Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a method for measuring
parameters relating to stability of joints in the human
body. More specifically, the invention relates to such a
method which includes the step of soft tissue compensation.
The inventive method also includes the step of digitization,
that is, the step of locating it three dimensional space
and in an appropriate coordinate system, the position of
parts of the human body related to the joint. The method
also includes the step of presenting data relating to motion
and force in the appropriate coordinate system.
In order to determine the stability of a human
joint, it is necessary to gather data of the motion of the
relative portion of the joint (that portion ox the joint
farthest removed from the body) relative to the reference
portion of the joint (that portion of the joint closest to
the body), with different forces applied to the relative
portion. For example, in determining the stability of the
knee joint, tibio/femoral~motion motion of the tibia
relative to the femur) must be observed, with the apply-
cation of different, known, forces to the tibia.
With present methods, as discussed in our co-
pending Canadian application Ser. No. 469,958, an examiner,
e.g. a physician or a physiotherapist subjectively observes
the motion so that comparison with the state of the joint
between observations separated in time is a function of the
memory of the examiner and the accuracy of his description
viz-a-viz his observations. Furthermore, objective and
accurate knowledge of the magnitude of applied forces is
not available to the examiner.
It is apparent that a set of reproducible,
objective measurements, would be superior both for recording
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a present condition and for comparison purposes.
It is also known that the application of force
to the relative portion of a join-t will cause motion of the
reference portion thereof due to the soft tissue surrounding
the reference portion. Present methods measure the total
motion of the relative portions plus the reference portions
due to applied forces, without consideration of the motion
of the reference portion. As the critical data which is
necessary for determining the stability of a joint is the
motion of the relative portion relative to the motion of
the reference portion thereof/ such measurements are
insufficient.
For example, and in considering specifically the
knee joint, the ability to measure complete tibia/femoral
motion is limited by the existence of soft tissue surround-
in the femur. Thus, when force is applied to the tibia
to cause motion thereof, force is also inevitably applied
to the femur. This force will cause motion of the femur
in the soft tissue surrounding the femur. A surface
measuring device, which measures only the motion of the
tibia, will therefore not be measuring the motion of the
tibia relative to the femur which is also moving.
A technically ideal method of providing accurate
bone versus bone measurement is to attach the measuring
instruments by bolt screws to the bones. This it clearly
unacceptable from a clinical point of view.
A further problem of measurement systems presently
available is that they present data in the coordinate system
of the measuring instrument rather than in the coordinate
system of the relative portion of the joint. Thus, the
examiner must attempt to visualize what is going on in
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the coordinate system of the relative portion of the joint
from data which is displayed in the coordinate system of
the measuring instrument.
Referring once again to the knee joint, the
measurement of total motion of bones in this joint requires
accuracy on the order of one millimeter and one degree.
The total motion (three translations and three rotations)
is ideally required because of the complex sliding and
rotational motions occurring in the knee. These motions
must be measurable at all desired flaxen angles. measure-
mint devices available range from accurate unidirectional
transducers to much less accurate six degree of freedom
transducers. Roy poor accuracy and lack of completeness
of the measurement throughout normal ranges of motion of
the knee are major deficiencies.
In conducting tests, it is also of course nieces-
spry to have indications of the magnitude of the forces
being exerted on the joint during the test. In order to
correctly assess the extent of ligaments damage, it is
necessary to have data concerning the stiffness or stability
of the joint under a great variety of external forces and
moments. Accuracy of the order of ON with ranges as high
as Lyon may be required for such tests.
Devices presently available typically are single
axis force transducers which are usually interposed between
the examiner and the joint being examined. Thus, the
examiner's ability to pursue his normal method of assess-
mint, including palpitation and visual observation of
measured motion, is restricted. In addition, patient
apprehension, which is detrimental to well performed
laxity testing, is significantly increased by devices
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attached directly to an injured and sensitive joint.
Finally, attachments of measuring devices near the joint
may affect the joint stiffness itself.
It is therefore an object of the invention to
provide a method for measuring parameters relating to the
stability of joints in the human body which overcomes the
above disadvantages.
It is a further object of the invention to pro-
vile such a method which includes the step of soft tissue
compensation.
It is a still further object of the invention to
provide a method which includes the step of digitization.
It is a still further object of the invention
which provides a method which permits presentation in the
coordinate system of the relative portion of the joint.
In accordance with the invention, a method for
measuring parameters relating to the stability of joints
in a patient's body includes the steps of securing a
reference portion of a joint to a fixed location and per-
forming a soft tissue compensation procedure by applying forces to the reference portion to thereby determine the
amount of reference portion motion, in the soft tissue
surrounding the reference portion, due to the forces. A
variety of forces are then applied to the relative portion
and the total motion of the reference and relative portions
associated with the forces are measured. Thus, by sub-
treating the motion determined in the soft tissue compel-
station from the motion measured by the application by a
variety of forces, it is possible to determine the motion
of only the relative portion relative to the reference
portion .
