The standard clinical gait analysis (Vicon Clinical Manager) protocol
uses the so-called Davis marker set, which consists of Sacrum, right
and left: ASIS, a lateral mid-thigh marker on a wand, a knee marker
on
the "flexion axis" of the knee, a lateral mid-shank marker on a wand,
lateral malleolus and foot (dorsum). The knee flexion axis is determined
during a static trial by applying what is called a knee alignment
device, which is a 3-marker pod which one aligns by with where the
axis
of the knee is thought to be (by asking the patient to alternately
flex
and extend their knee). This static trial also calculates the angles
between the thigh and shank wands for use in anthropometric calculations
to derive the knee joint centre. Once the knee axis has been defined,
the alignment device is removed, the knee marker is attached in its
place, and the gait is then ready to be analysed.
I wonder what people think of this procedure? It worries me, and I
suspect I'm not the only one, that such an important part of the
analysis process is highly dependant on a subjective judgement (defining
the knee axis of flexion). As you know, the knee axis is known to change
quite markedly depending on the knee angle, travelling a few centimetres
up the femur in an evolute as the knee flexes.
So I wonder if you could answer the following questions for me...
1.1. If you use a Vicon yourself, do you get good results
from the knee alignment device?
2.2. If you use another marker set, can you please describe
it, and tell us whether you think it is superior to the Davis set?
3.3. Is anybody routinely using a method for calculation
of the instantaneous axis of the knee, rather than simply predicting
it from anthropometry and a static trial?
Please don't think I'm criticising the Davis set at this stage - I'd
just like to know what people think of it, since it seems to be such
a
crucial part of the analysis. I'd also be willing to conduct some
experiments to assess the realtive merits of any other marker sets
people are using. Seems to me we should know the strengths and
weaknesses of each before we can interpret results.
Look forward to your replies.
Chris
--
Dr. Chris Kirtley MD PhD
Dept. of Rehabilitation Sciences
The Hong Kong Polytechnic University
The Davis marker set I know of is different from the one you described.
(See Davis et al. (1991) A gait analysis data collection and reduction
technique, Human Movement Science, 10/5, pp 575-587).
The ELITE system (BTS, Milan, Italy) uses the marker set in the above
paper. I have serious reservations with the method, but I've learned
to
live with it.
The knee varus/valgus and rotation angles are said to be inaccurate
by
the authors, but I also have problems with the hip rotation angles
too.
I am looking forward to the discussion on this topic.
Gabor
--
Dr Gabor Barton MD
CGA@gaitlab.demon.co.uk
Manager, Gait Analysis Laboratory UIN (ICQ): 2625928
Alder Hey Children's Hospital
tel: +44 (0)151 252 5949
Eaton Road, Liverpool, L12 2AP, UK fax: +44 (0)151 252
5846
Chris:
You have asked some very
interesting questions
about the marker set used by Vicon. It seems that everyone
who gets one of their systems goes through some wrestling
with the model and how to cope. Thanks for bring this to
the list for discussion.
We have a Vicon 370 system
here in Portland. I
have used it for about 4 years and love it. It does have
its limitations, but to me they are well balanced with it
accuracy and speed.
Vicon user group meetings
always include a session
on marker placement and discussion of the limitations of
the model. I was just leading a marker placement group at
the Gait and Clinical Movement Analysis Society meeting in
San Diego, California last month so these topics are fresh
in my mind.
Here in Portland we capture
a static trial without
the knee alignment devise (KAD) prior to collecting walking
trials and another with the KAD following walking.
We place the makers by aligning
the thigh wand to
the knee axis. We ask the patient stand and to lift their
foot off the floor by flexing their knee to 90 degrees while
keeping the thigh vertical. The patient holds a railing for
added stability. We align the thigh wand perpendicular to
the shank with the knee flexed to this 90 degree position.
We then place the knee marker
on what appears to be
the center of rotation. We ask the patient to squat and
rise, flexing the knee. It is important to have the
patient face straight ahead during marker placement.
Looking down at the action makes wand placement impossible
due to rotations. We have an assistant who's job it is to
engage the patient and keep then facing forward.
We use a eye liner pencil
(rubs off easily) to mark
the proposed center of rotation, and watch the knee flex
and extend. This first mark is placed by feeling the joint
line and determining the center of the condyle in
extension. The manual says about 1 cm above the joint line
but it is of course different for smaller of larger
patients. If the dot appears to move excessively, we try
another mark above or below the first to see if one rotates
about the other. We shoot for greatest accuracy in
extension. After about 300 or so patients all the variants
will be seen and even the impossible knees will just get
hard. :-) I have moved a marker 1 cm in each direction and
found very minimal changes in the hip and knee motions in
all planes. Knee extension changed less than 2 degrees
for example.
After walking tails are
captured, we take an
additional static trial with the knee alignment device.
This allows us to determine if all wands and markers remain
correctly aligned and allows calculation of an offset which
can be entered into the Vicon Clinical Manager (VCM)
calculations. There are some risks associated with this
approach (Micah Forstein of Los Angeles Childrens Hospital
has suggested that it is possible to enter an "impossible"
offset, one which alters the thigh rotation without
changing the knee axis to a more posterior position for
example.) but our offsets are usually less than 10 degrees,
making the risks acceptable.
