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Claims  |
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I claim:
1. A method for the detection of mechanical injuries in the lumbar spine of
a patient and the identification of said injuries, comprising the steps
of:
(a) providing a mathematical model of spine which is applicable to the five
lumbar vertebrae and to their disks and is capable of calculating the
distribution of moments, compression and shear forces between the
ligaments and muscles at the intervertebral joints of a human being as a
function of a load to be pulled up and of the spinal geometry and muscle
activity of said human being, said model including the pelvis as a
supporting base for the entire spine and assuming, as fundamental
hypothesis for its calculation, that any healthy person will perform a
task in such a way as to substantially minimize and equalize the stress or
an approximation thereof at each intervertebral joint;
(b) measuring with a set of surface electrodes the electromyographic (EMG)
activities of the erectores and abdominals of a patient in a bilateral and
symmetrical manner with respect to the spine of said patient while he is
flexing forward in the median plane and pulling up a small load;
(c) simultaneously measuring the angle of flexion .alpha. of said patient
while he is flexing forward, said angle .alpha. being the dihedral angle
between a plane passing through the hips and shoulders of the patient and
a vertical plane parallel to the frontal plane of said patient;
(d) supplying the measured angle .alpha. as variable input to the model and
running said model with said input to calculate the EMG activities of the
erectores and abdominals that would normally be used by a healthy person
to produce the same task;
(e) comparing the calculated EMG activities with the EMG activities
measured on the patient with the surface electrodes;
(f) tuning parameters of the model to fit the calculated EMG activities to
those measured on the patient until their differences are minimized; and
(g) detecting and identifying the mechanical injuries, if any, that may be
present in the lumbar spine of the patient as a function of the amount and
type of tuning that was necessary to complete step (f).
2. The method of claim 1, wherein the measurement of .alpha. is carried out
by:
monitoring and recording with a stop-action camera the relative positions
in the median plane of the shoulders and hips of the patient while the
same is bending forward;
digitizing with an X-Y digitizer said relative positions;
determining the angle between a line passing through said digitized
positions and a vertical line; and
using the so determined angle as angle .alpha..
3. The method of claim 2, wherein the relative positions of the shoulder
and hips are monitored at least twice per second.
4. The method of claim 2, comprising the additional steps of:
(h) measuring the lumbo-sacral angle .lambda., said angle .lambda. being
the angle between the bissector of the L.sub.5 -S.sub.1 disc and the
bissector of the T.sub.12 -L.sub.1 disc of the patient;
(i) determining with said measured angle .lambda. the angle .alpha..sub.o
at which the lordosis of the patient's spine is so reduced that the
midline ligament is activated when said patient is pulling up the small
load, said angle .alpha..sub.o being the one at which the variation of the
rate of change of .lambda. versus .alpha. is maximum; and
(j) using the so determined angle .alpha..sub.o as a parameter of
interpretation in the mathematical model, said angle .alpha..sub.o indeed
corresponding to the angle at which the patient switches the balancing of
the load being pulled up from his muscles to his ligaments in order to
reduce the stress level which the patient's spine has to sustain.
5. The method of claim 4, wherein the measurement of .lambda. is carried
out in a non-invasive manner by:
detachably fixing a string of skin-markers to the skin of the back of the
patient in the midline of his spine from at least thoracic vertebra
T.sub.10 down to at least sacral vertebra S.sub.3 ;
monitoring with a high resolution T.V. camera the relative positions of
said skin-markers in the median plane of the patient while the same is
flexing forward, said skin-markers altogether defining a curve with an
inflexion point;
locating the inflexion point on the monitored curve and tracing tangents to
said inflexion point;
measuring the angle .psi. between said tangents; and
using said angle .psi. as angle .lambda..
6. The method of claim 4, wherein the measurement of .lambda. is carried
out in a non-invasive manner by:
pressing a pair of inclinometers against the lumbar spine of the patient;
measuring with said inclinometers a range of motion of the spine (ROMS) and
a range of motion of the pelvis (ROMP) when the patient is flexing
forward; and
deriving from said ranges the lumbo-sacral angle .lambda..
7. The method of claim 4,
wherein the mathematical model provided for in step (a) makes use of a
criterion function (OF) to be optimized and minimized when the model is
run, said function (OF) having the following structure:
OF= C1*(shear)+ C2*(comp)+ C3*(mid)+ C4*(fascial)
in which:
C1, C2, C3 and C4 are parameters characterizing the relative importance of
each component of the function:
shear is the square of the enclidian norm (SEN) of the shear vector at each
joint;
comp is the SEN of the compression vector at each joint;
mid is the SEN of the midline ligament vector at each joint; and
fascia is the SEN of the abdominal muscles group vector; and
wherein the parameters to be tuned in step (f) are the parameters C1, C2,
C3 and C4 of function (OF).