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The invention will be better understood by an
examination of the following description, together with
the accompanying drawings, in which:
FIGURE 1 illustrates the preferred apparatus for
carrying out a knee laxity evaluation in
accordance with the inventive method and
also illustrating the positions and
attitude of a patient and an examiner;
FIGURE PA illustrates a format for presenting
applied forces and motions;
FIGURE us illustrates a format for presenting test
results; and
FOGGIER 3 is a graph useful yin explaining the
function of soft tissue compensation
measurements.
The method of the present invention is preferably
performed with the apparatus as described in our co-pending
application Ser. No. 469,958.
Although the present method is applicable to any
joint in the human body where it is possible to restrain
the reference portion of the joint, for ease of description,
the method will be described with respect to the knee joint.
Turning now to Figure 1, the apparatus for knee
laxity evaluation comprises an examination chair 1 with a
tilting back. The examination chair includes thigh restraints
3 and an electrogoniometer 5. The examination chair also
includes a seat 7 which incorporates a dynamometers (not
shown). The electrogoniometer and the dynamometers could be
of the type described in our co-pending application Ser.
No. 469,958.
icky
The apparatus also preferably includes a computer
hot shown) and a keyboard 9 for addressing the computer.
A display screen 11 is provided for instantaneously disk
playing the data, and a printer 13 can be provided to
provide the hard copy of the data.
The patient 15 is placed in a conformably reclining
state in the examination chair, as shown in Figure 1, with
the hip at no greater than 30 of flaxen and the knee joint
presented in a manner appropriate to physical examination by
the examiner 17. The reference portion of the joint, that
is, the femur, is secured by restraining the thigh of the
patient, and the bracing system must not unduly influence
the mechanical properties of the joint of interest.
tibia support 19 is fitted onto the leg of
interest. The tibia support includes means for attaching
the free end of the electrogoniometer.
Before the free end of the electrogoniometer is
attached, it is necessary to perform a three-dimensional
digitization of the reference and relative portions of a
joint (the femur and the tibia) for accurate measurement of
reference points and coordinate definition and placement.
For this purpose, the electrogoniometer is equipped with a
pointer which can be used, when connected to the electron
goniometer, to digitize points in three-dimensional space
as described in our co-pending application Ser. No. 469,95~.
When the knee is the join-t under consideration, seven points
are digitized in order to provide a clear picture of the
flaxen angle as well as the definition of points con-
stituting the center of the knee for the purposes of force
descriptions, and the centers of both the medial and lateral
tibia plateaus in the case of dual tests, as well as the
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coordinate systems for both the reference and relative
portions. These points, and the order of which they
digitized, are:
i) two points on -the tibia crest which are
used to define the flaxen angle with respect to
the apparatus and hence the femur;
ii) a point on the tibia tubercle at the
proximal end of the tibia;
iii) the medial tibia plateau at the proximal
end of the tibia; !'
iv) the medial femoral condole at its half
width;
v) the lateral Eemoral condole at its half
width; and
vim the lateral tibia plateau at the proximal
end of the tibia.
Obviously, if a different joint is under con-
slderation, different points will be digitized.
In order to enter the digitization data into the
computer, the pointer of the electrogoniometer is pointed
at the point of interest, and the data thus registered is
entered into the computer by, for example, pressing on a
foot-switch 21.
When the digitization process has been completed,
the soft tissue compensation procedure is begun. The first
step of the soft tissue compensation procedure is performed
with digitizer tips still attached to the electrogoniometer.
This is to provide a measurement of the distal motion of the
femur due to an applied force. In this procedure, the
digitizer tip is placed on the surface of the pettily at
approximately the point through which the femoral axis would
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pass. The opposing hand is placed behind the tibia in order
to apply an anterior load. The load applied is of the order
of 100 N. During the application of the load, the ankle of
the patient is stabilized either between the examiner's knee
or against the examiner's chest. When the force is applied,
the data for the soft tissue compensation of the femoral
distal motion is entered into the computer.
The free end of the electrogoniometer is then
connected to the tibia support 19 for the completion of
the soft tissue compensation procedure.
Femoral motion with the thigh muscles can be
defined in the following ways: proximal/distal translation,
medial/lateral translation, anterior/posterior translation,
as well as the three rotations: varus/valgus, internal/
external and flexion/extension. These femoral motions are
compensated for by remeasuring thigh stiffness within the
described restraints by applying loads to the proximal end
of the femur in the following four directions:
MEDIAL/LATERAL TRANSLATION:
A force is applied with the palm of the hand to
the bony prominence of the femoral condoles at the distal
end of the femur, first in the medial and then the lateral
directions.
ANTERIOR/~OSTERIOR TRANSLATION:
The posterior translation of the femur is measured
by applying a force posteriorly on the femoral condoles,
towards the angle as the leg is hanging at 90 degrees.
Anterior force is applied by firmly grasping the heel of the
foot and lifting the leg in an effort to displace anteriorly
the proximal end of the femur.