Or you can choose to use
the KAD trial as the
static and process all trials by this method in VCM. We
don't like this because it forces the tibial wand
parallel to the the knee axis. We don't feel that the
transmalloelar axis is parallel to the knee axis, but
rather feel that it is more external. Or you must enter
a measure offset to correct this, which almakes me wonder
if I can measure the tibial rotation why can't I place a
wand there just as accurately? And which one do I accept
if they are different? Which one is wrong?
We are one of the few labs
to include a transverse
plane knee rotation graph (tibial rotation) in our reports.
Many others consider these too error prone to include but
we have compared our to CT scans of transmalloelar axes and
found high R squared values. We feel that the tibial
rotation on the gait report is at least as accurate as
clinical evaluation or CT scans. (see Aktas et al. (1998)
Gait and Posture 7:2 (March)p. 182)
Including this graph allows
us to visually confirm
the interaction of the proposed hip rotation, knee
flexion/extension axis, knee varus/valgus axis and tibial
rotation. If the coronal knee motion begins to have the
appearance of the knee motion in the sagittal plane, then
wand placement error is suspected. The closer to neutral
(zero) it is the better we feel about the accuracy of the
hip rotation and knee axis. Provided we feel that there is
no "true" coronal motion of the knee.
If we enter an offset into
VCM to alter the thigh
wand placement and therefore the calculation of the knee
axis, we can reprocess the trial and watch the changes to
the transverse plane thigh rotation, the sagittal knee
motion, the coronal knee motion and the transverse plane
tibial rotation. It is the interaction of these which
makes one feel confident about the offset or uncomfortable
with the offset.
Now that I have gone on
forever, what was your
question? Oh yeah. Yes the knee alignment device assists
in placing the knee joint center in many patients.
I need to point out that
VCM uses the thigh wand to
perform a cord function which moves the knee marker from
the lateral position on the skin into the virtual space
within the knee. After the hip joint center is established
by the leg length calculation (now if you want to talk
accuracy and error, just open this can of worms) VCM uses
the hip joint center, the knee marker and the thigh wand to
establish the plane which defines the thigh. The knee
marker is moved medially along this plane (defined by the
thigh wand) into the center of the knee.
You are correct to point
out the skin movement at
the knee and its effect on knee motion, mainly in the
sagittal plane. I have routinely calculated the distance
between the hip joint center and knee joint center and the
distance between the knee joint center and the ankle joint
center on a series of about 70 patients. This distance
between the knee and ankle looks suspiciously like the knee
flexion/extension axis and varies about 3 cm (shorter in
flexion).
I asked Roy Davis about
instantaneous knee joint
centers from non-connected thigh and shank segments and he
said they had tried it but that it was so error prone that
it was useless. Perhaps someone else has done more work on
this issue recently. Did you get BodyBuilder with your
package? You could always develop your own maker set in
your spare time. :-)
I had better stop and get
something done today.
Thanks for keeping the list going and confirming the web
site existence. We true believers never doubted.
Michael
Michael Orendurff, MS
Clinical Biomechanist
Gait Analysis Laboratory
Portland Shriners Hospital
MSO@shcc.org
First and Foremost, I appreciate this type of discussion. My objective
is to assist the Clinical Gait Community in
further making systems better so that the children are the main beneficiary.
I have the most respect to all
researchers.
I have only used a Vicon system a few times in recent months but use
a Motion Analysis system daily. I head up the
Biomechanics Division for Motion Analysis Corporation.
Marker sets are among the most controversial topic in gait
assessment. As a former researcher with an advanced degree in
Biomechanics, I am amazed what restrictions clinicians will
accept based upon the limitations of their system.
To our knowledge, there are two markers sets used in Clinical Gait
Analysis.
Full Body marker set,
and
Lower body (Sacrum
R/L Asis and below) with and/or without heel markers
Now the Full Body marker set is then subdivided into two options:
Modified Helen Hayes Marker set
utilizing wands at the l/r thigh and lower leg and surface markers at other
locations
Cleveland Clinic Marker set utilizing
triads at the l/r Thigh and lower leg (Used exclusively by Motion Analysis
Corporation) and based upon cadavour studies
at Cleveland Clinic Foundation.
The Cleveland Clinic Marker set has always been the more difficult
marker set since it requires a static left and right leg
video record in order to calculate and determine joint centers for
the ankle, knee and hip. The Modified Hayes marker set,
relied upon Knee Alignment Deviced, Ankle devices, Clinicians eyeballing
the locations and or utilizing some device specifically
designed by the clinician. Again, Motion Analysis takes a static
video recording of the legs even with the Hayes marker set to
calculate and determine joint centers.
One problem that I personally see with the alignment devices is that
if the alignment device is not properly aligned, IE:
Aligned 3 degree upward or downward, Is the patient's gait assessment
then measured in this 3 degree clinician error. It
would seem practical and simple that the video system should be able
to calculate automatically a static measurement for Joint
Centers calculations and the use such calculations for the dynamic
assessment.
The Second problem that I see is with a "standard wand" located at
the l/r thigh and lower leg. I know with the Triad
(Cleveland Clinic Markers) the patient has three markers at the thigh
instead a single wand and the same with the lower leg.
