CGA FAQ: Varus/Valgus Artifact & Cleveland Marker Set
Dear Sir,
We received from Mr. Richard Baker the following message concerning flaws in our protocol
for collection of data in normal children (database available in CGA site). We are also sending our
reply to Mr. Baker. We are interested in hearing the CGA's opinion about this issue. We would
like to present these two letters for discussion.
Message One:
from: Mr. Richard Baker (mailto:richard.baker@greenpark.n-i.nhs.uk)
Paulo,
Hope you'll take this e-mail the right way! I'm on a mission to raise awareness of the
critical importance of thigh markers/knee alignment jigs amongst the VICON users community.
I've been looking in detail at the normal datasets posted to the CGA site with references to a
presentation I'll be giving at the VICON user group meeting in Dallas on the importance of
accurate and repeatable placing of thigh markers (or knee alignment jigs if used). If these
aren't placed repeatably then the normal data shows high standard deviations for hip rotation
profile and knee varus/valgus angle.
I'm afraid that your data exhibits this (the set from Dr Sang Hyun also shows it but
unfortunately his data is a picture so I have no access to the underlying data). The average
standard deviation of your hip rotation profiles is marginally over 9 degrees whereas Jeremy
Linskell's and our data shows just under 6 degrees. You also pick uo quite a large mean signal
on the valgus/varus trace which looks very similar to the knee flexion extension trace. The
knee varus/valgus trace should be fairly close to zero throughout its rage and what you are
actually picking up is "cross-talk" from knee fexion which is caused by a systematic error in
the placement of the thigh markers/knee alginment jigs. It is our experience that if you place
the Knee alignment jigs exactly on the medial and lateral epicondyles rather than using these
as a guide for locating the knee axis then you will get data looking like this. This means your
hip rotation profiles are about 10 degrees more internally rotated than if the thigh markers
are placed correctly. Of course if you are consistent and compare your pathological data wth
your own normal database then the offset in varus-valgus and hip rotation is less of an issue.
However the large standard deviations you are showing suggests that the data collection
protocol is not all that consistent either.
Hope you don't mind me poking my nose into your data like this but I've been looking for a way
to illustrate these problems for the talk in Dallas. Do you mind if I use your data for this
purpose. I will not make any reference to the source of the data (and will even be positively
misleading if you like!) and by the time I've played around with scaling factors etc the data
will not be recognisable as that posted on the CGA site.
From the positive side you'll get an independent peer-review of your data collection protocol
which should be useful!
Best wishes
Richard
Message two (reply):
from: Paulo Selber, MD
Gait Lab. AACD
Brazil
Dear Richard
Not only have I taken your e-mail the right way, I would also like that your mission in raising awareness
amongst VICON users include the awareness of how much KAD used model is misleading, non precise and human error
dependent.
Before beginning our gait exams here, we already knew VICON's embedded models and understood the
KAD's proper alignment importance. The problem, was and still is aligning it in some patients.
Well, our Pt's have heard from different Lab's staffs and you wouldn't imagine how many techniques are
posted in the name of the proper positioning of this jig. Which is the center of rotation of the knee or the instantaneous center if
you prefer (as a good engineer), at all?
We hate this jig so much and the model behind it that our engineer is himself developing a new one, a
virtual KAD as he will. This virtual data, will definitely define to VICON where the knee axis of rotation is at a certain time or amount
of flexion during gait.
Instead of raising awareness of the "jigs" importance, I would rather suggest you to raise the will for its
substitution and the model with it, to a new "jig method" which can be human error free.
We have now done about 800 full gait analysis mostly in CP. kids whom as you may well know have one of
the major problems concerned to the hips transverse plane. Our staff knows and knew already the error we carried in
our learning curve, to the point that when we compared our data to a source of other gait lab the only statistical difference
found was exactly the one related to hip rotation, we had expected that.
These days, we compare our patients data in all respects to our normal data except the hip rotation. This
figure, we know it from the literature.