8. An equipment for the detection of mechanical injuries in the lumbar
spine of a patient and the identification of said injuries, comprising:
(a) a programmed computer containing, in its programm, a mathematical model
of a spine which is applicable to the five lumbar vertebrae and to their
discs and is capable of calculating the distribution of moments,
compression and shear forces between the ligaments and muscles at the
intervertebral joints of a human being as a function of a load to be
pulled up and of the spinal geometry and muscle activity of said human
being, said model including the pelvis as a supporting base for the entire
spine and assuming, as fundamental hypothesis for its calculation, that
any healthy person will perform a task in such a way as to substantially
minimize and equalize the stress or an approximation thereof at each
intervertebral joint;
(b) a set of surface electrodes fixable onto the patient in a bilateral and
symmetrical manner with respect to his spine for measuring the
electromyographic (EMG) activities of the erectores and abdominals of said
patient while he is flexing forward in the median plane and pulling up a
small load;
(c) means for measuring the angle of flexion .alpha. of the patient, said
angle .alpha. being the dihedral angle between a plane passing through the
hips and shoulders of the patient and a vertical plane parallel to the
frontal plane of said patient;
(d) means connected to the computer for supplying the measured angle
.alpha. as variable input to the mathematical model;
(e) means forming part of the computer for running the model with its
variable input to calculate the EMG activities of the erectores and
abdominals that would normally be used by a healthy person to produce the
same task;
(f) means for comparing the calculated EMG activities to the EMG activities
measured on the patient with the surface electrodes; and
(g) means for tuning parameters of the model to fit the calculated EMG
activities to those measured on the patient until their differences are
minimized;
whereby any mechanical injuries that may be present in the lumbar spine of
the patient is detected and identified as a function of the amount and
type of tuning that was necessary to minimize the differences between the
calculated and measured EMG activities.
9. The equipment of claim 8, wherein said means for measuring the angle of
flexion .alpha. comprises:
a stop action camera for monitoring, in the median plane, the relative
positions of the shoulders and hips of the patient when the same is
bending forward;
an X-Y digitizer for digitizing said relative positions; and
means forming part of the computer programm for determining the angle
between a line passing through said digitized positions and a vertical
line, said determined angle being used as angle .alpha..
10. The equipment of claim 9, further comprising:
(i) means for measuring the lumbo sacral angle .lambda., said angle
.lambda. being the angle between the bissector of the L.sub.5 -S.sub.1
disc and the bissector of the T.sub.1 2-L.sub.1 disc of the patient;
(j) means forming part of the computer programm for determining the angle
.alpha..sub.o at which the lordosis of the patient's spine is eliminated
when said patient is pulling up a small load, said angle .alpha..sub.o
being the one at which the variation of the rate of change of .lambda.
versus .alpha. is maximum; and
(k) means forming part of the computer programm for supplying said angle
.alpha..sub.o as an interpretation parameter to the mathematical model.
11. The equipment of claim 10, wherein said means for measuring the lumbo
sacral angle .lambda. comprises:
a plurality of skin-markers attachable to the skin of the back of the
patient to form a string along the midline of his spine from at least
thoracic vertebra T.sub.1 0 down to at least sacral vertebra S.sub.3 ;
a high resolution T.V. camera for monitoring the relative positions for
said skin-markers in the median plane of the patient while the same is
flexing forward, said skin-markers all together defining a curve with an
inflexion point;
means forming part of the computer programm for locating the inflexion
point on the monitored curve;
means forming part of the computer programm for tracing tangents to said
inflexion point; and
means forming part of the computer programm for measuring the angle .psi.
between said tangents, said angle .psi. between said tangent being used as
angle .lambda..
12. The equipment of claim 10, wherein said means for measuring the lumbo
sacral angle .lambda. comprises:
a pair of inclinometers to be pressed against the lumbar spine of the
patient;
means for measuring with said inclinometers a range of motion of the spine
(ROMS) and a range of motion of the pelvius (ROMP) when the patient is
flexing forward; and
means forming part of a computer programm for deriving from said ranges the
lumbo-sacral angle .lambda. of the patient.