With the three points specifically located on the segment, you would
get a more accurate assessment of thigh and lower leg
rotation especially in children with severe internal or external rotation
of the thigh and lower leg. I think Stanhope and Holden
studied a similar marker set as reported in Gait and Posture.
Of course the wand vs the Triad tracks faster but with today's
systems of auto identification, the clinician needs to emphasize accuracy
since time has been negated.
The third issue that I see is that there is "NO Standard" marker set
even with in the Vicon users. I know that I can name
several Vicon users that with the same system their markers sets are
different. Now, I am not attacking Vicon nor Promoting
Motion Analysis. The fact remains that years ago these systems
created by Oxford Metrics couldn't track a cluster of markers
and Motion Analysis systems could not track markers without editing.
But today's systems are different, and are on the verge
of Passive Marker Real-time display.
Now is the time where the Clinical Gait Community needs to come to
an agreement on:
1) A full body marker set,
( I question the validity of measuring only the lower body only to determine
surgical
outcomes).
2) Data reporting format.
Once determined, tell the manufacturers and let them further design
the hardware and software to accept your
agreed upon standards.
Motion Analysis is listening to your needs (and sure others are too)
and are making changes in its software to accommodate
research and collaboration in Clinical Gait by all users of systems.
Whether you produce a C3D or a P3D (which we produce
both), we hear that the researcher wants the binary data to collaborate
and share data and it all starts with the "acceptable"
marker sets (nomencalture included) and we (manufacturers) make changes.
Again, I respect this forum, my competitors, clinicians and not advertising
but providing input from a manufacturer's
perspective.
Motion Analysis Corporation
Daniel India, Vice President
3617 Westwind Blvd
Santa Rosa, CA 95403 USA
HQ Tel: 707-579-6500 Direct 847-945-1411
HQ Fax 707-526-0629 Direct 847-945-1442
dan.india@motionanalysis.com
Hi Chris
Glad to hear that you are happy with the Vicon.
As you point out Vicon Clinical Manager VCM utilises an exact
implementation of the Davis model as published in:
Kadaba, Ramakrishnan and Wootten of Helen Hayes Hospital
(J Orthop. Res., Vol. 8, No., 3, 1990)
Davis, Ounpuu, Tyburski and Gage of Newington Children's Hospital
(Human Movement Sciences #5, Volume 10, 1991)
If you don't have these papers to hand I will be happy to fax you
copies.
As for the 'Davis Marker set' the beauty of this if how it reduces
the 7 segments to a mere 13 markers during the dynamic trial.
As you
have rapidly noticed one of the limitations of this is the need for
very careful marker placement.
Roy Davis recently spoke at our Vicon Users meeting at NASCGMA and
presented some work on the errors created by misplaced markers.
It is
quite remarkable how robust this technique is! Also many studies
have
been done to compare marker placement repeatability between staff in
various labs and between labs. The findings again show how comparable
data from this model is. Also I have seen work comparing skin
markers
versus transcutaneous (patients with Ilizarhov frames ready to
be
removed) that show pleasingly consistent results.
So what is clear is that this is a good, easy to use and repeatable
tool and therefore universally applicable. However this raises
the
question: is this sufficient for the new millennium? To which
the
answer is: we don't really know! Until recently there have been
few
other options for comparison and certainly no models using enough
markers to give a rigorous (3 or more markers per segment)
measurement.
Now for the ADVERT. You have a 370 and BodyBuilder. (I am
aware of
some other labs who have been putting multiple markers on segments.
Up
to 43 in total for their full body model. They are comparing this to
VCM type data.) With your kit it will be easy to produce alternative
models.
Geoff Shaw <geoff.shaw@metrics.co.uk>
Chris,
We have a 5 camera, Vicon 370 system that we use for bilateral
lower
extremity gait analysis. We do use the Vicon Clinical Manager
(VCM) with the
marker set you described in your posting. We also use the KAD
and enter a value
for tibial torsion. Regarding your first question:
1. If you use a Vicon yourself, do you get good results from the knee
alignment device?
I am not very confident in the hip and foot rotation measures as well
as knee
varus and valgus, that we get using VCM and the KAD. Transverse
plane rotations
of the thigh and shank are always interpreted with caution. We
attempt to
define the knee flexion axis accurately by marking the axis during
a physical
exam. But, as you pointed out, it is still subjective.
You can increase your
confidence by watching the output knee varus and valgus curve.
If there is an
increase in knee varus and an internal hip rotation offset during swing,
when
there is an increase in knee flexion, you may enter an external thigh
rotation
offset in your session file and reprocess your data. This may
help correct
errors in placement of the KAD. I have additional documentation
that may not be
included in the manual regarding this adjustment process. Hope
this helps.
Greg Lange
Gregory W. Lange, M.S.
(210) 705-6597 voice
Engineer, Biomechanics Laboratory
(210) 705-6567 fax
Santa Rosa Outpatient Rehabilitation Center
greg_lange@srhcc.org
for Children and Adults
2701 Babcock Road, San Antonio, Texas,
78229, USA
http://www.santarosahealth.org/BioMechanics.html
Although we do not use a passive marker system, we do use a protocol
for
locating the knee and hip center similar to the one Chris has described.
