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OCR for page 48
4
Injury Biomechanics Research and
the Prevention of Impact Injury
Improved protection against injury can be realized
through a better understanding of the biomechanics of
injury and disability. This chapter reviews the current
state of knowledge on injury biomechanics and presents
recommendations to advance the field by forming university
centers for research, by training engineers and scientists
in injury biomechanics, and by ensuring a long-term com-
mitment of funds and leadership. By investing in the
needed research, we can reduce injuries and preserve the
well-being of countless people.
STATE OF INJURY BlOMECHANICS RESEARCH
Impact injury of the human body occurs by deformation
of tissues beyond their failure limits, which results in
damage of anatomic structures or alteration in function.
Even if there is recovery from structural injury, normal
physiologic function does not always return. For example,
bony fractures can heal, but associated damage to the
central nervous tissue might result in a permanent loss
of motor and sensory function.
Injury b~omechanics research uses the principles of
mechanics to explore the mechanisms of physical and
physiologic responses to mechanical forces. Injury is
caused by penetrating or nonpenetrating blows to the
body, and the energy delivered and the area of contact
are important determinants of the results. Penetrating
injuries are caused by high-speed projectiles, such as
bullets, or by sharp objects moving at lower speeds, such
as knives.) ~. Penetrating injury generally involves
a concentration of mechanical energy in a small area of
the body. Nonpenetrating injuries are caused by blunt
48
OCR for page 49
49
ob jects that distr ibute energy over larger areas at a
wide range of speeds. . 3 ~ a ~ ~ 7 7 ~ 9 3 Although in jury
can occur by slow deformation of the body, such as in
crushing, the predominant features of impact in jury are
speed and violence, as in the rapid impact of the chest
on the instrument panel of an automobile or a bullet's
penetration into the chest cavity.
Research in biomechanics involves a variety of disci-
plines, including engineering, physiology, medicine,
biology , and anatomy. Thus, there is no unified approach
or single area of training, education, or exper fence .
Research is often conducted by teams of engineering,
medical, and other personnel, and the combining of such
disciplines is an important element of a successful study.
Detailed reviews of the literature on var ious aspects of
in jury biomechanics are available,. 2 9 6 ~ O ~ ~ ~ ~ ~ 9 ~ 2 0 S
but the research is so broad that no publication can cover
the entire scope of this subject.
MECHANISMS OF INJURY
The severity of an impact depends not only on the ve-
locity of the collision that produces it, but also on the
shapes of the colliding objects and their rigidity. It
can be reduced by energy-absorbing structures and padding
material by allowing simultaneous deformation of the body
and of the surface collided with. Il7 l.9 l'l i93
This extends the duration of impact and reduces the r isk
of injury.
Because of the inertial resistance of the body tis-
sues and the elastic and viscous compliance of body struc-
tures, force is developed on the body during impact.
Force deforms and accelerates the body.
4-1) can be caused by:
Injury (Figure
· Crushing deformation of the body, such as through
chest compression, rib fracture, and sortie laceration.
· Impulsive impact, such as by violent sternal
motion that deforms the heart beyond its viscous
tolerance and causes contusion and rupture.
· Acceleration of the skeleton and tearing of
internal organs, because of their inertia; for example,
during head impact, the skull accelerates and the loosely
attached brain lags,.. 'I 12] 129 \~l So injury is due
in part to deformation of brain tissues beyond their
limit of recovery.
OCR for page 50
so
~1
Elastic
At,
~ ~0
V iscous
my,
~ \
~ /
I/
SN/`
I nertial
FIGURE 4-1 Three pr incipal mechanisms of impact in jury:
left, compression of the body and injury when the crush
exceeds elastic tolerances; center, high-speed impact
with in jury when violence exceeds viscous tolerance; and
right, body acceleration when internal organ motion lags
the skeleton with injury due to organ inertia.