You may freely mention our staff's name,that's god to us anyway, remember we're south of equador... you
may also ask the folks from VICON (Oxford Metrics),about the number of e-mails they have received from us in the past
recent years, concerning this issue and also about our intent to know the model profoundly so that we eventualy will be
able to change or even improve it.
Once more thank you for your analysis, don't even think of using it as example of how accurate one should
be in placing the jig only, use it also and please as an example of how the model is not reliable, I can tell you even today, after all
our lab's experience (for I don't consider us to being on the learning curve any longer), and awareness, problems in
placing this damn jig still exist. We in fact need to somehow abolish it and not raise it.
Thank you very much for your input.
Warm regards.
Paulo Selber, MD
Clinical Coord.
Gait Lab AACD
Sao Paulo, Brasil
Thank you for your attention.
Best Regards,
Gait Laboratory - AACD
Brazil
http://www.aacd.org.br
mailto:aacd.labmarcha@aacd.org.br
(Written by: Paulo Selber, MD / Wagner de Godoy, Eng)
Dear Paulo, Richard, and I'm sure many more interested subscribers,
I'm so glad you've raised this issue, and I admire your honesty, and I
hope others will follow your lead! This was the actual reason why CGA
was started - to provide a forum for such open discussion.
We of course noticed the artifact problem when we started using our
Vicon system, and after much helpful advice from Richard, Jeremy
Linskell (Dundee) and Michael Orendurff (Portland Shriner's), decided to
abandon the use of the KAD.
The problem is, as anyone who has used it will know, that placement of
the KAD is extremely critical and difficult to get exactly right. We now
use the mirror technique (see
http://www.rs.polyu.edu.hk/gaitlab/fyp98/mirror2.jpg) suggested by
Jeremy to align the thigh wands as straight as possible - I've been
quite happy with the results from this method so far (varus/valgus
artifacts less than 10 degrees, which I consider to be acceptable).
I would be much happier, though, to find a more satisfactory and
objective method. I guess the basic problem is the lack of suitable bony
landmarks on the thigh. We really only have the femoral condyles to play
with, and they are not very well-defined.
I wonder about the experiences of people who are using the alternative
Cleveland marker set, since I noticed that Andreas, who has supplied
most of the Cases of the Week, seems to achieve quite small varus/valgus
artifacts with his Motion Analysis Corp. system in Vienna - see, e.g.
/archives/29-06-98/kinem.gif
/archives/25-9-97/kinem.gif or
/archives/01-03-98/kinem.gif
As you may know, the Celeveland set does not rely so heavily on bony
landmarks, since it uses triads (although, of course, we don't usually
like such things in Hong Kong!). However, I have never really understood
the static calibration of this marker set - perhaps someone could
enlighten me?
Once again - I'm so glad this issue has been raised!
Chris
--
Dr. Chris Kirtley MD PhD
Dept. of Rehabilitation Sciences
The Hong Kong Polytechnic University
Chris,
In reponse to your email concerning the cleveland clinic methods and use
of static trials:
I have been quite interested in this problem for the past 5 years. We
currently see about 350-400 patients per year in our laboratory and
looked closely at the differences between the "Wand" marker set and the
"Cluster" marker set (cleveland clinic) to asses what differences, if
any. We us a MAC high-res system with 6 cameras and the cleveland
clinic marker set. The new OrthoTrak software allows for 3 different
marker sets (cleve clin, helen hayes without static trial, and helen
hayes with static trials). Orthotrak has been programmed to use the
same medial and lateral knee and ankle markers to determine the
respective joint centers. The wand set (HH with static) uses the pelvic
arrangement of R and L ASIS and Sacrum to determine the hip joint center
during static trial to complete the "triad" of points needed to
construct the local coordinate system for the thigh segment (hip center,
thigh wand, knee lateral marker). The cleveland clinic set does not use
the hip center as part of the triad but rather the 3 points on the
lateral cluster (triad) to construct the local system. With this local
thigh orthogonal coord sys, the CC marker set references the lateral and
medial knee point to the this system during the static capture and uses
this reference to reconstruct the knee center during the dynamic
trials. This being the case, lateral and medial knee and ankle points
are not needed during walking for the CC set.