We
presently employ a Selspot II optoelectric system where arrays of LEDs,
mounted
on rigid plastic disks, are strapped to the mid section of the feet,
shanks,
thighs, pelvis, trunk, arms and head. The knee and hip centers are
located by
first using a hand held pointer, upon which is mounted an array of
LEDs, to
specify the sagittal plane upon which the joint center is assumed to
lie. The
subject then performs a large range of motion task - chair rise. An
axis of
rotation method is then used to compute directional vectors of proximal
and
distal segment endpoints; the intersection of the directional vectors
with the
sagittal pointing plane is assumed to be the joint center of rotation
(see Riley
et al., J Biomech 23, 503-506, 1990).
We recently had an opportunity to examine the accuracy of this technique
by
comparing repeated measures of hip center location in a single subject
(several
times over three years), using the above protocol, to accurate photogrammetric
measures of the same subject's hip centers from his explanted pelvis
following
death (see McGibbon et al., Clin Biomech, 12, 491-495, 1997). The pointing
technique was, on average, within 1.2 cm of the "true" hip center.
Anatomical
scaling techniques for locating the hip center were also applied; errors
ranged
from 1.5 to 5 cm using various published scaling factors. However,
we have not
had the opportunity to repeat these experiments on other humans, so
generalization is limited. Nevertheless, the pointing method does appear
satisfactory.
Chris A. McGibbon, PhD <ronny@gait52.mgh.harvard.edu>
MGH Biomotion Laboratory
Boston MA
Thanks for all your comments.
It seems to me that there are two separate problems (right?):
1.1. To define an axis for each segment: e.g. hip joint
centre to knee joint centre
2.2. To define an axis of rotation for one segment wrt
another
Could someone enlighten me why we actually need (2). In reality there
is
no axis - there may be several or none - so why don't we simply measure
the motion of, say, shank with respect to thigh? An axis is really
superfluous to the problem, isn't it? Or is it simply to express the
result in clinically meaningful terms (sagittal plane flexion/extension,
frontal plane ab/adduction)?
I can see that (1) implicitly requires the joint centres to be defined.
Could we not, therefore, find some method to get away from the joint
centres as definitions for the local axes?
Please correct me if I'm not understanding the problem - 3D kinematics
was never my strong point!
Chris
--
Dr. Chris Kirtley MD PhD
Dept. of Rehabilitation Sciences
The Hong Kong Polytechnic University
The question you ask is a fairly fundamental one about kinematic
analysis and the linked segment model concept. If you use unlinked
segments (with no common axes) it becomes harder to describe the motion
in terms of three rotations as we do now. Not impossible, but difficult
to do such that the answers can be visualised. It is well known that
the
concept of the knee flexion axis is an approximation, but it's a very
useful one, and over the range of motion used in gait, quite a good
one.
There are much more significant sources of error.
The concept of a centre of rotation in the knee is less obviously
valid than at the hip. At the hip, the mechanical ball-and-socket
structure means the problem reduces to estimating the centre of the
socket. At the knee, the choice of the midpoint of the flexion axis
is
rather an arbitrary one and less obviously related to anatomical
structure. As usual, the response to such questions is - can anyone
suggest a better method which is as practical for routine use? We hope
that people will experiment with the alternatives, and that the better
alternatives will emerge over time.
Martin Lyster <martin.lyster@metrics.co.uk>
If you have two linear segments floating freely in space, you can always
define a common plane and measure the angle between them (this is what
370 does if you define an angle using four points and make a pop-up
graph). Alternatively you could do some kind of projection into fixed
planes. However it is less easy to interpret than the linked segment
model in which there is assumed to be a common axis about which the
two
segments rotate wrt each other. Also, if the segments are not linked
in
your model, I don't know how you can do kinetics.
I've been thinking some more about the free body problem. Its not a
simple question. There is a related problem in mechanics which goes
something like, if a car drives into a lamp post and the bottom of
the
lamp post snaps off the ground, what is the subsequent motion of the
lamp post. It's not an easy problem.
Martin <martin.lyster@metrics.co.uk>
Chris, you pose some interesting questions. Consider your two problems:
1.1. To define an axis for each segment: e.g. hip joint
centre to knee joint centre
2.2. To define an axis of rotation for one segment wrt
another
For determining joint angles or segmental kinematics in general, the
joint
center (2) is not required. The center of rotation, however, does have
utility.
For instance, knowing where the center of rotation of the knee is in
relation to
the the femoral and tibial surfaces allows prediction of the contact
kinematics
(the Oxford school of thought that the knee's instant center in the
sagittal
plane is located at the intersection the ACL and PCL "isometric" fibres
is one
application in the design of prosthetic components where contact kinematics
have
been inferred from knowing the joint center - see for example Goodfellow
and
O'Connor, JBJS, 60B, 355-369, 1978). The instant center is also a convenient
place to sum moments because it is generally assumed that articular
forces in
normal synovial joints (under low coefficient of friction - ice on
ice) do not
have shear components and hence the line of action of the articular
force acts
through the center of rotation: this allows elimination of the articular
force
in the moment equation for computing tendon or ligament forces (which
provide
the shear restraint).
I can see that (1) implicitly requires the joint centres to be defined.