13. The equipment of claim 8 wherein the mathematical model comprises a
criterion function (OF) to be optimized and minimized when the model is
run, said function (OF) having the following structure:
OF=C1*(shear)+C2*(comp)+C3*(mid)+C4* (fascia)
in which:
C1, C2, C3 and C4 are parameters characterizing the relative importance of
each component of the function;
shear is the square of the enclidian norm (SEN) of the shear vector at each
joint;
comp is the SEN of the compression vector at each joint;
mid is the SEN of the midline ligament vector at each joint; and
fascia is the SEN of the abdominal muscle group vector; and
wherein the parameters to be tuned in step (g) are the parameters C1, C2,
C3 and C4 of function (OF). |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and to an equipment for the
detection of mechanical injuries in the lumbar spine of a patient and for
the identification of these injuries using a mathematical model
representative of the physiological behavior of the spine of a human
being.
2. Brief Description of the State of the Art
It is well known in the medical art that common back disorders have a
mechanical etiology. It is also well known from pathological studies that
there are two common patterns of disc injury which correspond to two
different types of mechanical failure of the spine.
The first type of common injury hereinafter referred to as "compression
injury", usually starts by a central damage to the disk with fracture of
varying magnitude of the end plates of the adjacent vertebrae, sometimes
followed by injection of part of the nucleus into the vertebral body. In
this particular case, the injured end plate permits the invasion of the
avascular nucleus and of the avascular inner portion of the annulus by
granulation tissue ingrowing through the fractured end plate, such an
invasion leading to gradual destruction of the avascular nucleus and inner
annulus. In the early stages, the facet joints of the vertebrae are not
affected and the outer annulus survives while the center portion of the
disc is destroyed. With progression, the disc loses its thickness while
the outer layer of the annulus remains relatively well preserved. With
lost of disk thickness, the facet joint subluxates and develops a moderate
degree of osteoarthritis.
Usually, the fracture of the end plate of a vertebra is an undisplaced
fracture of cancellous bone which heals rapidly. The symptoms are short
lived, typically lasting two weeks. The facet joint arthritis appears
late. At this stage, symptoms may also arise from the reduction in size of
the spinal canal (lateral or central spinal stenosis).
The other type of common injury hereinafter referred to as "torsional
injury", is characterized by a damage to the annulus occuring
simultaneously with a damage to the facet joints. The annulus is avulsed
from the end plate and its laminae become separated while the central disk
and the end plate remain intact. At the later stage, the annulus develops
radial fissures while the nucleus remains relatively untouched. The
changes in the facet joints are severed with massive joint destruction and
osteophytosis similar to hypertropic arthritis. Relatively late in the
process, there may be changes in the end-plates and central disks, with
consequent collapse of the articular surfaces and chronic synovitis.
In this particular case, the basis injury is to collageneous ligamentaous
tissue which requires six weeks to regain 60% of its strength. Because the
injury involves both the disk and facet joints, it is more difficult for
the joint to stabilize itself and recurrence is frequent. The condition is
progressive and may lead to spinal stenosis, instability and degenerative
spondilolisthesis.
Tests conducted in laboratory have shown that a compression injury is
easily produced by compressing a joint between 2 Mpa to 6 Mpa. A torsional
injury can be seen with as little as 2 to 3 degrees of forced rotation
requiring only 22 to 33 Newton-meters of torque.
Statistically, in a group of patients suffering from back disorders, 64%
exhibit torsional injuries whereas 35% exhibit axial compression injuries.
Statistics have also shown that torsional injury occurs mainly at the
L.sub.4 -L.sub.5 level (almost 76% of forth joint problems are of
torsional nature). Statistics have also shown that almost 98% of the
compression injuries occur at the L.sub.5 -S.sub.1 level. Statistics have
further shown that double injuries where the joint is injured both in
compression and torsion, occur in 22% of the cases, invariably at the
L.sub.5 -S.sub.1 level.
The following Table I reflects the probabilities of injuries among patients
complaining from backache and sciatica, or sciatica alone. As can be seen
from this Table, the important frequency of torsional injury cannot be
overlooked. As can also be seen, the probability of a third type of injury
giving symptoms is very remote.
TABLE I
______________________________________
CLINICAL DETERMINATION OF THE VARIOUS
PROBABILITIES OF INJURIES
JOINT P (injury) P (compression)
P (torsion)
______________________________________
L.sub.5 /S.sub.1
47% 98% 22%
L.sub.4 /L.sub.5
47% 1%< 76%
L.sub.3 /L.sub.4
5%< 1%< 1%<
L.sub.2 /L.sub.3
1%< 1%< 1%<
L.sub.1 /L.sub.2
1%< 1%< 1%<
100% 100% 100%
______________________________________
It is also well known that health professionals are trained to use symptoms
in the determination of diagnoses, the large numbers of known symptoms
being quite naturally associated with a large number injuries and
diagnoses. Unfortunately, as can be understood from the above short
description of the pathology in the case of back disorders, both the
compression and torsion injuries give rise to identical symptomology.