I am completing research comparing the 2 sets during gait. I put both
markers on the body that would satisfy the algorithm of both marker sets
and collected a static trial for each leg. The same static trial was
used in the processing of both marker sets. Prelim results show very
close kinematic and kinetic comparisons between the 2 sets over 60
normal children. These data will be available in poster form at the
Gait conference in Dallas this week.
I would be interested in hearing any feedback on what you use in your
lab and ideas for improvement
Patrick
--
Patrick W. Castagno
Manager/Biomechanist - Gait Analysis Laboratory
duPont Hospital for Children
1600 Rockland Road
Wilmington, DE 19899
Chestnut@udel.edu
(302-651-4615)
Dear suscribers,
I would like to try to use the "Cleveland marker set" for gait analysis
but was unable to find any published paper describing this marker set.
Could someone help me to find informations on this subject ?
Many thanks by advance,
Stephane
Stephane BOUILLAND
Ingenieur Biomedical - Clinical Engineer
Fondation Franco-Américaine
Hopital Calve
62600 Berck sur mer
tel : 03-21-89-31-99 (bureau)
03-21-89-21-89 (laboratoire)
fax : 03-21-89-33-18
email : sbouilland@hopale.com
http://www.hopale.com
I have received many inquires on the Cleveland Clinic Marker Set and I thought I
would publically answer the question since this set is only used by Motion
Analysis gait and sports researchers alike.
The Cleveland Clinic Marker set is a proprietary marker set own by Motion
Analysis Corporation, Santa Rosa CA. The CC marker set developed in conjunction
with the Cleveland Clinic Foundation for Motion Analysis in the 1980's allows
the gait researcher to place a three point marker triad on the shank and thich
segment of the child and adult. Its purpose is to assess the segment's
rotational factors more precisely, especially in children with severe rotation
of these segments. The data captured on the CC marker set is automatically
tracked by the Motion Analysis: HiRES system and then the software OrthoTrak
automatically takes the data and calculates joint kinetics and kinematics,
Ankle, knee and hip forces, Varus/Valgus of the femur, tibia rotation... The
Cleveland Clinic marker set was based using cadaevors and the research
associated with cadaevor bone segments and mechanical properties of the segment,
etc.
The Cleveland Clinic marker set requires a static trial of the left and right
legs to automatically calculate joint center information for the lower body.
The disadvantage of using the CC marker set with a non-Motion Analysis type of
system is that the other systems (Vicon, Peak, Qualisys, Elite) cannot identify
the triad markers and would see them a one large marker. (Maybe their systems
have changed, but check your specific manufacturer for proven data). Hence
defeating the purpose of placing a 3 point cluster in a plane parallel to the
long axis of the bone to capture the motion. That is why all other systems use
a Helen Hayes, Modified Helen Hayes or some aspect of a single point at the
location marker set to calculate the motion. They have poor resolution on their
system, secondly the CC requires an extra trial (static), therefore is it deemed
extra work (5 minutes) for the technician (versus spending tens of minutes with
a Knee Alignment device and it potential inaccuracies of misalignment) and
thirdly, fewer markers on the segment tracks faster and may be deemed easier to
edit with non-Motion Analysis systems. So accuracy was sacrificed for speed.
Now I do not see why you can not place three - 10mm sized markers on a small "t"
shaped jig, but insure that your tracking software can track it and the
reporting software can report it.