Not really. It was common to see 2D motion analysis labs place markers
on
external landmarks approximating joint centers (such as lateral epicondyle
or
lateral malleolus), but these are really just segment reference markers
for
determining angles (and in my opinion a mistake to assume they are
also joint
centers). In modern 3D systems, any three (or more) markers forming
a reference
plane can be used to determine the 6 DOF kinematics of the segment,
provided one
knows the transformation between the reference plane and the assumed
location of
the segment coordinate axis. The origin of the coordinate system does
not have
to be located at a joint center (although it usually is).
Could we not, therefore, find some method to get away from the joint centres as definitions for the local axes?
I believe this to be one issue at the center of the debate for standardization
of gait lab measures (along with other issues such as sequence of rotation
matrix decomposition into angles, etc), and I'm sure there is someone
out there
who can shed some light on the current status of implementing these
standards.
Regards,
Chris A. McGibbon, PhD
MGH Biomotion Lab
Boston, MA <ronny@gait52.mgh.harvard.edu>
This has turned into an essay, but I think the gait analysis community
is
getting really lazy in its widespread and largely uncritical acceptance
of
the Kadaba/Davis model. I'm off for two weeks leave beginning
today so
I'll not be able to give any feedback, but this may be no bad thing.
I haven't really got time to contribute to this discussion but it is
one
that concerns/interests me. Chris McGibbon's comment that the present
drive
for standardisation might be addressing the use of marker sets is
particularly concerning because if this standardisation takes place
in the
near future we will undoubtedly be stuck with some variant of the
Kadaba/Davis model as the standard. There is no doubt that these models
have achieved an immense amount in informing the way we analyse, present
and interpret gait analysis data but I feel strongly that the time
is
coming when the limitations of the models are becoming more fully understood
and when work should be being done on better alternatives.
Most of the limitations are acceptable as long as they are understood
but
as gait analysis becomes a more and more standard procedure I suspect
a
smaller proportion of staff involved in its application are actually
fully
aware of these limitations. There are particular problems when the
output
of standard gait analysis packages are being used to drive other
biomechanics models such as muscle length calculations.
Chris Kirtley is correct in that there are essentially two requirements,
which I would state slightly differently:
1.1) to determine the position and orientation of each
segment (mathematically, to define an axis system)
2.2) to determine the relative orientation (and for some
applications position) of one segment with respect to another (He is
also correct in that this in itself does not
actually require the definition of a joint centre).
One of the problems with the Kadaba/Davis model is that it seeks to
use the
minimum number of markers possible and in doing so merges the two steps
above into one. This was important when the model was developed as
the
technology available then could not cope with very many markers. Modern
technology however does not impose such a limitation.
In order to determine the position and orientation of a segment it is
necessary to define three points on the segment. For the pelvis these
points are three markers. To determine the thigh you also need three
points
but Kadaba/Davis spotted that the hip joint is fixed in both the pelvis
and
thigh axis systems. You can therefore get away with one less marker
by
using the hip joint centre as one of the points for the thigh. A similar
argument uses the knee joint centre as one of the points for the shin
and
the ankle joint centre for the foot. Thus we save ourselves three markers
on each side. This is technically irrelevant given the capacities of
modern
data capture systems.
However, in using this approach we have introduced several problems.
Firstly, the system is hierarchical so any inaccuracy in determining
one
segment's axis system will carry on to an inaccuracy in determining
the
axis systems for all distal segments. Secondly, by definition the system
cannot cope with polycentric joints where the joint centre is not fixed
in
the axis system of both the proximal and distal segment. Thirdly, the
only
sensible way of determining joint angles is as rotations about the
fixed
joint centres. Fourthly, because all the information captured by the
system
is used in deriving the model there is no redundant information which
can
be used to check its validity.
These are theoretical limitations. The most important practical limitation
has already been alluded to by Chris Kirtley, the difficulty in placing
the
thigh wands accurately/repeatably. This is an art not a science and
those
who do place markers repeatably do it after many months (if not years)
of
practice, thinking about the process and critically reviewing their
results. Many centres, I am sure, simply do not place them repeatably.
It
doesn't need an alignment device, there are other methods. It is quite
easy
to place the markers repeatably on adults who can stand still and
co-operate it is quite a different matter on kids who can't (or won't)
stand still. Having said this it is comparatively easy to spot. If
the knee
has medio-lateral stability and the varus-valgus trace has the same
shape
as the knee flexion-extension curve then the most likely explanation
is
that this is an artefact of poor thigh marker placement leading to
incorrect definition of the thigh axis system. For able bodied
subjects
knee varus-valgus is essentially zero throughout the gait cycle. If
you get
a varus-valgus signal for such subjects which looks anything at all
like
the flexion-extension graph (or a reflection of it in the x-axis) then
you
are not putting the thigh markers on correctly. Because of the fact
that
the thigh marker is used to determine the knee joint centre this may
also
have a significant effect on hip flexion-extension, knee flexion-extension,
ankle dorsi/plantar flexion. Much more concerning when dealing with
CP is
that if you see this artefact then the hip rotation profile will also
be
wrong. Beware the surgeon who performs proximal derotation osteotomies
of
the femur based on gait analysis data without an understanding
of this
limitation of the Kadaba/Davis model!
