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SHARP FORCE INJURIES

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PostPosted: Sat Oct 18, 2008 5:53 pm   Post subject: SHARP FORCE INJURIES   

SHARP FORCE INJURIES



Lecture by Derrick Pounder, lecturer at Dundee University's Forensic Medicine Department, on the nature of sharp force injuries caused by a variety of different methods and the means of establishing the weapon involved as well as the limitations in doing so, from such injuries. Ref: D. Pounder, Lecture Notes in Forensic Medicine, (p. 5-8, Ch. 2) http://www.dundee.ac.uk/forensicmedicin ... 8pages.pdf


Derrick Pounder wrote:
SHARP FORCE INJURIES

Sharp force injuries caused by objects which are keen-edged
or sharp-pointed are the second major class of injuries. There
are two types: incised wounds (cuts) and puncture wounds
(stabs).

An incised wound is a breach of the skin resulting from
contact with a keen edge. Penetration of at least the full
thickness of the skin is usual, but shallow, partial-thickness
skin cuts can occur, such as the common ‘paper cut’. In
clinical practice it sometimes happens that incised wounds and
true lacerations are both loosely referred to as lacerations. This
may create confusion when clinical medical records are used
in legal proceedings because the diagnosis of an incised
wound implies contact with a sharp-edged object or weapon,
whereas the diagnosis of a laceration implies contact with a
blunt object or weapon as the cause of the injury.
A puncture wound or stab wound is a penetrating injury with a
depth greater than the wound length on the skin surface.
Puncture wounds are caused by objects which are long, thin,
and usually sharp-pointed, such as a knife. Both incised
wounds and stab wounds typically retain no trace evidence
from the weapon, but they are associated with blood staining
of the weapon, clothing and scene.

Incised wounds

Most incised wounds are knife wounds, but this is not a
conclusion that can be safely drawn from the examination of
the wound alone. Similar wounds are produced by shards of
glass, sharp edges of machinery and other sharp-edged objects.
The key features of incised wounds reflect the fact that they
are produced by keen-edged objects rather than blunt objects.
Incised wounds typically have well defined margins with no
associated abrasion or bruising of the adjacent skin and there
is always complete severance of all tissues within the depth of
the wound, so that there are never any of the tissue bridges
which characterise lacerations.

Occasionally incised wounds have irregular or ragged margins
because either the causative object had an irregular edge, such
as broken glass, or the cut was made across skin folds, such as
on the neck or palm, so that the wound assumes a different
shape when the folds flatten out. A single incised wound may
be discontinuous if it involves surface ridges but misses
troughs of soft tissue, for example across the helix of the ear,
or across the tip of the nose and adjacent cheek. Similar
discontinuous injuries can occur across joints so that the
degree of joint flexion at the time of injury can be established
by lining up the wounds, as with a cut across the anterior
forearm and upper arm sparing the cubital fossa.

Incised wounds inflicted with heavy-bladed instruments, such
as an axe, adze, sharpened spade, machete, meat-cleaver,
hatchet, sabre, and boat propeller characteristically show
abrasion, and sometimes bruising, of the wound margin.
This results when the edges of the first-formed incised wound are
crushed by the entry of the following thick blade. The
prominence of the abrasion is highly variable and usually less
prominent if the blade is very sharp and not very thick, such as
a sabre. However, the large size of the wounds and the usually
associated cuts and fractures to underlying bone are more
obvious pointers that the weapon used was heavy-bladed. An
associated blow with the blunt poll at the back of an axe blade,
or similar weapon may leave an imprint bruise-abrasion.

The shattering of the tempered glass side windows of vehicles
during collisions produces a shower of small dice-like
fragments which strike the vehicle occupant producing a
characteristic pattern of scattered, small, irregular incised
wounds associated with small irregular abrasions, the wound
shapes reflecting the different angles of impact of the glass
fragments. The incised wounds result from the impact of the
sharp edges and corners of the dice-like fragments, and the
abrasions result from the impact of the flat sides of the
fragments. These dicing injuries occur on the side of the face
adjacent to the shattered window, so that it is possible to
determine whether the individual was seated on the left or
right hand side of the vehicle at the time of impact,
distinguishing driver from front-seat passenger.

Cuts produced by a serrated knife blade may give an
indication of the nature of the blade if the cut is tangential to
the skin surface so that the serrations leave shark-fin skin tags
at the wound edge, or parallel scratch abrasions on the
adjacent skin. Light contact with the skin by serrated blades
can produce a line of punctate abrasions or parallel scratches
without an associated incised wound. Blade serrations will
leave corresponding striations on the cut surfaces of the
cartilage of the ribs or larynx.

Accidentally inflicted incised wounds are common. They may
result from the use of knifes in the home or workplace, broken
window glass in burglaries, broken drinks glasses, and sharpedged
machinery or parts of vehicles.