But the fact remains, the CC marker set provides a true 6 degree of freedom
cluster at each of the four lower body segment takes less time than the KAD's
technique, is more accurate for joint center calculations and can be tracked
automatically and identified at no difference in time than the Hayes single
point wand technique (well at least with our system). It has been used in
Hundreds of Gait, sports, Rotational studies, Animation, Neuroscience, etc.
investigations. It can also be used on the upper body too and processed with
our KinTrak Software system.
I will ask my colleague at Motion Analysis Murali Kadaba, Ph.D, who heads up
our Engineering/Biomechanics Research applications in Santa Rosa for a second
opinion on the Cleveland Marker set and it use with Non-Motion Analysis systems.
I do suspect the answer would be: the specific company or user would need to
write specific software for the cluster, but first the system needs to see the
extra 12 markers.
The Cleveland Clinic Marker Set is and always has featured upper body markers
and now being expanded to the head too to assess head rotation, lean and tilt.
The orginal data and concurrent work on the Cleveland Clinic marker set has
never been published and it will not be published. It is the main advantage of
our system over all others: accuracy and precision. Like the best x-ray machine
on the market, their data is kept under security to maintain their competitive
edge over its competitors too.
It has been validated and continues to be validated by our hundreds on users
worldwide.
Further discussions on this topic can be sent directly to me at the following
coordinates.
Quick references:
http://www.ivanhoe.com/docs/backissues/3dgaitanalysis.html
http://www.ivanhoe.com/docs/backissues/slippingstudy.html
http://www.ivanhoe.com/docs/backissues/lightscameraaction.html
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
www.motionanalysis.com
Dan.India@motionanalysis.com
Dan
I appreciate the high quality of the system which you are rightly
proud of, but I would like to take you up on one point which you
could possibly answer for me and that is in relation to the static
test. It strikes me that if the subject cannot adopt the 'neutral'
standing position for the static, then even your model will not be
able to correctly define the anatomical axes accurately. We
all have the same dilemma in that we are trying to estimate the
position of underlying rigid bony segments from markers placed on
surface tissue. All data produce from surface markers, on bony
segments, is inferential - i.e we can never know that our markers
actually reflect the true bone orientation/position. Different
protocols have different advantages and disadvantages and yes, Prof
Capozzo has demonstrated objectively that there are advantages to be
gained from using clusters of markers. However the inferential nature
of our results cannot be ignored - all we can ever do with our
different approaches is to shuffle the pack, in terms of the sources
of error. I really do not feel it is reasonable to claim that one
approach is inherently superior to another.
regards
Jeremy Linskell, Clinical Engineer
Manager, Gait Analysis Laboratory
Co-ordinator, Electronic Assistive Technology Service
Dundee Limb Fitting Centre
Dundee, DD5 1AG, Scotland
tel +1382-730104, fax +1382-480194
email: j.r.linskell@dth.scot.nhs.uk
(backup email: j.r.linskell@dundee.ac.uk)
web: http://www.dundee.ac.uk/orthopaedics/dlfc/gait.htm
To avoid commercial advertisement in a public forum, let me rephase the
statement, that in the past when such systems were going through development,
and tracking of markers, resolution of cameras, and speed of processing etc.
were time considerations, companies including Motion Analysis and all its
competitors, Vicon, Elite, Peak, Qualysis, Ariel.....etc.. sought to provide the
most meaningful 3D data to the technicians. Markers were large, systems were
slow, camera resolution was not as it is today. And those are facts. Today,
all vendors strive for excellance, we work hard to meet your perfomance needs
and I am not seeking public criticism to my vendor colleagues and respect their
systems and performances.
A second fact is that all motion capture companies developed technology to read
a single marker at the femur and shank that was provided by researcher. One
also developed a cluster based upon research. Hence, years ago tracking triads
may have been difficult for motioncapture companies. Today it should be
standard and a mute point.
Third fact is that today all such systems, and I'll refer to Dr. Jim Richards
presentation at July 3rd, 1998 ISB 3D Comparison Symposium, can identify a
cluster of markers, and if not have some intelligence to edit the unnamed or
unidentified marker pathway. ( I have copies of this report for those
interested and will send.)