I am looking at a technique which uses this varus/valgus artefact to
determine a correction factor for thigh wand alignment, similar to
that
calculated when using the knee alignment device. It works well for
those
with a good range of knee flexion during gait but still requires work
for
those with limited knee flexion (ie real patients!). Unless you suspect
medio-lateral instability of the knee or are supremely confident in
your
placement of thigh markers of knee alignment devices I suspect that
the
most reliable definition of the sagittal plane of the thigh is that
in
which the knee flexes.
Of course this is being a little unfair on the model. The main problem
is
that the thigh does not have sufficient well defined anatomical landmarks
to define internal/external rotation. However the model's requirement
of
the markers chosen leading to a fairly reliable definition of the knee
joint centre restrict choice here. Freeing ourselves from the hierarchical
definition of segments to an independent definition of segments could
prove
a distinct advantage.
Getting away from the requirement to use as few markers as possible
also
allows the possibility of giving us extra information. Cappozzo's group
in
Italy have pioneered the use of more markers not less. This gives redundant
information which can be used to as the basis for optimisation techniques
which lead to more reliable determination of segment axis systems.
This is
a particularly good way of working with skin movement artefact. If
the
effect of skin movement is different at different points then "averaging"
the readings from several markers can reduce those effects.
It has been pointed out that independent determination of segment axis
systems leads to more noisy data for joint angles. An alternative and
equally valid expression of this is to say that, by forcing artificial
constraints on the data, the Kadaba/Davis suppresses noise at the expense
of the reliability of the signal. Determining the instantaneous centre
of
rotation is notoriously noisy (it is not, however, actually required
to
obtain joint angles). However the noise is essentially a measure of
the
validity of the model. By seeking to minimise the number of markers
required the Kadaba/Davis model is very poor at giving such measures
of
data validity.
Does anyone else fancy discussing the limitations of the model for
determining hip joint centre from pelvic markers?
This has turned into a bit of an essay rather than a response to the
original question. I do hope it puts the questions regarding the widespread
adoption of this marker set on the agenda and makes people think more
critically about its application.
I would end by saying that until someone comes up with a better model
which
is easy to use clinically, we will, like the vast majority of gait
labs
throughout the world, continue to use the Kadaba/Davis model. Better
the
devil you know ...
Richard Baker <baker@unite.co.uk>
Gait Analysis Service Manager
Musgrave Park Hospital
Stockman's Lane
BELFAST BT9 7JB
Tel: 01232 669501 ext 2155
Fax: 01223 683816
I haven't really got time to contribute to this discussion but it is
one that concerns/interests me. Chris McGibbon's comment that
the present drive for standardisation might be addressing the use of
marker sets is
particularly concerning because if this standardisation takes place
in the near future we will undoubtedly be stuck with some
variant of the Kadaba/Davis model as the standard. There is no doubt
that these models
have achieved an immense amount in informing the way we analyse, present
and interpret gait analysis data but I feel strongly
that the time is coming when the limitations of the models are becoming
more fully understood and when work should be being
done on better alternatives.
Most of the limitations are acceptable as long as they are understood
but as gait analysis becomes a more and more standard procedure I suspect
a
smaller proportion of staff involved in its application are actually
fully
aware of these limitations. There are particular problems when the
output of standard gait analysis packages are being used to drive other
biomechanics models such as muscle length calculations.
Chris Kirtley is correct in that there are essentially two requirements,
which I would state slightly differently:
1.1) to determine the position and orientation of each
segment(mathematically, to define an axis system)
2.2) to determine the relative orientation (and for some
applicationsposition) of one segment with respect to another (he is
also correct in that this in itself does not
actually require the definition of a joint centre).
One of the problems with the Kadaba/Davis model is that it seeks to
use
the minimum number of markers possible and in doing so merges the two
steps
above into one. This was important when the model was developed as
the
technology available then could not cope with very many markers. Modern
technology however does not impose such a limitation.
In order to determine the position and orientation of a segment it is
necessary to define three points on the segment. For the pelvis these
points are three markers. To determine the thigh you also need three
points but Kadaba/Davis spotted that the hip joint is fixed in both
the pelvis
and thigh axis systems. You can therefore get away with one less marker
by
using the hip joint centre as one of the points for the thigh. A similar
argument uses the knee joint centre as one of the points for the shin
and the ankle joint centre for the foot. Thus we save ourselves three
markers on each side. This is technically irrelevant given the capacities
of
modern data capture systems.
However in using this approach we have introduced several problems.
Firstly, the system is hierarchical so any inaccuracy in determining
one
segment's axis system will carry on to an inaccuracy in determining
the
axis systems for all distal segments. Secondly, by definition the system
cannot cope with polycentric joints where the joint centre is not fixed
in the axis system of both the proximal and distal segment. Thirdly,
the
only sensible way of determining joint angles is as rotations about
the fixed joint centres.
Fourthly, because all the information captured by the system is used
in deriving the model
there is no redundant information which can be used to check its validity.
These are theoretical limitations. The most important practical
limitation has already been alluded to by Chris Kirtley, the difficulty
in placing
the thigh wands accurately/repeatably. This is an art not a science
and
those who do place markers repeatably do it after many months (if not
years)
of practice, thinking about the process and critically reviewing their
results. Many centres, I am sure, simply do not place them repeatably.
