
cally
in
4
pairs
of
femora,
despite
positioning
in
the
custom
frame
without
rotation.
This
normal
anatomic
variation
may
limit
its
usefulness
in
assessing
rotation
of
the
femur.
Additionally,
because
the
lesser
trochanter
is
a
proximal
structure,
distal
femoral
torsion
may
affect
its
radiographic
observation.
External
torsion
of
the
distal
femur
can
con-
tribute
to
MPL
6
and
would
cause
the
lesser
trochanter
to
be
less
visible
during
appropriate
positioning
without
rota-
tion.
In
our
study,
when
the
lesser
trochanter
was
visible,
the
amount
protruding
was
consistent
throughout
the
range
of
elevation.
Based
on
our
results,
the
lesser
trochanter
was
determined
to
be
a
limited
radiographic
landmark
in
deter-
mining
rotational
position
or
distal
femoral
elevation.
The
nutrient
foramen
should
be
centered
between
the
medial
and
lateral
cortices.
11
In
our
study,
only
6/14
were
centered.
The
remainders
were
either
at
the
lateral
(7/14)
or
medial
(1/14)
extent
of
the
middle
1
/
3
of
the
femoral
width.
In
individual
femora,
positioning
was
again
consistent
at
all
elevations.
We
felt
the
nutrient
foramen
was
less
useful
as
a
rotational
landmark,
and
was
a
poor
landmark
for
evaluation
of
distal
femoral
elevation.
The
walls
of
the intercondylar
fossa
should
be
paral-
lel
to
each
other
and
centered
in
an
unrotated
femur.
5
'
6'
10
In
our
study,
this
positioning
was
noted
and
consis-
tent
throughout
the
range
of
distal
femoral
elevation
(Fig
6A—D).
This
supports
them
as
a
landmark
for
assess-
ment
of
rotation
of
the
femur,
rather
than
distal
femoral
elevation.
In
a
previous
study,
the
trochlear
ridges
were
noted
to
be
centered
between
the
femoral
condyles
on
appropriately
positioned
radiographs.
9
In
our
study,
this
position
was
confirmed,
supporting
use
of
the
trochlea
as
a
landmark
for
assessment
of
rotation.
The
trochlear
ridges
were
also
identified
as
potential
landmarks
assessing
distal
femoral
elevation.
On
the
true
CrCd
projection
(0°
distal
femoral
elevation),
the
trochlear
ridges
were
observed
extending
be-
yond
the
distal
aspect
of
the
femoral
condyles
(Fig
6A
and
B).
This
protrusion
was
typically
noted
until
approximately
15°
of
distal
femoral
elevation.
To
our
knowledge,
evalua-
tion
of
the
femoral
trochlear
ridges
in
this
way
has
not
been
previously
reported.
In
our
clinical
experience,
this
land-
mark
is
not
identified
on
typical
hip
extended
CrCd
femoral
projections,
potentially
because
of
variable
amounts
of
dis-
tal
femoral
elevation
with
this
projection.
In
this
in
vitro
study,
radiographic
observation
of
the
distal
aspect
of
the
trochlear
ridges
beyond
the
femoral
condyles
may
have
been
because
of
either
level
positioning
of
the
femora
or
due
to
centering
the
radiographic
beam
closer
to
the
joint
than
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
999
B
Effect
of
Femoral
Elevation
on
aLDFA
Jackson
and
Wendelburg
A
Figure
6
(A)
Distal
portion
of
the
radiograph
of
the
right
femur
from
specimen
2
at
elevation
illustrating
the
trochlear
ridges,
walls
of
the
intercondylar
fossa,
and
fabellae.
(B)
Identical
image
to
(A),
with
the
trochlear
ridges
(yellow),
walls
of
the
intercondylar
fossa
(blue),
and
fabellae
(green)
identified
in
comparison
to
the
femoral
condyles
(red).
Note
that
observation
of
the
trochlear
ridges
beyond
the
femoral
condyles
distally.
The
proximal
extent
of
the
trochlear
ridges
overlaps
the
condyles.
The
fabellae
are
transected
by
the
femoral
cortices
and
the
walls
of
the
intercondylar
fossa
are
parallel
and
centered
within
the
femoral
condyles.
(C)
Distal
portion
of
the
radiograph
of
the
right
femur
from
specimen
2
at
25°
elevation.
(D)
Identical
image
to
(C)
with
structures
marked
as
in
(B).
Note
that
approximately
30%
overlap
of
the
trochlear
ridges
and
femoral
condyles.
The
trochlear
ridges
are
not
visualized
beyond
the
condyles
distally.
The
position
of
the
walls
of
the
intercondylar
fossa
and
the
fabellae
are
essentially
unchanged.
is
typically
performed
in
a
clinical
situation.
Centering
the
beam
closer
to
the
stifle
projects
a
more
tangential
view
of
the
distal
femur
than
when
the
beam
is
centered
at
the
mid-diaphysis.
It
should
be
noted
that
care
must
be
taken
in
assuring
consistent
use
of
the
distal
most
portion
of
the
condyles
as
the
anatomic
location
for
the
TCA,
rather
than
the
trochlear
ridges.