Hence, symptoms cannot be used to diagnose a type of injury because
identical symptoms may arise from different injuries.
It is also well known in the art that low back pain is the leading cause of
disability in North America today, affecting from 8 to 9 million people.
It is the most common disability in persons under the age of 45 and the
third only after arthritis and heart disease in those over 45. It is also
estimated that two of three persons will have a low back pain at some time
of their lifes, usually between the ages of 20 and 50. The fact that
problems are so common in people of working age is not coincidental.
Indeed, most of the back problems are work-related. As the injury caused
by a certain task cannot be identified from the patient's symptoms, it is
of course not possible to relate directly a given task to an injury mode,
although such a relationship is central to the definition of tasks that
will not injure a specific worker.
The economic effects of back pain and injuries are staggering. Back
problems are second only to the common cold as a cause of absenteism in
the industry. It is moreover responsible for 93 million lost workdays
every year and is a leading cause of reduced work capacity. Hence, an
incentive for prevention of back injury is very large.
In order to unequivocally relate a given task to a given injury in the
absence of any measurement of the effect of the task on a given joint, it
has already been suggested to use mathematical and/or biomechanical model
of spine, like the one suggested by J. M. Morris et al in their article
"The Role of trunk in stability of the spine, J. Bone and Joint Surg.,
43A, 1961. However, a major problem with the known models of spine,
including the widely used model of J. M. Morris et al, is that they do not
truly reflect the physiological behaviour of the spine over the full range
of capacity.
Thus, by way of example, the model of J. M. Morris et al which assumes, as
fundamental hypothesis, that the moment generated by the body weight and
any external load carried by the patient is balanced by the combined
action of the erectores spinae and the intra-abdominal pressure, is a very
poor representation of physiological behaviour which is not supported by
observations. By way of example, such a model predicts a total failure of
the spinal mechanism at about one fourth of the known potential of a
healthy spine.
The major reason why all of the models known to the inventor are defective
is essentially because they give an incomplete representation of the
actual anatomy of a human being. It is true that a moment-supporting
member is required in such a model but this cannot be the abdominal
pressure only, as suggested by J. M. Morris et al.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the detection
of mechanical injuries in the lumbar spine of a patient and for the
identification of these injuries, which method makes use of a new kind of
mathematical model of spine applicable to the five lumbar vertebrae and to
their disks.
Another object of the invention is to provide an equipment especially
designed for carrying out the above mentioned method.
A further object of the invention is to use the method according to the
invention for determining the mechanical conditions of the lumbar spine of
a patient as an indication to determine the optimal method of
rehabilitation of said spine by either a conservative method or a surgical
repair.
In accordance with the present invention, it has been noted that any
mathematical representation of the anatomy of a spine must include the
posterior ligamentous system which has indeed the strength to support any
moment generated onto the spine by the body weight and any external load
carried by the patient. It has also been noted that the extensors of the
hip have the bulk and the lever arm necessary to supply all the moment
requirements to flex the spine.
In greater details, it has been noted that, under normal circumstances,
most of the motion of an individual flexing forward from zero upright down
to about 45.degree. (for an unloaded spine), is due to spinal flexion.
From about 45.degree. to full flexion, the motion is mostly due to the
rotation of the pelvis at the hips.
In the range of 0.degree. to about 45.degree. (for an unloaded spine), the
posterior midline ligament system is inactive and, in its place, the
erectores spinae and/or the abdominal muscles support most of the moment
due to the body weight. From about 45.degree. to full flexion, this moment
can be also supported by the midline ligament system without muscular
activity. This relaxation phenomenon from muscular to ligamentous support
was already noted in the art by W. F. Floyd et al in their article "The
Function of the Erector Spinae Muscles in certain Movements and Postures
in Man", J. Physiology, volume 129, pp. 184-203, 1955.
Using electromyographic (EMG) measurements, W. F. Floyd et al clearly saw a
relation between the moment to be supported and the angle of forward
flexion, and realized the meticulous coordination of muscle, ligament and
joint movement. They hypothetized that in the case of injury to an
intervertebral joint, this delicate coordination will be upset and this
would be reflected in change of the E.M.G. pattern. Then, they embarcated
on an E.M.G. study and tried to compare statistically the E.M.G. pattern
of normal individuals to that of those with common back problems in the
performance to a standardized simple weight lifting task. However, they
gave up after testing 140 cases because the results were inconsistent.
