Advanced Trauma Life Support ATLS Student Course Manual 2018

402
BIOMECHANICS OF INJURY
Ejection
The likelihood of serious injury increases by more
than 300% when the occupant is ejected from the
vehicle. Injuries may be sustained within the vehicle
during the collision and on impact with the ground or
other objects.
Organ Collision
Types of organ collision injuries include compression
injury and deceleration injury. Restraint use is a key
factor in reducing injury.
Compression Injury
Compression injuries occur when the torso ceases
to move forward, but the internal organs continue
their motion. The organs are compressed from
behind by the advancing posterior thoracoabdominal
wall and the vertebral column, and in
front by the impacted anterior structures. Blunt
myocardial injury is a typical example of this type of
injury mechanism.
Similar injury may occur in lung parenchyma and
abdominal organs. In a collision, it is instinctive for
the vehicle occupant to take a deep breath and hold it,
closing the glottis. Compression of the thorax produces
alveolar rupture with a resultant pneumothorax and/or
tension pneumothorax. The increase in intraabdominal
pressure may produce diaphragmatic rupture and
translocation of abdominal organs into the thoracic
cavity. Compression injuries to the brain may also
occur. Movement of the head associated with the
application of a force through impact can be associated
with rapid acceleration forces applied to the brain.
Compression injuries also may occur as a result of
depressed skull fractures.
Deceleration Injury
Deceleration injuries often occur at the junction of fixed
and mobile structures. Examples include the proximal
jejunum, distal ilium, and proximal descending thoracic
aorta. The fixed structure is tethered while the mobile
structure continues to move. The result is a shearing
force. This mechanism causes traumatic aortic rupture.
With rapid deceleration, as occurs in high-speed frontal
impact, the proximal descending aorta is in motion
relative to the distal aorta. The shear forces are greatest
where the arch and the stable descending aorta join at
the ligamentum arteriosum.
This mechanism of injury also may cause avulsion of
the spleen and kidney at their pedicles, as well as in the
skull when the posterior part of the brain separates from
the skull, tearing blood vessels with resultant bleeding.
Numerous attachments of the dura, arachnoid, and pia
inside the cranial vault effectively separate the brain
into multiple compartments. These compartments
are subjected to shear stress from acceleration,
deceleration, and rotational forces. The vertebral
column can also be subjected to shearing between
fixed and mobile elements such as the junction of the
cervical and thoracic spine and that of the thoracic
and lumbar spine.
Restraint Use
The value of passenger restraints in reducing injury has
been so well established that it is no longer debated.
When used properly, current 3-point restraints have
been shown to reduce fatalities by 65% to 70% and to
produce a 10-fold reduction in serious injury. At present,
the greatest failure of the device is the occupant’s
refusal to use the system. A restrained occupant who
is not properly positioned in the vehicle does not reap
the full benefit of the 3-point restraint system.
The value of occupant restraint devices can be
illustrated as follows: A restrained driver and the
vehicle travel at the same speed and brake to a stop
with a deceleration of 0.5 × g (16 ft/sec 2 , or 4.8 m/
sec 2 ). During the 0.01 second it takes for the inertial
mechanism to lock the safety belt and couple the driver
to the vehicle, the driver moves an additional 6.1 inches
(15.25 cm) inside the passenger compartment.
Air bags were widely available in most vehicles in the
mid-1990s. The most common are front impact, but
head curtain and side-impact air bags are also available
on many newer models. The increasing availability of
air bags in vehicles may significantly reduce injuries
to the head, chest, and abdomen sustained in frontal
impacts. However, air bags are beneficial only in
approximately 70% of collisions. These devices are
not replacements for the safety belt and are designed
as supplemental protective devices. Occupants in
head-on collisions may benefit from the deployment
of an air bag, but only on the first impact. If there is a
second impact into another object, the bag is already
deployed and deflated and thus is no longer available
for protection. Frontal air bags provide no protection in
rollovers, second crashes, or lateral or rear impacts. The
3-point restraint system must be used. Side air bags are
generally seat mounted, are smaller, dissipate energy
in a side-impact collision, and provide some protection
in a lateral crash. Curtain air bags deploy from the roof
rails, are larger, and stay inflated longer. They provide
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