A mechanism of injury involves the mechanical deforma-
tions and physiologic responses that cause an anatomic
lesion or functional change. Knowledge of injury mechan-
isms is fundamental to the science of in jury biomechanics,
because it points to the appropr late biomechanical mea-
surements that characterize injuries.
The human body has viscoelastic tissues that absorb
energy and protect vital organs from the effects of
impact. As long as the energy delivered to the tissue is
below the limit of injury--whether it be the crush
limit for for 137 20\ Me viscous limitless 106 203
or the acceleration limit ~ ~ 6 ~ 9 S--tbe energy will be
absorbed without causing injury. The resistance of the
human body to impact i'; responsible for survival of falls
from extreme heightsS. ~° ~62 am -t'~~;*'-' me -~''~^
". - - i. ~ Ace, is, - all. ~
motor-vehicle crashes. ~ ~ 6 ~ ~ 7 ~ a ~ Even though the body
can survive great impact, the frequency and variety of
impact incidents are so great that they constitute one of
the leading causes of ser ious in jury , death , and disabil-
ity in the United States. Ef fective in jury-prevention
strategies must be based on knowledge of the mechanisms
of injury and disability, as well of the body's biomech-
anical responses and tolerances and techniques for
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51
assessing the injury-prevention benefits of safety
technology.
Deformation of tissues beyond a recoverable limit is
the most common origin of in jury. s g 6 5 ~ 2 ~ From an
engineering point of view, deformation of a tissue or
structure is measured according to change in shape (such
as the change in length divided by the in itial length ) or
strain. 6 2 The two main types of strain (Figure 4-2)
that can damage tissue are tensile strain and shear
strain; a third type is compressive strain, which is
responsible for crushing injuries. Stretching of an
artery increases its length and increases strain. If the
strain is too great, the tissue will break . There are
many ways to stretch tissue and thus produce tensile
strain. For example, the motion of the heart during
TENSILE STRAIN
SHEAR STRAIN
-
-
.%, If:
cV~O Hemorrhage
~50:__
( ~ ~Adventitial Seal
fit
Pseudo~aneurism
(pulsating hematorna)
lit' do,
-or ~`
ado'
i"\ (A Hemorrhage
A) Adventitial Seal
Pseudo~neurism
(pulsating hematomaJ
FIGURE 4-2 Stretch of vessel can tear tissue
(partial tear shown) with loss or containment of
blood. Opposing forces across vessel can cause
shear injury (complete tear shown) with or
without loss of blood.
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S2
chest compression stretches the aorta near points of
attachment; the stretch is primarily along the axis of
the vessel and generally leads to a transverse laceration
when the recoverable limit of tissue strain is
exceeded.~9 An increase in vascular pressure dilates
blood vessels and produces tensile strain in the tissue;
in this case, the strain is in both axial and transverse
directions (i.e., it is biaxial), and pressures beyond
the recoverable limit cause a bursting of the tissue.
Axial impact on the femur causes an increase in its
curvature, tensile strain on its enter for surface, and
compressive strain on its posterior surface. Midshaft
failure of the femur occurs when the tensile strain
exceeds the recoverable limit. s 2 2 o ~ This mechanism is
common in the r ibs, where compression of the chest causes
tensile strain on the outer surfaces. Bending failure of
bone and failure of vessel walls are two common examples
of tensile failure due to tissue stretch.
Shear strain occurs when forces oppose each other
across a tissue (see Figure 4-2 ) . The movement of tissue
in opposite directions separates it when the recoverable
limit is exceeded. For example, the dif ferential
movement of the brain with respect to the accelerated
skull during head impact causes a combination of shear
and tensile strain at the interface between brain and
skull. ~3 Damage occurs when the strain exceeds the
resistance of the tissue. Differential motion of lobes
of the liver can shear and lacerate hepatic vessels when
the strain exceeds the recoverable limit.~°` to 6
Stretch and shear of tissue are the primary mechanisms
of laceration, fracture, rupture, and avulsion in the
human body. The strain mechanisms commonly occur when an
organ moves relative to its attachment during deformation
of the body. This strain explains major vessel ruptures
in the liver during deformation of the abdomen and
laceration of the aorta due to compression of the chest.