The Fourth Fact is that only one motion capture uses such a cluster today in its
gait software and the others do not. Therefore if a cluster triad is used in a
gait analysis, would the commerical software be able to generate the gait report
and use the data? I can speculate that the users and commerical vendors are
quite satisfied with their outcomes. I am aware of several commercial vendors
developing cluster marker sets for the future and also further enhancing their
technology. I can suspect with high level of confidence that Peak's Motus,
Ariel's APAS, Qualisys 3D Program and Vicon's Body Builder can use cluster data
to transform the information for kinematics and kinetic measurements.
(January 98: Gait and Posture 7 (1998) 1-6 Holden and Stanhope identified a
cluster of targets on the Femur and Shank to seek moment calculations. This was
done with a Vicon system and Move 3D from NIH). Jeremy Linskell of Dundee
Scotland reminded me that: "Different protocols have different advantages and
disadvantages and yes, Prof Capozzo has demonstrated objectively that there are
advantages to be gained from using clusters of markers. However the inferential
nature of our results cannot be ignored - all we can ever do with our different
approaches is to shuffle the pack, in terms of the sources of error". But my
statement was to check it out and get proof.
So my point is before you start tossing extra markers on the segments to
calculate joint centers or whatever, you need to understand your system's
limitations and capabilities to be altered and accept the data. Can you avoid
static trials using medial and lateral markers at the knee and ankle, can you
use KAD's or related devices? One research group takes a static series of
pictures and manually digitizes the pcitures with one system and sends that data
into another system to create 3D joint centers and final reports.
Personally, I would rather see non-commerically funded researchers conducting
such investigations. For your information, In Japan on July 3 and 4th, 1999 will
be the 3rd motion capture 3D Comparison Meeting at which vendors have the
ability in a public forum to have their system compared. Maybe this activity
should be repeated, with what is scenarios, (what if I create virtual markers?,
what if I add a cluster of markers?, what if the raw data is bad, how can I
correct it)?
Sincerely,
Dan India
Motion Analysis Corp.
Chris,
I look forward to communicating with you more about this stuff. By the way, I have
had issues with the way MAC's OrthoTrak 2.5 (1986-1995) calculated the helen
hayes model without the use of any static capture or knee axis jig. This
model's accuracey was completely dependant on how well you could align the
thigh and shank wands to the flx/ext axis of the knee and ankle
respectively. That's like trying to align a pencil to the side of a coke
can. For this reason I went with the cleveland clinic set since we started
our lab in 1991. Since Jim Richards, myself, Dr Freeman Miller and MAC got
together and created OrthoTrak4.1 which has programmed into it a helen hayes
model for use with static trials, my feeling about this model is much
better. Same exact model as in 2.5 except now the alignment of the wand has
no bearing on accuracy because of the static trials for each leg. I am
probably babbling on here because I am fading fast so I think I will catch
up with you upon my return from the conference.
Anyway, have a great week.
Patrick W. Castagno
Manager/Biomechanist - Gait Analysis Laboratory
duPont Hospital for Children
1600 Rockland Road
Wilmington, DE 19899
Chestnut@udel.edu
(302-651-4615)
Dear suscribers,
First I would like to thanks those who answered my first question
concernin Cleveland marker set. I would like to enlarge the debate
concerning advantages and disavantages of marker sets used for gait
analysis. Looking in Biomch-l archives I identified the four following
marker sets :
-Helen Hayes marker set
-Ohio State University marker set
-Cleveland Clinic marker set
-Mayo Clinic marker set
I was unable to find informations concerning those marker sets on the
web ( I searched on the concerned Clinic's web sites and medline). I
would be very grateful if our community could share knowledge and
experience on this topic. I would like to know what are the differences
between these marker sets and what are advantages and disavantages.