The mathematical model used in accordance with the present invention takes
it from granted that the pelvis acts as a "supporting base" for entire
spine, and assumes as fundamental hypothesis, that any healthy person will
perform a task in such a way as to minimize and equalize the stress at
each invertebral joint.
In this model, the main power for a lift is assumed to be generated by the
extensors of the hip, such as the Gluteus Maximae.
The moment generated by these muscles is transmitted to the upper
extremities by the trunk musculature and the posterior ligamentous system
(PLS) which, for the purpose of this discussion, is composed of the
midline ligament and the lumbodorsal fascia. Regardless of the inclination
of the trunk, the moment generated by the extensors of the hip must equal
the sum of the moment generated by the trunk musculature and PLS.
Therefore, for any given hip extensor moment one can find an infinite
number of combinations to distribute this moment between trunk muscles and
the PLS.
Because of the reserve capacity in performing a small weight lift, a normal
individual may select a combination of ligaments and muscles which is not
optimum from a stress minimization and equalization point of view.
However, the reserve is reduced in the presence of injury. The option of
selecting a non-optimum strategy is also reduced. Therefore one can expect
a certain amount of variation in EMG pattern in a normal individual and a
very limited variation in those with injury.
Assuming that the distribution of moment between ligaments and muscles is
controlled by the requirement that stress be minimized and equalized at
all lumbar joints, stress at one intervertebral joint will be defined as
the ratio of the resultant compressive force acting perpendicular to the
bisector of the disk to the area of the disk. In general, when muscles are
used, the stress is higher than when either ligament systems are used,
because the lever arms of the ligament systems are longer than those of
any of the muscles. The midline ligament system can be activated only when
the spine is sufficiently flexed. The hip/shoulder angle .alpha. at which
this ligament takes up tension is called .alpha..sub.o which is about 45
degrees for no load. This ligament system is strong enough to support the
heaviest lift and hence, when this ligament system is activated, the
spinal musculature is no longer required and therefore the muscles are
electrically silent. As aforesaid, this is the muscle relaxation
phenomenon observed by W. F. Floyd et al.
The thoracodorsal fascia can be activated by the contractions of the
abdominal muscles, in particular the internal oblique and T. abdominis,
which exert a pull at its lateral margin only when the abdominal pressure
is at sufficient value to maintain a rounded abdominal cavity. This
ligament system can therefore be activated for any angle of flexion. This
is an essential difference when compared to the midline ligament.
Based on such a mathematical model, the method according to the invention
as claimed hereinafter for the detection of mechanical injuries in the
lumbar spine of the patient and the identification of these injuries
comprises a basic step of:
(a) providing a mathematical model of spine which is applicable to the five
lumbar vertebrae and to their disks and is capable of calculating the
distribution of moments, compression and shear forces between the
ligaments and muscles at the intervertebral joints of a human being as a
function of a load to be pulled up and of the spinal geometry and muscle
activity of said human being, said model including the pelvis as a
supporting base for the entire spine and assuming, as fundamental
hypothesis for its calculation, that any healthy person will perform a
task in such a way as to minimize and equalize the stress at each
intervertebral joint;
(b) measuring with a set of surface electrodes the electromyographic (EMG)
activities of the erectores and abdominals of a patient in a bilateral and
symmetrical manner with respect to the spine of said patient while he is
flexing forward in the median plane and pulling up a small load;
(c) simultaneously measuring the angle of flexion .alpha. of said patient
while he is flexing forward, said angle .alpha. being the dihedral angle
between a plane passing through the hips and shoulders of the patient and
a vertical plane parallel to the frontal plane of said patient;
(d) supplying the measured angle .alpha. as variable input to the model and
running said model with said input to calculate the EMG activities of the
erectores and abdominals that would normally be used by a healthy person
to produce the same task;
(e) comparing the calculated EMG activities with the EMG activities
measured on the patient with the surface electrodes;
(f) tuning parameters of the model to fit the calculated EMG activities to
those measured on the patient until their differences are minimized; and
(g) detecting and identifying the mechanical injuries, if any, that may be
present in the lumbar spine of the patient as a function of the amount and
type of tuning that was necessary to complete step (f).