Deformation or strain is also a principal factor in
contusion injury (Figure 4-3). In this case, the surface
of the tissue is not damaged, but the deformations stretch
and shear internal vessels and increase intraluminal pres-
sures, which can damage vessels and initiate hemorrhage.
The rate of loading, or strain rate, is important in
the production of injury. Biologic tissues are visco-
elastic, and their response and tolerance depend on both
strain and strain rate. 6 ~ 6 S 2 °° For example, compact
bone fails at a lower strain applied at higher rates,
even though the load it carries at failure is higher.
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53
), -rl, If' Spasm
-I (O)J ~ ~ \: ~ j Local Thrombosis
\
57 True Aneurism
FIGURE 4-3 Vessels beneath sk in can be torn by
stretch or shear with laceration of skin,
resulting in bruise, or contusion.
The rate of loading is critical in soft-tissue injury,
particularly when the viscous tolerance of the tissue is
exceeded.~° 6 2 0 3 This tolerance is proportional to the
product of loading rate and amount of compression. The
faster a tissue is loaded, the lower is its tolerance to
compression. The viscous in jury mechanism is important
in cases of high-speed impact, particularly when rupture
and contusion occur.
RESEARCH NEEDS
Injury Mechanisms
Gaining knowledge of the mechanisms of injury is the
first step in injury biomechanics research. It permits
an understanding of the deformations associated with gross
anatomic lesions or damage to biologic tissues that result
in functional change. The study of injury meabanisms
makes use of clinical and accident data and data from
experiments with human cadavers and anesthetized
animal`;. Mechanisms of many of the important in jury-
related anatomic lesions have been studied. For example,
the mechanisms of hear t and great-vessel rupture have
been discussed extensively; ~ ~ ~ ° ~ the sequence of
events that follow blunt impact and lead to rupture and
laceration has been descr ibed ; and f racture of the femur
and dislocation of the knee are generally well under-
stood,~39 2 o 2 although much less is known about
injuries of the other long bones and many joints.
OCR for page 54
54
Cerv ical spine in jury has been emphas ized, because of
the ser ious functional consequences associated with spinal
cord damage . 3 ~ Neck in juries occur in rollover automo-
bile crashes, shallow-water diving, and contact sports.
Although the literature and hypotheses on fracture-
d islocation of the neck are extensive, 8. 94 iso ~ 7 2
little of the underlying sequence of injury events has
been verified. The mechanisms of functional damage to
the central nervous system (CNS) are less well estab-
lished, primarily because the problem has not been
studied from the point of view of impact deformation and
functional loss. Although isolated tissue preparations
have provided information on the underlying mechanisms of
functional change, the problem is complicated by the
abundance of physiologic responses to injury. But CNS
injury is paramount, because it so often causes functional
disability 99 ~ 3 ~ ~ S 7 ~ 9 2 We need to study the sequence
of events and biomechanics of impact injury to the CNS.
Many of the mechanisms responsible for functional change
are speculative and generally unknown, but they deserve a
balance of biomechanical and physiologic research.
Measurement of Biomechanical Responses
The first step in understanding biomechanical
responses is to measure changes in shape of the body,
organ, or tissue caused by impact. ~ ° ~ ~ 2 ~ ~ 9 ~ The
elastic and viscous resistance of biologic tissue to
impact deformation and the inertial resistance of the
body to motion must be characterized. Measurement of
biomechanical responses is used to analyze the injury
process. Studies of volunteers, limited by rigid
regulations and guidelines, can be conducted with impacts
below the pain threshold. Several military research
facilities use military volunteer subjects, but the
United States has only 8 few research facilities for
civilian impact experiments. Although noninjurious
responses can be measured in volunteer experiments, the
basic study of impact responses must use surrogate humans.