Many thanks by advance,
Stephane
------------------------------------------
Stephane BOUILLAND
Ingenieur Biomedical - Clinical Engineer
Fondation Franco-Américaine
Hopital Calve
62600 Berck sur mer
tel : 03-21-89-31-99 (bureau)
03-21-89-21-89 (laboratoire)
fax : 03-21-89-33-18
email : sbouilland@hopale.com
http://www.hopale.com
Talking about Marker Sets, I would like to include in the list the Cappozzo
/ Istituti Ortopedici Rizzoli marker set, supported by the following
publications:
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;171-178
Position and orientation in space of bones during movement: experimental
artefact; A.Cappozzo, F.Catani, A.Leardini, M.G.Benedetti, U.Della
Croce;Clinical Biomechanics, Vol.11(2) 1996; 90-100
Data management in gait analysis for clinical applications; M.G. Benedetti,
F. Catani, A. Leardini, E. Pignotti, S. Giannini; Clinical Biomechanics,
Vol.13(3) 1998; 204-215
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; Journal of Biomechanics; 1999; 32(1):
99-103
Nice to talk to you all,
Alberto Leardini M.Eng.
Movement Analysis Laboratory
Istituti Ortopedici Rizzoli
Via di Barbiano 1/10, 40136 Bologna ITALY
tel: +39 051 6366522
fax: +39 051 6366561
email: leardini@ior.it
http://www.ior.it/movlab/
Address in Oxford
Oxford Orthopaedic Engineering Centre
Nuffield Orthopaedic Centre
Windmill Road, Headington, Oxford OX3 7LD ENGLAND
tel: ++ (0)1865 227684
fax: ++ (0)1865 742348
email: alberto.leardini@ooec.ox.ac.uk
You'll find the source code for processing the cleveland clinic
marker set inside the distribution of ANZ on the biomechanics
website. There are a lot of comments in there that explain how the
markers need to be placed and how they are processed to compute 3d
motion of the segments. I think there are also comments on what it
can't do. I wrote the software about 10 years ago so I can't say what
exactly is in there, but I know that we used it a number of times
when I was post-docing at mayo clinic. We also put together some
other modified versions of the marker set that are in the software
also. I think you will also find references to literature explaining
the marker set.
--dwight
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dwight Meglan, PhD Mitsubishi Electric
Lead, Medical Applications Group Information Technology Center America
dwight@merl.com 201 Broadway, 8th Floor
http://www.merl.com Cambridge, MA 02139
617 621 7522 / 7550:fax
Hi Chris
Your last Biomch-L posting has me confused! You stated "the relative merits
of the Davis versus Cleveland marker set". I understand the Cleveland Clinic marker
set---I work at the Cleveland Clinic, and I know of the work done by Kevin
Campbell prior to me moving to Cleveland. As an aside, none of the current
gait-related work at the Cleveland Clinic uses the "Cleveland Clinic" marker
set, although we do use a system we purchased from Motion Analysis Corp. I
personally use the Helen Hayes system.
By "Davis", I'm assuming you are not referring to me! I did do some work
with Kit Vaughan, and together we published a book that uses a marker set
different from the Cleveland Clinic, Helen Hayes, and any other marker set
that I know of. However, if anyone should get the credit for our marker
set, it should be Kit, since he did 99% of the development. Peak
Performance sells a gait system that is based on Kit's marker set, though
this system has been expanded to use other marker sets too. I'm not exactly
sure what Kit called his marker arrangement, but at one stage we called it
the "Charlottesville, Cleveland, Cape Town (CCC)" system to reflect the fact
that the three authors of the package were at that time in three different
cities. (In contrast to both the Helen Hayes and Cleveland Clinic marker
sets, the CCC system does not use triads or wands for marker placements.
The CCC system places all the markers directly on to anatomical landmarks.)
Regards, Brian
Dear Brian & Kit,
Now you've got ME confused too! I thought the VCM marker set was
designed by ROY Davis!
Anyway, I'd be glad if Kit can clarify the whole business.