It
doesn't need an alignment device, there are other methods. It is quite
easy to place the markers repeatably on adults who can stand still
and
co-operate it is quite a different matter on kids who can't (or won't)
stand still. Having said this it is comparatively easy to spot. If
the
knee has medio-lateral stability and the varus-valgus trace has the
same
shape as the knee flexion-extension curve then the most likely explanation
is
that this is an artefact of poor thigh marker placement leading to
incorrect definition
of the thigh axis system. For able bodied subjects knee varus-valgus
is essentially zero throughout the gait cycle. If you
get a varus-valgus signal for such subjects which looks anything at
all like the flexion-extension graph (or a reflection of it in the
x-axis) then you are not putting the thigh markers on correctly. Because
of the fact that the thigh marker is used to determine
the knee joint centre this may also have a significant effect on hip
flexion-extension, knee
flexion-extension, ankle dorsi/plantar flexion. Much more concerning
when dealing with CP
is that if you see this artefact then the hip rotation profile will
also be wrong. Beware the surgeon who performs proximal
derotation osteotomies of the femur based on gait analysis data
without an understanding of this
limitation of the Kadaba/Davis model!
I am looking at a technique which uses this varus/valgus artefact to
determine a correction factor for thigh wand alignment, similar to
that
calculated when using the knee alignment device. It works well for
those with a good range of knee flexion during gait but still
requires work for those with limited knee flexion (ie real patients!).
Unless you suspect
medio-lateral instability of the knee or are supremely confident in
your placement of thigh markers of knee alignment devices I
suspect that the most reliable definition of the sagittal plane of
the thigh is that in
which the knee flexes.
Of course this is being a little unfair on the model. The main problem
is that the thigh does not have sufficient well defined anatomical
landmarks to define internal/external rotation. However the model's
requirement of
the markers chosen leading to a fairly reliable definition of the knee
joint centre restrict choice here. Freeing ourselves from the
hierarchical definition of segments to an independent definition of
segments could
prove a distinct advantage.
Getting away from the requirement to use as few markers as possible
also
allows the possibility of giving us extra information. Cappozzo's group
in Italy have pioneered the use of more markers not less. This gives
redundant information which can be used to as the basis for optimisation
techniques which lead to more reliable determination of segment axis
systems. This
is a particularly good way of working with skin movement artefact.
If the
effect of skin movement is different at different points then
"averaging" the readings from several markers can reduce those effects.
It has been pointed out that independent determination of segment axis
systems leads to more noisy data for joint angles. An alternative and
equally valid expression of this is to say that, by forcing artificial
constraints on the data, the Kadaba/Davis suppresses noise at the
expense of the reliability of the signal. Determining the instantaneous
centre
of rotation is notoriously noisy (it is not, however, actually required
to
obtain joint angles). However the noise is essentially a measure of
the
validity of the model. By seeking to minimise the number of markers
required the Kadaba/Davis model is very poor at giving such measures
of
data validity.
Does anyone else fancy discussing the limitations of the model for
determining hip joint centre from pelvic markers?
This has turned into a bit of an essay rather than a response to the
original question. I do hope it puts the questions regarding the
widespread adoption of this marker set on the agenda and makes people
think more
critically about its application.
I would end by saying that until someone comes up with a better model
which is easy to use clinically, we will, like the vast majority of
gait labs
throughout the world, continue to use the Kadaba/Davis model. Better
the
devil you know ...
Richard Baker <baker@unite.co.uk>
Gait Analysis Service Manager
Musgrave Park Hospital
Stockman's Lane
BELFAST BT9 7JB
Tel: 01232 669501 ext 2155
Fax: 01223 683816
ronny@gait52.mgh.harvard.edu wrote:
I believe this to be one issue at the center of the debate for
standardization of gait lab measures (along with other issues such
as sequence of rotation
matrix decomposition into angles, etc), and I'm sure there is someone
out
there who can shed some light on the current status of implementing
these standards.
In the last 9 years I have been involved in several projects for Gait
Analysis standardization, within the CAMARC II European project and
now
within the North American Society of Gait and Clinical Movement Analysis
-
Subcommette for Gait Analysis Modelling (NASGCMA). Because I haven't
seen
reply from any member of the commettee, I would just like to say that
several efforts have already been made to try to propose to the Gait
Analysis comunity a common reference model for marker placement and
joint
kinematics and kinetics calculation. The members have already agreed
many
points and are still thinking about many others. I think we will be
able to
submit a preliminary proposal of the model at the next NASGCMA meeting
in
Dallas, March 1999. Most of the agreed issues are present in the paper:
Position and orientation in space of bones during movement: anatomical
frame definition and determination A.Cappozzo,
F.Catani, U.DellaCroce,
A.Leardini Clinical Biomechanics, Vol.10(4)
1995.
I would suggest the reading of such paper also because it may help at
least
in the definition of the most critical points in clinical gait analysis.
The proposed use of the anatomical landmark calibration seems also
to be
now well accepted, because applied (althought in different contexts
and
applications) in many gait analysis models.
I would be glad to receive comments, suggestions, criticism on that
paper,
which could be considered as a first draft for future models. It has
already been accepted in the CAMARC II project as the official model
of the
European comunity.