The primary research tools to evaluate injurious bio-
mechanical responses are human cadavers and anesthetized
animals that are exposed to impact and detailed response
measurement. Cadavers are suitable research models that
simulate gross geometric and material properties of living
humans, and they are often used to study kinematics, such
as the motion of a body during deceleration, or the
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55
mechanical response of a body segment, such as deformation
of the chest. Animal research is vital in obtaining
information on physiologic responses tr iggered by in jury;
there is no other way to simulate pathophysiologic
responses or itical to developing information on disabling
brain and spinal cord injuries and life-threatening
arrhythmia or shock.
Mathematical models9' i 8 8 ~9° and anthropomorphic
dummiesS 7 ~ 2 ~ ~ 2 ~ are used extensively as predictive
tools, particularly in connection with body k inematics
and acceleration during impact. However, simulations are
only as accurate as the biomechanical information used in
their formulation. Mathematical models generally lack
accuracy for interactive forces developed during contact.
In many instances, the lack of basic research data on
biomechanical responses has retarded the development of
more sophisticated models or dummies.
Tolerance
The threshold of injury is a degree of deformation,
energy absorption, or biomechanical response beyond which
damage occurs to the tissue or structure. Damage in this
case can be a gross anatomic lesion or an injury that
results in a permanent alteration of function. The
threshold is not fixed, but is a function of a variety of
characteristics, including the type of tissue and the
type of test subject. Our current knowledge concerning
human impact tolerances is sparse, and experimentally
derived data on women, children, and other segments of
the population are highly limited.
Determination of human tolerances to impact is compli-
cated by many factors, including the magnitude, direction,
distribution, duration, and pulse shape of the force of
the impact; the body orientation; tightness and configura-
tion of restraint; and structure of the striking object.
Biologic factors may also influence human tolerance,
including sex, age, physical and mental condition, and
body size.70 ~° i94 Individual variability must be
considered, because tolerance under identical test
conditions can vary in the same person, as well as from
one person to another. Furthermore, although data are
available on impact forces in some body orientations,
such as forward or rearward, less is known about the
effects of lateral or multiple-direction impact.
OCR for page 56
::
S6
i
Thus, it is not possible to state the human tolerance
to impact definitively without knowing specific condi-
tions, because tolerance depends on so many conditions.
In fact, there is likely to be a distr ibution of toler-
ances for a given population and a given impact.
Falls provide a means of estimating human tolerances
to impact, and particularly to extreme impact beyond
which volunteers may not be subjected.S. '8° I.2 l8.
Data from falls have limited value, however, because
responses are not measured during the event, and informa-
tion on the impact dynamics must be reconstructed after
the fact. Other estimates of impact tolerance are
obtained from clinical studies of impact injury and from
reconstruction of automotive crashes and aircraf t
crashes. i , 6 ~ 7 9 In these cases, although the
resulting injuries are known, impact conditions can only
be estimated.
Assessment of Safety Technology
A research goal is to develop a test tool, such as a
dummy, and a test method, such as a crash simulation, to
study the effectiveness of safety technology. ~ 2 ~ ~ ~ 9
If a test is sufficiently representative of a range of
exposures in which injury might occur, the test tool and
method can be used to a';sess the r isk of in jury and
disability.
Automotive crashes are violent events of short dura-
tion, so they are typically studied in the laboratory
with dummies. The purpose of such studies is to evaluate
methods for reducing the overall risk of impact injury to
havens, but dummies provide only the most basic informa-
tion, which usually cannot be correlated accurately with
human r esponse and injury. For anatomic injur ies , the
predictive capacity of dummy tests is marginal. More
important, current dummies cannot be used to evaluate
functional changes that result in severe cognitive
dysfunction, in quadriplegic, or in fatal ventricular
fibr illation .
Advances have recently been made in the mathematical
simulation of crashes, - ~° and this simulation can
be used to ';tudy the effects of a wide range of design
changes and improvements.