Chris
--
Dr. Chris Kirtley MD PhD
Dept. of Rehabilitation Sciences
The Hong Kong Polytechnic University
Hong Kong
Dear Brian and Chris
Here's my take on all of this:
(1) When I did a post-doc with Mike Whittle at the Oxford
Orthopaedic Engineering Centre (OOEC) in 1983, I developed a
15 marker set. In fact, we gathered data on Ros Jefferson at
that time and her data set is included in the package called
Gait Analysis Laboratory which was published by Human Kinetics
in 1992 (the software was written in 1988-89) where the
co-authors were Vaughan, BRIAN Davis and O'Connor. The marker
set is illustrated in Figure 3.4 (page 23) in the book "Dynamics
of Human Gait".
(2) In the late 1980s, Murali Kadaba and colleagues at the Helen
Hayes Hospital in upstate New York developed a 13 marker set. It
was published in the Journal of Orthopaedic Research in 1990
(Volume 8, pp. 383-392). This set is sometimes expanded to 15
markers by the addition of markers on the heels. It is referred
to as the Helen Hayes (Hospital) marker set and is essentially
the same as the set used by the group at the Children's Hospital
in Newington, Connecticut,where ROY Davis was the lead engineer.
His main paper on their approach was published in Human Movement
Science in 1991 (Volume 10, pp. 575-587). It is on these two
papers that Oxford Metrics have based their VCM model. The
marker set has never been referred to as the Davis set as far as
I know.
(3) In the mid- to late-1980s, Kevin Campbell was at the Cleveland
Clinic Foundation (CCF), and the CCF had just purchased a system
from Motion Analysis Corporation (MAC). The CCF contracted to
MAC and, with the clinician Chet Tylkowski from Florida, they
developed their marker set and the OrthoTrak product. During
my first few years at Virginia (1989-92), we had a MAC system
and used the CCF marker set and I can tell you it was a pain
in the butt! We switched over to the HHH marker set and, in
time, to Vicon370 and VCM.
(4) The Gait Analysis Laboratory package referred to in (1) above,
will be released on CD-ROM shortly (yes, Brian, we are nearly
there!) and it supports the 15 marker HHH system. Watch this
space for an announcement, Chris!
Well, that's it from me. I hope you're now up to speed, Chris.
Regards.
Kit
Having read all the discussion so far on the marker set issue, i.e CC
vs. HH sets it strikes me that 2 issues are being merged into one
here. The 2 items appear to be the benefit of clusters of markers and
the best method to obtain reasonable estimation of femoral
orientation.
As far as I understand it, the benefit of clusters in is relation to
the reduced susceptibility of their combined output to skin movement
artefact, compared to single markers. The issue of why VCM uses the
hip joint centre co-ordinates as one of the points to define the
thigh segment is really a diversion.
The reason we developed the mirror technique was mainly because we
did not like the idea of defining a bi-condylar axis relying on the
medial condyle and the reason the for this was that we felt that the
medial condyle was too vague an anatomical landmark to rely upon -
wether with KAD or 2 condylar markers or any other protocol -
especially in a pathological knee. We feel much more comfortable
using our clinical experience to identify and replicate sagittal
plane knee motion.
I think separating the 2 issues out will allow us to gain more
value from the dissussions.
regards
Jeremy Linskell, Clinical Engineer
Manager, Gait Analysis Laboratory
Co-ordinator, Electronic Assistive Technology Service
Dundee Limb Fitting Centre
Dundee, DD5 1AG, Scotland
tel +1382-730104, fax +1382-480194
email: j.r.linskell@dth.scot.nhs.uk
(backup email: j.r.linskell@dundee.ac.uk)
web: http://www.dundee.ac.uk/orthopaedics/dlfc/gait.htm
Firstly it would like to inform that my academic
formation was not in the biomechanics area.
I don't also possess specialization in this branch,
therefore I would like to ask excuses for the primary
level of my doubts.