SUE
PROXI~AL/DISTAL TRANSITION:
The proximal/distal translation is identified by
the application of a proximal force on the pettily in the
direction of the femoral axis. It has been observed that
in the range of the forces required for most laxity testing
that the motion of the femur in the thigh in both the
proximal and distal direction is equal. Note that this is
only true in the case where the patient is reclined. Should
the patient be seated this assumption is no longer correct.
FEMORAL AXIS ROTATION:
The femoral axis rotation is caused by the creation
of an internal/external rotation in the coordinate system
of the femur. This motion can be created by applying a
medial lateral force to the ankle while the leg is hanging
at 70 degrees.
In summary, the soft tissue compensation procedure
is designed to measure the amounts of motion occurring in
the reference portion of the joint as forces are applied
directly to it. This provides a measurement of the amount
of motion of the reference bone within its surrounding tissue
as a function of applied force. This motion will, of course,
also occur when forces are applied indirectly to the
reference portion of the joint, i.e., when forces are applied
to the relative portion of the joint which in turn will apply
forces to the reference portion thereof.
The soft tissue compensation procedure is performed
by generally measuring six degrees of freedom forces and
motion generated in the reference portion of the joint using
remote transducers, in this case, the electrogoniometex and
the dynamometers Because remote transducers are used,
instrumentation is not placed between the hand of the examiner
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and the knee of the patient which may disrupt both natural
soft tissue and the proper force application.
After the soft tissue compensation procedure
has been performed, it is now possible to proceed with a
series of joint evaluation tests. This series of tests
are performed by applying a variety of forces to the
relative portion of the joint (the tibia in the case of the
knee joint) in order to generate relative displacements
between the tibia and the femur of the knee joint which is
subsequently measured by the six degree of freedom electron
goniometer while six degree of freedom forces are being
measured by the dynamometers The tests are performed
under the classical passive and functional stability test
protocol.
By using a high speed processor, in accordance
with the preferred embodiment of the invention, there can
be presented simultaneously to the examiner the applied
forces and motions as shown in Figure PA so that he can
properly control the test in three dimensions. A preferred
format for presenting the test results is illustrated in
Figure 2B herein which is self-explanatory. Preferably,
the same format is used both for the screen display and
for the hard copy output.
The forces and motion presented in the displays
should be in the coordinate system of the relative portion
of the joint the tibia) in order to permit easy and under-
stand able representation for the examiner. A detailed
explanation of how to transform the motion and forces to
the coordinate system of the relative portion of the joint
is included in our co-pending application Ser. No. 469,958.
However, briefly, in order to perform the transformation,
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the following measurements must be made: the exact position
of the relative portion of the joint measured by the electron
goniometer and the six degree of freedom forces being applied
to the joint measured at the remote dynamometers Standard
mathematical techniques are used to convert the remotely
measured forces to the coordinate system of the relative
portion of the joint using the information from the six
degree of freedom electrogoniometer.
Simultaneously, motions of the reference portion
of the joint as they were previously measured in the soft
tissue compensation procedures are subtracted out. Prefer-
ably, the processor is programmed to automatically perform
the subtraction step. The total motions of the relative
portion (the tibia) less the motions of the reference portion
(femur) previously mentioned, results in the relative motion
between the relative and reference (tibia and femur) portions
of the joint as shown, for example, in Figure 3.
The data concerning the applied forces and the
resulting motions are then presented in graphical and
tabular format as above discussed. The format must contain
at least the following details:
1. The true displacement and associated forces of
the principle loading directions.
2. A tabular summary of all of the forces and disk
placements which occurred in the joint during the examine
anion as well as the time, date and test type. The latter
is to permit the examiner to ultimately compare accurate
measurements by ensuring that all other aspects of the test
to be compared are equal or similar.
The test data results are then stored. In a system
which includes a computer, the data can be stored on a
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storage medium, for example, a floppy diskette. In addition,
in the illustrated system, hard copies can be made and stored.
The above procedure would be carried out for both
legs of the patient so that the test results can be evaluated
by bilateral comparison, that is, by comparison of the
results for one leg (presumably an injured leg) with respect
to the other leg (uninjured). If both legs are injured,
then the results can be compared to normal population
results which would be determined by independent research.
The entire procedure must not take longer/than
; thirty minutes in order to avoid exceeding comfort limits
of the patient or practical clinical requirements.
As above mentioned, although a knee laxity evil-
anion procedure was above described, the inventive method
can also be applied to other joints of the human body. For
example, the inventive method can be applied to evaluating
the elbow joint. In this case, the upper arm would be
restrained, and forces would then be applied to the lower
arm. Soft tissue compensation and digitization steps would
also be taken.
Although a particular embodiment has been described,
this was for the purpose of illustrating, but not limiting,
the invention. Various modifications, which will come
readily to the mind of one skilled in the art, are within
the scope of the invention as defined in the appended claims.
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