In
this
in
vitro
study,
lack
of
observa-
tion
of
the
trochlear
ridges
distal
to
the
femoral
condyles
indicated
femoral
elevation
of
more
than
15°.
The
potential
for
clinical
use
of
this
radiographic
landmark
has
not
been
evaluated.
Additionally,
the
proximal
aspects
of
the
trochlear
ridges
were
noted
within
the
confines
of
the
femoral
condyles
in
femora
without
distal
elevation.
With
increas-
ing
elevation,
the
proximal
aspects
of
the
trochlear
ridges
were
observed
proximal
to
the
femoral
condyles
(Fig
6C
and
D).
Approximately
30%
overlap
was
noted
at
approx-
imately
25°
of
distal
femoral
elevation.
This
may
also
be
useful
in
determining
positioning
to
radiographically
as-
sess
the
weight-bearing
portion
of
the
stifle,
which
is
lo-
cated
tangent
to
the
radiographic
beam
at
approximately
25°
of
distal
femoral
elevation.
Observation
of
the
proxi-
mal
portion
of
the
trochlear
ridges
was
difficult
in
some
of
the
cadaveric
specimens
in
this
report,
and
the
presence
of
additional
overlying
soft
tissues
may
make
evaluation
of
this
structure
difficult
in
some
clinical
cases.
When
vis-
ible,
use
of
the
percentage
of
the
proximal
aspect
of
the
trochlear
ridges
overriding
the
femoral
condyles
may
allow
assessment
of
distal
femoral
elevation.
Clinically,
radiographic
positioning
for
a
true
CrCd
femoral
projection
may
be
affected
by
multiple
factors
in-
cluding
patient
tolerance,
temperament,
pain,
conforma-
tion,
or
concurrent
orthopedic
disease.
Heavy
sedation
or
anesthesia
is
recommended
and
may
be
necessary
to
ob-
tain
appropriate
positioning.
However,
conformational
dif-
ferences,
including
thigh
muscle
mass,
may
prohibit
full
1000
Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
Jackson
and
Wendelburg
Effect
of
Femoral
Elevation
on
aLDFA
extension
of
the
femur,
despite
sedation
or
anesthesia.
In
a
clinical
report
of
dogs
with
elevated
distal
femoral
varus
in
association
with
MPL,
80%
had
a
grade
III
or
IV
MPL.
9
Pathology
associated
with
these
higher
grade
MPLs
include
periarticular
fibrosis
and
muscle
contracture
1,19
and
may
limit
stifle
extension
in
these
patients.
Inability
to
obtain
appropriate
radiographs
for
perioperative
mea-
surements
in
a
dog
with
grade
IV
MPL
has
also
been
reported."
Additionally,
concurrent
pathology
in
the
hip
or
stifle
can
limit
appropriate
positioning,
either
because
of
pain
or
decreased
range
of
motion
secondary
to
os-
teoarthritis.
Concurrent
hip
dysplasia
was
noted
in
up
to
41%
and
concurrent
cranial
cruciate
ligament
rupture
in
46%
of
dogs
with
increased
distal
femoral
varus,
7,8
indi-
cating
a
high
prevalence
of
concurrent
joint
pathology
in
clinical
populations
in
which
assessment
of
femoral
varus
is
needed.
As
our
results
demonstrate,
distal
femoral
elevation
leads
to
a
statistically
significant
elevation
in
measured
radiographic
aLDFA.
The
literature
has
indicated
a
high
prevalence
of
factors
that
may
limit
appropriate
and
re-
peatable
radiographic
positioning
of
patients
with
exces-
sive
femoral
varus.
Therefore,
description
and
evaluation
of
alternative
methods
to
obtain
appropriately
positioned
radiographs,
or
the
use
of
advanced
imaging
modalities,
such
as
computed
tomography,
are
crucial
to
standardized
assessment
of
distal
femoral
varus.
Based
on
our
results
and
clinical
experience,
we
recom-
mend
thorough
evaluation
of
radiographic
positioning
of
the
femur
for
measurement
of
distal
femoral
varus.
Verifi-
cation
of
parallel
position
of
the
femoral
long
axis
and
the
radiographic
cassette
is
necessary
to
obtain
a
true
CrCd
projection.
Additionally,
the
available
radiographic
land-
marks
should
be
assessed
to
confirm
appropriate
and
re-
peatable
radiographic
position.
Future
focus
should
include
evaluation
of
the
normal
aLDFA
across
a
wider
variety
of
breeds,
larger
scale
evaluation
of
dogs
with
increased
femoral
varus,
further
assessment
of
the
accuracy
and
preci-
sion
of
various
radiographic
techniques
or
imaging
modal-
ities
for
measurement
of
aLDFA,
and
the
evaluation
of
the
effect
of
femoral
rotation
on
aLDFA.
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Veterinary
Surgery
41
(2012)
994-1001
©
Copyright
2012
by
The
American
College
of
Veterinary
Surgeons
1001