I would like just to give my 2 cent contribution to this point
1.1. To define an axis for each segment: e.g. hip joint
centre to kneejoint centre
2.2. To define an axis of rotation for one segment wrt
another
Could someone enlighten me why we actually need (2). In reality there
is
no axis - there may be several or none - so why don't we simply measure
the motion of, say, shank with respect to thigh? An axis is really
superfluous to the problem, isn't it? Or is it simply to express the
result in clinically meaningful terms (sagittal plane flexion/extension,
frontal plane ab/adduction)?
In order to calculate clinical meaningful variables we first need to
determine position and orientation of reference coordinate systems
rigidly
associate to the bones, and oriented according to the functional anatomy.
Potitions of joint centres and other anatomical landmarks are necessary
for
this purpose. We then need to define axes about which we can consider
that
the joint rotations occur. These axes ought to be selected according
to
traditional way of thinking of clinicians, providing an objective
instrument to describe flexion/extension, ab/adduction and
internal/external rotation. The mechanical convention from Grood and
Suntay
1983 seems to be a good way for this representation. To calculate joint
rotations we can select whatever axis. However, to make the final results
with some clinical meaning we do need a precise definition of the axis
of
rotation. A standardisation of these definition would allow for result
comparison and data exchange.
More problems are instead involved in the accurate and repeatable
identification of the anatomical landmarks, and their artefact-free
tracking in space by external markers. Many investigations are still
in
progress to address this problems.
Richard Baker wrote:
Does anyone else fancy discussing the limitations of the model for
determining hip joint centre from pelvic markers?
We have in press on J Biomechanics the following paper:
Validation of a functional method for the estimation of hip joint centre
location A.Leardini, A. Cappozzo,
F. Catani, S.Larsen, A. Petitto, V.
Sforza, G. Cassanelli, S. Giannini
Here is the abstract:
The present study deals with the accuracy with which the hip joint
center
(HJC) is estimated using different methods. The functional approach
has been applyed by
calculating the center of the best sphere described by markers placed
on the thigh in a pelvic
anatomical frame during several trials of hip rotations. Two different
prediction approaches, for
which HJC is estimated by regression equations using antrophometric
measurements (Bell 1990, Davis
1991), were also tested. The accuracy of the three techniques was investigated
by comparing their predictions with
measurements obtained using the Roentgen Stereophotogrammetric Analysis
(RSA), who was
assumed to provide the true HJC location. In contrast with some previous
studies, the functional approach seems to be
more reliable than the prediction approaches proposed so far. If care
is taken in
maximizing the amplitude of rotations and minimizing the skin movement
artefact, then this approach may
provide local HJC coordinates with a minimum error of few millimeters.
The method is
therefore suggested in gait tests in which is applicable, as it allows
a reliable evaluation of
the subject-specific HJC location even in cases of pelvis and hip deformities.
Alberto Leardini
Movement Analysis Laboratory
Department of Orthopaedic Surgery - Istituto Ortopedico Rizzoli
Address:
Via Di Barbiano, 1/10
40136 Bologna
ITALY
Tel & Fax: ++39 51 6366561 (secretary)
E-mail: <VI6BOQ71@ICINECA.CINECA.IT>
Can I also throw in a question related to a commen Richard Baker threw
in
to the marker set debate: why do people think that the medial
rotation on
a motion graph is related to bony torsion?? Surely the model
is trying to
measure relative motion of the thigh in relation to the pelvis?
In the
Vicon static trial with KAD's, you try to get a handle on the amount
of
torsion, so that the motion graph will indeed reflect movement.
In many
cases of increased medial femoral torsion plus femoral head anteversion
the
hip is actually, anatomically laterally rotated (although it still
looks
medially rotated from the outside, due to the bony configuration).
You can
do proximal derotation to correct torsional alignment in the long bones,
but it will not necessarily alter hip rotation curves, even though
things
like foot progression angle may improve towards normal. Portland
Shrine
presented a paper on that showed this at AACPDM 2 years ago in Minneapolis.
I think the assessment for torsional correction via bony surgery
is mainly
via clinical exam +/- 3D or 2D CT or other imaging study, not via hip
rotation motion graphs!
Jenni
Jenni Dabelstein, Consultant Physiotherapist
Technology Unit (Equipment Technology Services)
Cerebral Palsy League of Queensland
354 Bilsen Road, Geebung 4034
AUSTRALIA
ph: 61 7 3874 2050
fax: 61 7 3874 2051
e-mail: techunit@iname.com
I'm worried too about the knee flexion axis definition with the
Vicon. I'm using it with children, and other problems apart, the KAD
won't stay on a child's knee in any where near the right place.
I
use the single knee marker instead, but I'm not convinced that I can
place it within 5 maybe 8 mm of the 'correct'? place. Add a similar
error to the thigh marker, just 50 / 70mm off the mechanical axis of
the femur (100 if you use a wand, but they are too easy for kids to
catch and move) and the knee flexion axis is 30 degrees off.
Having said that, I usually have no more than 10 degrees of knee
valgus showing in the kinematic report, but its always going to mask
any real varus / valgus.
We are examining this problem and others related to Vicon /
children, having just established our 'team' at Sheffield Children's
lab.
Rob
J.R.Strachan <J.R.Strachan@sheffield.ac.uk>
Royal Hallamshire Hospital
Sheffield
UK