Dear colleagues
I am very much interested in this topic both from a scientific and
medical point of view, for the reasons summarized by Alberto Leardini in
his introduction to the topic.

Basically I think, like Ton van den Bogert, that the answer to the
problem depends on the resolution you need to describe the knee motion,
therefore on the application or problem you are addressing. And in my
opinion the "resolution" of the investigation is mainly determined by
the method used to observe the knee motion.

In most clinical diagnosis and examiniations the knee laxities and the
knee passive range of motion is tested manually by the physicians, and
all my colleagues at the hospital agree to say that the passive range of
motion (PROM) is repeatable for all individuals, even if the amount of
internal-external rotation or the anterior-posterior displacement during
PROM is probably "different" in each patient. Therefore I would say
that in manual kinematic tests for diagnosis purposes the normal knees
show 1 DoUF, because a unique internal-external or varus-valgus rotation
and displacement is associated to each flexion angle.
However in pathological knees the DoUF seem to increase: in ACL -
deficient knees there is an evident anterior - posterior laxity with
makes PROM less contrained in this direction during PROM, especially in
flexion, while after collateral ligament injuries the variability of the
varus - valgus rotation associated to flexion is much more than in
normal knees (as reported also by wolf Schweitzer yesterday). It is
difficult to say if in this case the DoUF are 2 or more, as the debate
on the effects of secondary restraint for ligaments is still open, and
in ACL or MCL-LCL injuries the internal - external laxity is sometimes
considered significant. However let's say that pathological knees have
more than 1 DoUF, and a successful treatment (either rehabilitation,
trining or surgery) is able to recover the normal 1 DoUF in PROM.
This conclusion is true within the accuracy reached by manual test,
which I think is certainly not less than 3° and 3mm , measured in our
cadaveric studies of knee kinematics by means of spatial linkages and
optical trackers. Standard clinical scores, such Muller score, or forms
for clinical trials report ranges of 5° and 5mm.

As a scientist I feel that recent instrumentation can and therefore
should certainly improve the accuracy of the description, reasonably to
1-2 deg for rotations and mm for displacements. Unfortunately a few
reliable data are available in literature reporting instrumented tests
with equipment able to contrain or control more than 1 dof like KT
machines, and not only to monitor the motion executed manually like
spatial linkages and navigation systems. I am aware only of the work
done in Pittsburgh with a 6dof robot, who's method is reported by van
den Bogert in the previous message.
Our team has done some experiments on pig and human knees (mainly
cadaver, up to now occasional intraoperative tests on patients under
anesthesia) and we are collecting data about the passive kinematics in
normal knees (cadaveric specimens), arthrotic knees undergoing total
knee replacement and ACL-deficient knees undergoing ACL reconstruction.
Although results are not yet available for a definitive conclusion we
are trying to verify whether 2 DoUF are enough to define the knee
behaviour, that is try to verify whether flexion angle and
internal-external rotation only can determine the PROM, possibly both in
normal and pathological knees. This would be simpler (i.e. less) than in
robotic tests and would reflect the feeling that these two degrees of
freedom in the knee are more important than the others. Whether the
translations are independent or not from these rotation is still not
clear to me, but the work by Hollister, Churchill, Pinskerova and M.
Freeman suggest than they may be dependent provided appropriate axes of
rotation or reference frame for the knee joint is choosen.
Probably the interpretation of sexperimental data and therefore the
problem of how many DoUF has the knee joint is closely related to the
problem of the deomposition of the motion in a suitable joint reference
frame, which affects the accuracy of the conclusions more than the
resolution of the spatial linkages used for acquisitions. It would be
interesting to discuss also this issue, within this or in another topic.

At present I think that attempts to achieve a submillimetric accuracy in
the description of the knee motion is certainly beyond the technical
capability of current equipment, either 3D imaging like MRI or RSA or
optical-mechanical like patial linkages. In fact if the equipment
capability can attain 0.2-0.3 mm accuracy its application on human
joints imply tioo deal with soft tissues, difficult fixation to the
bone, the manual adjustment of the frame to image the knee, or similar
problems drastically enhancing the uncertainty of acquired data, that is
reaching the accuracy mentioned above.

Sandra Martelli

Sandra Martelli e-mail:
Laboratorio di Biomeccanica URL:
Istituti Ortopedici Rizzoli
via di Barbiano 1/10 tel:
I-40136 Bologna, Italy fax:

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