In order to make interpretation easier, the method according to the
invention may advantageously comprise the additional steps of:
(h) measuring the lumbo-sacral angle .lambda., said angle .lambda. being
the angle between the bissector of the L.sub.5 -S.sub.1 disc and the
bissector of the T.sub.12 -L.sub.1 disc of the patient;
(i) determining with said measured angle .lambda. the angle .alpha..sub.o
at which the lordosis of the patient's spine is eliminated when said
patient is pulling up the small load, said angle .alpha..sub.o being the
one at which the variation of the rate of change of .lambda. versus
.alpha. is maximum; and
(j) using the so determined angle .alpha..sub.o as a parameter meter of
interpretation in the mathematical model, said angle .alpha..sub.o indeed
corresponding to the angle at which the patient switches the balancing of
the load being pulled up from his muscles to his ligaments in order to
reduce the stress level which the patient's spine has to sustain.
The equipment according to the invention for use to carry out the above
mentioned method comprises:
(a) a mathematical model of spine which is applicable to the five lumbar
vertebrae and to their disks and is capable of calculating the
distribution of moments, compression and shear forces between the
ligaments and muscles at the intervertebral joints of a human being as a
function of a load to be pulled up and of the spinal geometry and muscle
activity of said human being, said model including the pelvis as a
supporting base for the entire spine and assuming, as fundamental
hypothesis for its calculation, that any healthy person will perform a
task in such a way as to minimize and equalize the stress at each
intervertebral joint;
(b) a set of surface electrodes fixable onto the patient in a bilateral and
symmetrical manner with respect to his spine for measuring the
electromyographic (EMG) activities of the erectores and abdominals of said
patient while he is flexing forward in the median plane and pulling up a
small load;
(c) means for measuring the angle of flexion .alpha. of the patient, said
angle .alpha. being the dihedral angle between a plane passing through the
hips and shoulders of the patient and a vertical plane parallel to the
frontal plane of said patient;
(d) means for supplying the measured angle .alpha. as variable input to the
mathematical model;
(e) means for running the model with its variable input to calculate the
EMG activities of the erectores and abdominals that would normally be used
by a healthy person to produce the same task;
(f) means for comparing the calculated EMG activities to the EMG activities
measured on the patient with the surface electrodes;
(g) means for tuning parameters of the model to fit the calculated EMG
activities to those measured on the patient until their differences are
minimized; and
(h) means for detecting and identifying the mechanical injuries, if any,
that may be present in the lumbar spine of the patient as a function of
the amount and type of tuning that was necessary to minimize the
differences between the calculated and measured EMG activites.
As can be easily understood, the method according to the invention for
determining the mechanical condition of the lumbar spine and the extent of
breakdown of its mechanism, may be used to classify spines according to
their efficiency and thus help in matching jobs with spine conditions to
avoid needless exposure to injury, thus reducing the risk of said injury.
As a result, the overall cost of medical treatment should be reduced.
Moreover, the method according to the invention can be used as a diagnosis
tool to determine an optimal method or rehabilitation in the case of an
injured spine, by either conservative methods or surgical repair.
The method according to the invention is particularly interesting in that
it can be carried out at low cost and thus be used extensively for the
screening of individuals with common low back problems in order to
diagnose the exact mechanical fault for the purpose of prognostication,
treatment, effects of treatment and rehabilitation. The method according
to the invention may also be used to assess the individual capacity to
perform a given task or, inversally, to assess a particular job for its
risk to the individual. It may further be used to assess insurance of risk
and compensation and to select fitness or athletic training programs.
It is worth noting again that the mathematical model of spine used in the
method and equipment according to the invention assumes, as fundamental
hypothesis, that any healthy person will perform a task in such away as to
minimize and equalize the stress at each intervertebral joint. In
practise, this "stress to be minimized and equalized" can be divided into
a plurality of stress components each associated to a given source of
stress. Thus, by way of example, the stress may be divided into a
compression stress component, a shear stress component, a midline stress
component and the like.
In use, it is preferable to take into account all these possible components
when defining the stress value to be minimized and equalized. However, in
practise, use can be made in prime approximation of one or two of these
components only, as fully representative of the whole stress.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and its advantages will be better understood with reference
to the following, non restrictive description of a preferred embodiment
thereof, given in connection with the accompanying drawing in which:
FIG. 1 is a schematic view of a non-invasive equipment for the detection of
mechanical injuries in the lumbar spine of the patient and the
identification of these injuries;
FIG. 2 is a schematic representation of the identification procedure;
FIG. 3(a), (b) and (c) are schematic views of a patient in flexed position
and/or of his spine, in which views are defined the angle of flexion
.alpha., the lumbar curve angle LC and the lumbo-sacral angle .lambda.;
FIG. 4 (a) is a schematic representation of the relative positions of a
strain of markers attached to the skin along the spine in the saggital
plate, said markers being used to determine the angle .lambda. and
.alpha..sub.o ;
FIG. 4(b) is a schematic representation of the relative positions of two
inclinometers attached to the skin along the spine in the saggital plane,
said inclinometers being used to determine the angle .lambda. and .alpha..
