Advanced Trauma Life Support ATLS Student Course Manual 2018

403
BIOMECHANICS OF INJURY
improved protection for the head, neck, and chest. By
staying inflated longer, they protect vehicle occupants
in impacts with secondary impact and in rollovers.
Currently, maximum protection is provided only with
the simultaneous use of both seat belts and air bags.
When worn correctly, safety belts can reduce
injuries. When worn incorrectly—for example,
above the anterior/superior iliac spines—the forward
motion of the posterior abdominal wall and vertebral
column traps the pancreas, liver, spleen, small bowel,
duodenum, and kidney against the belt in front. Burst
injuries and lacerations of these organs can occur. As
shown in n FIGURE 1, hyperflexion over an incorrectly
applied belt can produce anterior compression fractures
of the lumbar spine and flexion-distraction injuries
through a vertebra (Chance fractures). Proper use
and positioning of the 3-point restraint system and
appropriate occupant position will minimize the risk
of injury in a collision.
Pedestrian Injury
It is estimated that nearly 90% of all pedestrian–auto
collisions occur at speeds of less than 30 mph (48 kph).
Children constitute an exceptionally high percentage
of those injured by collision with a vehicle, since they
often “dart” into the street midblock and are hit by a
vehicle at higher speed. Thoracic, head, and lowerextremity
injuries (in that order) account for most of
the injuries sustained by pedestrians.
The injuries sustained by a pedestrian involve three
impact phases: impact with the vehicle bumper,
impact with the vehicle hood and windshield as
the pedestrian rotates around the vehicle’s leading
edge, and a final impact with the ground. Lowerextremity
injury occurs when the vehicle bumper
is impacted; the head and torso are injured by
impact with the hood and windshield; and the head,
spine, and extremities are injured by impact with
the ground.
Injury to Cyclists
Cyclists and/or their passengers also can sustain
compression, acceleration/deceleration, and shearing
injuries. Cyclists are not protected by the vehicle’s
structure or restraining devices in the way occupants
of an automobile are. Cyclists are protected only by
clothing and safety devices such as helmets, boots, and
protective clothing. Only the helmet has the ability to
redistribute the energy transmission and reduce its
intensity, and even this capability is limited. Obviously,
the less protection the cyclist wears, the greater the
risk for injury. Concerns that the use of bicycle and
motorcycle helmets increases the risk of injury below
the head, especially cervical spine injury, have not
been substantiated.
Motorcyclists who are thrown forward often rotate
and land on their upper thoracic spine, fracturing
multiple thoracic vertebra. These patients commonly
complain of pain between the shoulder blades or have
a widened paravertebral strip on initial chest x-ray. Use
caution before sitting them up. Pelvic and long-bone
fractures are also common.
Falls
n FIGURE 1 Safety Restraints. When worn correctly, safety belts
can reduce injuries. When worn incorrectly, as shown here, burst
injuries and organ lacerations can occur. Hyperflexion over an
incorrectly applied belt can produce anterior compression fractures
of the lumbar spine.
Similar to motor vehicle crashes, falls produce injury
by means of a relatively abrupt change in velocity
(deceleration). The extent of injury in a fall is related to
the ability of the stationary surface to arrest the forward
motion of the body, the surface area on impact, and
tissue and bone strength. At impact, differential motion
of tissues within the body causes tissue disruption.
Decreasing the rate of the deceleration and enlarging
the surface area to which the energy is dissipated
increase the tolerance to deceleration by promoting
more uniform motion of the tissues. Characteristics
of the contact surface that arrests the fall are also
important. Concrete, asphalt, and other hard surfaces
increase the rate of deceleration and thus are associated
with more severe injuries.
Another factor to consider in determining the extent
of injury after a fall is the position of the body relative
to the impact surface. Consider these examples:
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