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The effect of mechanical trauma is related to the force transmitted to the
tissue, the rate at which the transfer occurs, the surface area to which
the force is transferred, and the area of the body that is injured.
The disruption of the continuity of the tissue results in a wound. The
transmission of the energy absorbed can produce alterations elsewhere in
the body.
Force expended:
The amount of energy released is related to the velocity and mass of the
object that strikes the person, or to that of the person who collides with
a stationary object.
In addition to the lateral displacement, many objects that strike people -
from bullets to car wheels - have rotational forces.
Prolongation of the period of impact dissipates some of the energy, as
when a boxer "rolls with a punch".
Transfer area:
The area over which transfer of force occurs is particularly important.
The intensity - that is, the force exerted per unit area - decreases with
the increasing area.
A protective helmet does not lessen the force of a blow or projectile, but
diffuses it over a large area.
Body area:
The area of the body that is
affected by physical trauma plays an important role.
The compressibility
of the tissue adjacent to the transmitted force in parts determines its
effect.
A blow over a large muscle mass, such as the thigh or upper arm,
is often less injurious than a direct blow to a poorly shielded bone, such
as the anterior tibia. Furthermore, the distribution of the force is
important.
Blows over a hollow viscus
can rupture the organ because of compression of the fluid or gas it
contains - organs located beneath the
skin, like the liver, can be easily ruptured.
An impact directly over the
heart can even disturb its electrical systems.
Contusions:
A force with sufficient energy may disrupt
capillaries and venules within an organ by physical means alone.
If this
occurs in the skin, a loss of blood into the tissue space occurs.
The
resultant altered coloration identifies a contusion, namely, a
localized area of mechanical injury with local hemorrhage.
The change may
be so limited that the only histologic change is hemorrhage in tissue
spaces outside of the vascular compartment.
The presence of a discrete
blood pool within the tissue is termed hematoma.
Initially the
deoxygenated blood renders the area blue to blue-black, as in the
classic "black eye".
Macrophages ingest the red blood cells and convert
the hemoglobin to bilirubin, thus changing the colour from blue to yellow.
Both mobilization of the pigment by macrophages and further metabolism of bilirubin cause the yellow to fade to yellowish green and then
to disappear.
Abrasions:
A direct or tangential impact can crush or
scrape the epithelial surface of the skin, producing a defect called
abrasion.
The disruptive force may destroy some of the cells by crushing them, and
thus may provide a portal of entry for microorganisms.
There may be disruption of the epidermis itself, and there may also be
vascular distortion of the cells within the dermis.
The impact of the agent and its configuration are frequently seen in these
wounds, and are of special interest to the forensic pathologist.
Lacerations:
Should the force be greater, and should the
tangential impact be stronger, the epithelium can split and tear,
resulting in a laceration.
Lacerations are usually the result of
unidirectional displacement, but they may have crushed margins, in which
case they are termed abraded lacerations.
Incisions:
The deliberate opening of the skin by a
cutting instrument, usually the surgeon's scalpel, is an incision.
Incisions have particularly sharp edges and
spare no tissue to the depth of the wound.
Deep penetrating wounds produced by
high-velocity projectiles, such as bullets, are often deceptive because
the energy of the missile as it passes through the body may be released at
sites distant from the space itself.
Bullets, because they rotate, produce a
well-defined and usually round entrance wound.
Once the projectile enters the flesh,
however, it may fragment, tumble, or actually explode, resulting in a
remarkable degree of tissue damage and a large, ragged exit wound.
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