FIGS. 5(a) and (b) are non-processed and processed E.M.G. signals of the
erectores spinae of a patient versus time;
FIG. 5(c) is a representation of the processed E.M.G. signal of FIG. 5(b)
versus .alpha.;
FIG. 6 is a simulation of the E.M.G. response for two different values of
the sacro-lumbar curve angle LC controlling the switch-over from muscle to
ligament;
FIGS. 7(a) to (c) are representation of a healty E.M.G. response of the
left erectores spinae of the patient as a function of the load to be
lifted in the case of an early extensor shut-off, a late extensor shut-off
and an intermediate strategy, respectively;
FIG. 7(d) is an overlay representation of the curves shown in FIGS 7(a) to
(c) for a 25 kg load;
FIG. 8(a) to (c) are examples of abnormal E.M.G. responses; and
FIG. 9 is a flow-chart of the model according to the invention, with
reference to its possible use.
DESCRIPTION OF A PREFERRED EMBODIMENT
A--Theoretical Considerations
As aforesaid, the mathematical model of spine used in the method according
to the invention includes the pelvis as a supporting base for the entire
spine and assumes as fundamental hypothesis for its calculation, that any
healthy person will perform a task in such a way as to minimize and
equalize the stress at each intervertebral joint. In other words, all
healthy persons are expected to perform a task in such a way as to
minimize and equalize the stress at each joint.
Of course, this does not mean that all healthy persons will execute a lift
in an identical manner. Differences in individual characteristics will
affect the overall response. However, it is essential to understand that
the overall response of any healthy individual will reflect the same
overall objectives, namely:
(1) lifting the weight; and
(2) minimizing and equalizing the stress at each intervertebral joints.
Because the control system strategy is totally independent of the task, it
is reasonable to analyze the problem by selecting a task that will
simplify computations and generating enough force at the intervertebral
joints to make measurement possible. For this reason, the choice of a
weight lifting as a model has been made, although such a choice is not
restrictive.
Based on such a choice of a weight lifting, the mathematical model used in
the method according to the invention calculates the distribution of
moments, compression and shear forces between the ligaments and muscles at
the intervertebral joints of the lumbar spine of a human being as a
function of the load to be lifted or pulled up and of the spinal geometry
and muscles activity of this human being. Originally, the range of motion
of the lumbar spine and the measurements required to locate the various
muscles and the ligaments were obtained from radiographs. The
cross-sectional area of the muscles were obtained from cross-sectional
anatomical slices. This allowed the representation of the muscles and
ligaments as vector forces with the resultant of all forces estimated as a
bissector of the disk. The forces generated by the task were estimated at
the line joining the hip and shoulder, the movement of which could be
followed in lateral photographs. The forces along this line were then
transferred to each of the five lumbar segments.
Using an optimization technique, an objective, criterium function OF
defined by its coefficients C1, C2, C3 and C4, was used to calculate the
distribution of moments between the ligaments and muscles, which
distribution produces a minimum of shear at the bissector of the
intervertebral joint. In this feedback hypothesis, the organism monitors
the shear and compression forces at the joint and uses this to select the
best combination of muscles and ligaments to accomplish a given task.
This, in practice, constitutes the control system of the model.
The model used in the method according to the invention makes use of
substantially the same function OF. A full description of this
mathematical model of spine is given in the Master Engineering Thesis of
Albert R. CARBONE which thesis is entitled "A muscular response model the
human lumbar spine the performance of a saggital plane dead lift",
Concordia University, Montreal, Mar. 1984. In this particular model, the
objective function OF to be minimized has the following structure:
OF =C1* (shear) +C2* (comp) +C3* (mid) +C4* (fascia) in which:
C1, C2, C3 and C4 are parameters characterizing the relative importance of
each component of the function:
shear is the square of the enclidian norm (SEN) of the shear vector at each
joint;
comp is the SEN of the compression vector at each joint;
mid is the SEN of the midline ligament vector at each joint; and
fascia is the SEN of the abodminal muscles group vector.
In the above formula, the expression "abdominal muscles" includes the
following muscles: external obliques, internal obliques and T. abdominis.