Thus, current Cools and techniques constitute only a
first step in the development of adequate evaluative
procedures based on understanding of trauma mechanisms,
biomechanical responses, and tolerance of humans .
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57
Other Factors in Injury
Many factors influence the occurrence, severity, and
outcome of impact. Hypertension and arteriosclerosis
tend to increase the severity of cardiovascular injury
and might be important in injuries among the older
population. Osteoporosis, a disease of the skeleton that
reduces the impact resistance of bone, s 2 S 9 6 S is a
factor in skeletal injury, particularly in compression
fractures of vertebrae and fractures of the femoral neck
and ribs. This disease is more likely to affect older
women, reinforcing the belief that age, sex, and well-
being influence injury.
Chronic use of alcoholic beverages interferes with
normal body repair processes and is important in injury
causation. It is now becoming evident that use of
alcoholic beverages predisposes to more severe and
extensive injury than would be experienced by nondrinkers,
given impact of the same severity S6 IS 10?
Alcoholic beverage use that produces even moderate or low
blood alcohol concentration can signif icantly increase
the fatality rate associated with cardiac injury and the
debilitating effects associated with CNS damage.
Physiologic experiments have demonstrated a higher
fatality rate associated with a combination of acute
alcoholic-beverage use and blunt thoracic impact than
that associated with impact alc~ne.~°' A. The
combination of alcohol and impact impairs cardiac
performance, and death is caused by electromechanical
dissociation, or decoupling, of the excitation-
contraction process. Because of the high incidence of
alcoholic beverage use by seriously and fatally injured
road-accident victims (33 percent of seriously injured
and 50 percent of fatally in jured), the reduced tolerance
associated with moderate blood alcohol concentration
might be one of the most important factors influencing
injury, but little is known about the mechanism for this .
CONCLUS IONS
The current state of knowledge of injury mechanisms,
the understanding of impact responses and tolerances, and
the availability of useful technology for injury evalua-
tion were analyzed by the Committee on Trauma Research
and are summarized in Table 4-1. There is a reasonable
understanding of mechanisms of anatomic damage, but the
OCR for page 58
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OCR for page 61
61
mechanisms of functional changes are less well understood.
Very little is known regarding the mechanisms of brain
and spinal cord damage. These tissues control life
processes and cannot be adequately protected if the
mechanisms of functional in jury to them are not better
understood. A better understanding of impairment of
cardiac function due to thoracic impact is also needed.
The understanding of impact responses and tolerances
is not as advanced as that of mechanisms of anatomic
injury, because of the lack of experimental models for
research to measure biomechanical responses and tolerances
of humans. The use of human cadavers has allowed the
assessment of the response of the ribcage to impact and
the response of the femur to axial loading at the
knee,~S ]03 200 but most recent research has emphasized
the biomechanical responses and tolerances of the chest
and extremities. Earlier cadaver experiments involving
head impact provided the basis for the current understand-
ing of the heads impact response and tolerance . ~ 2 ~ ~ 2 9
However, the current techniques are not sufficient to
assess the risk of severe and moderate in jury to the
brain with confidence. The measurement of head-impact
responses and tolerances is an urgent sub ject for research
and must use a physiologic model for study. The frequency
of head impact is high, and the consequences of brain
damage are severe.'s ' t ~ 2 ~ ~ ~ O ~ ~ ~ ~ S 7 Of similar
importance is development of response and tolerance data
on the spinal cord. The seriousness of quadriplegic is
obvious, and useful measures of the r isk of damage to the
central nervous system are needed. l.7 Although
hypotheses; are available for the mechanism of cervical
spine fracture-dislocation, there is a paucity of
experimental verification and a lack of correlation of
fracture-dialocation with spinal cord damage. Information
on the impact responses of the abdomen and external
tissues is also sparse.
Test dummies are the primary tools for predicting
injury, but only a few measures of potential injury are
assessed with current techniques during impact tests.