Subject: Questions about the determination of
flexion/extension axis of the knee for gait analysis
(angles of flexion/extension, rotation and valgus/varus)
with Vicon 370/VCM system.
I have used as reference the work of Mr. C. Frigo and Mr.
M. Rabuffetti (Multifactorial Estimation of Knee Joint
Centers for Clinical Applications of Gait Analysis, Gait
and Posture 8 (1998) 91-102).
page 92:
"A different situation applies at the knee. The joint
kinematics are determined by the geometry of the
internal surfaces and by the restraing forces from
muscles and ligaments [13]. A fixed centre of rotation
does not exist. Theoretically, knee motion should be
described usind an instantaneous axis of rotation, whose
position and orientation change in space ('helical axes
of motion') [14-16]. However, this kinematic definition,
that in some circumstaces places the axis outside the
body, does not relate readily to the concept of joint
centre as used in clinical practice. Moreover, its
estimation can be affected by measurement errors."
To analyse the efficience of KAD and possible methods for
its substitution, I would like to know the opinion of the
members of CGA about the followwing definitions:
Having the KAD the following finalities:
a)Determine the frontal thig plane, from of 3 points: hip
center calculed (equations of Davis,
Õunpuu and Tyburski), axial and virtual markers of KAD.
b)In the frontal thigh plane determine the knee flexion
axe, as well knee joint center, and the
longitudinal axe of the thigh, being these two axis
perpendiculars.
- Is there a definition for frontal thigh plane?
- If there is, could be this definition "translated"
to a mathematical model?
- Would it be possible to create a definition of
frontal plane - to thigh and legh - that could be
specific to the movement studies (gait analysis)? To this
finality, would it be possible to use any variables that
could be collected by the Vicon System?
- May be the movement of flexion/extension of the
knee considered always as predominant (much major
magnitude) in relation with the movement of rotation and
valgo/varus? If so, would it be possible to use this
movement as reference to determine the sagittal thigh
plane?
Thank you for your attention.
Best Regards,
Wagner de Godoy
Mechanical Engineer
Gait Laboratory
AACD - Brazil
I said I'd give you some feedback from the VICON user meeting in Dallas
where I spoke on the problems of determining the knee axis. I've taken this
opportunity of also copying this to CGA.
Several sorts of people seem to exist. First are those who don't realise
there is a problem (these are, thankfully, very few and far between but they
do exist). Second are those who recognise there is a problem but feel
powerless to do anything about it (these are by far the majority). Third are
those who recognise the problem but through sheer experience and attention
to detail have learnt how to put the KADs on reliably (restricted in my
conversations to one person!)
Conclusion: that KADs can be used reliably if you put sufficient time and
thought into learning how to use them. The emphasis here must be on the
thought. Those people that have mastered the KAD also have an in depth
knowledge of what the consequences of poor KAD placement are and how to spot
them in the gait data.
One tip was to try and place the thigh wands accurately and the KADs
accurately. If both are successful then the Thigh Offset calculated from the
static test will be small. If it is large then you must suspect something is
wrong somewhere.
It was reinforced that this understanding is around. Murali Kadaba's
original paper (Kadaba, M.P., Ramakrishnan, H.K. and Wootten, M.E.,1990.
Measurement of Lower Extremity Kinematics During Level Walking. Journal of
Orthorpaedic Research, 8, 383-392.) went at some length into the
consequences of poor definition of the knee rotation axis. Ed Cramp
(eac@emgsrus.com) pointed out that Motion Lab Systems who market the KAD
have a very comprehensive manual (since May 1998).
Perhaps the final word should go to that physio who had perfect confidence
in her own methods "Its an art, not a science".
Richard
Richard Baker
Gait Analysis Service Manager
Musgrave Park Hospital
Stockman'sLane
BELFAST
BT9 7JB
Tel: +44 (0)1232 669501 ext 2155
Fax: +44 (0)1232 382008
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