It should be noted that while the hypothesis used in this mathematical
model of stress minimization cannot be tested by direct measurements, the
deduction from theory can be subjected to experimental verification with
certain measurements reported from observations on volunteers performing
light tasks.
By way of example, the calculated, integrated E.M.G. value of the
sacrospinalis and the multifidus muscles are disclosed in the litterature
as being substantially linear for a range of weight listed from 0 to 40
kg. This linear relationship also exists in the model used in the method
according to the invention. In addition, it has been found that by using
an appropriate conversion factor, the muscle activity calculated with the
spine model according to the invention can be superimposed on the
experimental data.
In the litterature, it is also disclosed that there is a linear
relationship between the disk pressure and the weight supported by the
spinal column in the case of a small weight. This finding is confirmed
with the calculation of the mathematical model used in accordance with the
invention.
Similar correlations can be obtained between the calculated values of the
abdominal pressure, the moment of directores and the angle of spine
flexion at which the midline ligament is first brought under tension, and
the data collected in the litterature.
As a result, it may be taken for granted that the unique response of the
mathematical model used in accordance with the invention actually reflects
the basic relationship between the various components of the spine system
of a human being. This basic relationship relates the stress distribution
to the size and shape of the structural units composing the individual
spine. It also takes into consideration the degree of lumbar curve, the
elasticity of a ligamentous structure and the firing density of
contractile muscles.
It should be noted that in the mathematical model used in accordance with
the invention, a physiological loading system is yielded, where the
resultant force is always substantially maintained at 90.degree. to the
bissector of the disk. This allows the conclusion that the facets, dispite
current teaching, take negligible compression load because they are
oriented at 90.degree. to the bissector of the disk. However, the facet
joints and the disk are responsible for supporting all the shear stresses.
Asymmetry of an intervertebral joint may reflects an asymmetric shear
stress and the degree of asymmetry reflects the differences in shear
stress between right and left sides of the joint. The disk does not have
the structural design to support shear stress. The obvious protective
mechanism would be (1) asymmetric facet joint and (2) asymmetric muscular
response.
With an increased load, a change in the fixed structure cannot be expected
and therefore the only protective mechanism would be an asymmetric muscle
response small at low loads, but increasing with the magnitude of the
task. It is known that the compensation is not perfect because of the
asymmetric degeneration found in asymmetric joints. Therefore, there is a
weight range where the compensation works and, outside of this, it does
not.
In practise, the differences in E.M.G. output between right (R) and left
(L) sides are due to a variety of factors such as the skin resistance,
variability of muscle motor points and asymmetry. In there were no
asymmetry, then R should be equal to L. But factors other than asymmetry
introduce a difference between R and L. If we assume that there is a
constant gain difference between R and L because of asymmetric structure,
then this difference can be identified. The R and L integrated output is
collected at each millisecond. The sum of the square of all (R-aL) is
obtained and minimized with respect to "a". The value of this parameter
"a", which in the ideal case of perfect symmetry is equal to one,
characterizes the irreducible asymmetry between R and L and is arbitrarily
assigned to describe the asymmetry of the joint.
As a first approximation, the small contribution of the disk to the support
of shear force is ignored. Now, if it is true that stress if equalized,
then the geometrical differences between right and left facets must be
reflected in the differences in E.M.G. output of the two sides. The shear
induced by bodyweight and muscles in the asymmetric joint will induce a
torque which must be compensated by the size and distribution of the
facets joints. The bodyweight and muscles contribution to shear stress
(R+L) can be calculated. The moment produced by muscles must be sufficient
to balance bodyweight in relaxed upright stance.
When an external load is added, it becomes more difficult for the facet
joints to absorb the increased asymmetric stress. The torque at the IV
joint is increased and the joint tends to rotate. To compensate, an
asymmetric muscle response is required.
The simple joint model thus proposed relates EMG activity and joint
asymmetry to the shear component of stress induced by an axial compression
load. This joint model is particularly interesting in that it obviates the
need for complex IV joint models.
B--Equipment
As shown in FIG. 1, the equipment 1 necessary for the detection of
mechanical injury in the lumbar spine of a patient P and for the
identification of the so detected injuries comprises a computer 3
preprogrammed with the above mentioned mathematical model of spine
applicable to the five lumbar vertebrae of the patient B and to their
disk. As computer 3, use can be made of a standard IBM-PC or HP-9000
computer (trademark). The equipment 1 also comprises a first set of
surface electrodes 5 fixable onto the | | |