The most common ones are the acceleration responses of
the head and chest and the measurement of force applied
to the femur.5' i21 12. The measurement of femoral
forces is well accepted and used, but the assessment of
femoral injury is not based on the underlying mechanism
of injury, which is bending and not axial compression.
The criterion for evaluating the risk of head injury has
been well publicized, but it has limited exper isaental
OCR for page 62
62
verification and has not been correlated with the risk of
brain damage or facial injury. Limited experimental
criteria are available for evaluating neck in jury, but
they do not assess functional changes associated with the
risk of quadriplegic. Clearly, the two most.important
body regions, the head and neck, are inadequately
evaluated with the current testing technology. but they
are regions that suffer the most harm in motor-vehicle
crashes.29
RECO - ENDATIONS
1. A multidisciplinary approach to injury bio-
mechanics research should be coordinated to include:
· Injury investigation, in jury-mechanism study,
biomedical research on response and tolerance,
study of pathophysiologic response to impact, and
research on disabling injuries, particularly to
the central nervous system.
· Support for the training of scientists and
engineers in injury biomechanics, to overcome a
serious shortage of such workers.
.
Support and incentives for established
investigators on university faculties to develop
curricula in the mechanics and physiology of
in jury .
.
Development and nurturing of a core of
leaders by establishing university centers for
scientific study of injury and disability.
Adequate support for long-term and applied
research should be emphasized and an effort made
to curb the rapid loss of trained researchers
primarily to service as expert witnesses in
lawsuits.
· Maintenance of a balanced extramural research
program managed by engineers, physiologists, and
physicians.
· Foster ing of cooperation and exchange of
information with existing federal organizations.
.
Promotion of the development of information
on important problems in trauma, independent of
the regulatory function of the government.
2. High priority should be given to research that can
provide a clearer understanding of injury mechanisms.
The crucial subjects of research are as follows:
OCR for page 63
63
.
The relative contr ibution of linear and
angular acceleration of the head to deformation
and in jury of brain and spinal cord .
· Ver if ication of proposed mechan isms of in jury
to the neck--head motion can impose a variety of
combined axial, bending, and shear loads that
r esult in many k inds of in jury.
.
Verification of mechanisms of in jury to the
thoracic and abdominal viscera; these mechanisms
vary from unknown to slightly understood.
· Musculoskeletal in jury that leads to
functional joint disability.
3. Research in mechanical responses requires
sustained support. The responses of most body regions to
mechanical loading have not been measured adequately.
The critical regions are the following:
.
Central nervous system, where the accurate
measurement of linear and angular acceleration is
needed for use in b~omechanics exper iments .
· Thoracic viscera, including motion of
internal organs and vessels that leads to injury.
· Abdominal viscera.
.
knee .
Joints, with the possible exception of the
· Muscles and peripheral nerves.
4. High priority should be given to obtaining and
defining limits of human tolerance to injury, particularly
with regard to the following general subjects :
.
Segments of the population on which data are
extremely limited, including children, women, and
the aged.
· Both whole-body and regional tolerances, to
provide an improved basis for design of less
hazardous products and environments.
· The effect of variables that influence and
modify tolerance, such as substance abuse and
energy-attenuating and restraint systems.
· Physiologic tolerances, particularly in the
central nervous system.
· Long-term effects of rapid deceleration on
the body, particularly the brain, spinal cord, and
joints.
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64
· Survival of extreme impact, to provide a
better basis for understanding the limits of
tolerance.
5. Improvement in injury-assessment technology is
needed. Although some tools and techniques are available
for this purpose, they are inadequate, because of the
wide diversity of potential injury and disability. Work
in this field should include:
· Development of means of assessing the
important debilitating injuries and causes of
fatality.
· Improvement of anthropomorphic dummies.
· Development of computer models that can be
used to predict injury and response in complex
crash conditions.
Representative terms from entire chapter:
tensile strain