The ready availability of knifes and the relative painlessness
of incised wounds has encouraged their use in suicide since
antiquity. The Romans were said to have favoured opening the
veins of the arms whilst lying in a warm bath. The Japanese
ritual suicide of seppuku involves both self-stabbing and selfincision.
Self-inflicted incised wounds are directed towards
parts of the body where large blood vessels are close to the
skin surface, such as the neck, wrists and, less commonly, the
elbows, knees and ankles. Suicidal incised wounds are
commonly, but not invariably, accompanied by parallel,
shallow, tentative wounds, reflecting a testing of the weapon
as well as the indecision so often present in suicidal acts.
These tentative wounds, also referred to as hesitation wounds,
show a wide range of appearance between cases. Having
produced one deep cut, the person intent on suicide may then
produce repeated cuts in the depth of the wound, and these can
be seen as multiple trailing cuts arising from the ends and
sides of the principal wound.

The stereotypical suicidal cut throat is an incision starting on
the side of the neck opposite the dominant cutting hand, and
passing slightly obliquely downwards across the midline. Any
repeated cuts in the depth of the wound then produce sharply
angled tags of skin at the wound ends and edges. The cuts
may be so deep that the vertebrae are scored.

Self infliction of shallow cuts as a form of self harm falling
short of attempted suicide is seen as parallel, shallowly incised
wounds to the fronts of the wrists and forearms which heal
leaving multiple, fine, horizontal, linear white scars. Often
they are most prominent on the non-dominant arm. When seen
in clinical practice, the scars raise the possibility of a
personality or psychiatric disorder, with an attendant risk of
suicide in custody. If seen at autopsy, they raise the index of
suspicion for suicide, alcohol and drug abuse, and high risk
behaviour leading to accidental death.

Occasionally individuals self-inflict incised wounds in order to
falsely allege that they have been assaulted. These factitious
injuries have a pattern which reflects handedness, easy
accessibility of the injured parts, infliction by pressing the
sharp edge against the skin and then running it across the skin
surface rather than slashing movements, and the avoidance of
sensitive or critical areas such as nipples, lips and eyes.
Although usually superficial, these factitious injuries can be
extensive and mutilating. Some individuals have self-inflicted
such injuries in the pattern of the stigmata of Christ, claiming
that they appeared spontaneously.

Individuals defending themselves against a knife attack
commonly sustain defensive injuries to the hands and
forearms, which range from subtle to devastating. Defensive
wounds to the legs may be present in a victim who was
attacked when on the ground. Cuts to the palms and fingers
result from attempts to grab or deflect the weapon, while slash
and stab wounds to the backs of the hands and the forearms
result from shielding movements. By contrast with the pattern
seen in defensive wounds, if the knife slips within the hand of
the assailant then the blade typically cuts either the base of the
little finger or the web of the thumb of the assailant. Defensive
injuries to the hands and forearms although typical of knife
attacks may be absent in sudden, overwhelming attacks (so
called ‘blitz’ attacks), or if the victim is unable to offer a
defence as a result of the effects of alcohol and drugs,
unconsciousness, bindings, or other physical and emotional
circumstances. Since defensive wounds reflect anticipation of
injury and an attempt to ward off the harm, they may be seen
in accidents as well as assaults. For example, multiple cuts to
the palms of the hands can be produced in a fall onto a glassstrewn
floor, as well as during an attack with a broken bottle.

Assaultive incised wounds to the face may be inflicted with
the intention to disfigure. In a prison setting the classical
improvised weapon for this purpose is a plastic tooth-brush
handle into which is embedded two closely placed parallel
razor blades. The Stanley knife, a work knife which has a
very sharp retractable blade, is another weapon commonly
used.

Cleanly severed blood vessels within an incised wound bleed
profusely, and if large vessels are cut then the haemorrhage
may be sufficient to kill. Such extensive haemorrhage commonly
results in blood soaking of clothing together with
blood staining and spattering of surroundings, opening up the
possibility of reconstruction of the events through blood
spatter analysis. When large veins are severed, particularly in
the neck, air may be drawn into the circulation, obstructing the
flow of blood through the heart and killing by air embolism

Stab wounds

Stab wounds, like incised wounds, typically yield no trace
evidence, are associated with blood and tissue staining of the
weapon, as well as blood staining and damage to clothing, and
offer the opportunity for reconstruction of the events from
interpretation of blood spatter at the scene.
Stab wounds are penetrating injuries produced by a long, thin
object, which is typically pointed. Most commonly the
instrument is flat with a sharp point, such as a knife, a shard of
glass or a length of metal or wood. Other weapons may be
long and thin with a sharp point, such as a skewer, hypodermic
needle, ice pick, or old-fashioned hatpin. With sufficient force
even long, rigid objects which are blunt-ended will produce
puncture wounds, e.g. a screwdriver, wooden stake, or
protruding parts of machinery or motor vehicles. The
appearance and dimensions of the resulting wound often
provides useful information about the object which produced
it.

The skin surface appearance of a stab wound is influenced
both by the nature of the weapon and characteristics of the
skin. The skin contains a large amount of elastic tissue and
will both stretch and recoil. This elastic tissue is not randomly
distributed but is aligned so as to produce natural lines of
tension (Langer’s lines) which can be mapped out on the skin
surface. In survivors of knife assaults, the extent of wound
scarring will be influenced by the alignment of the wounds
relative to Langer’s lines. Wounds which have a long axis
parallel with Langer’s lines gape only slightly, a fact made use
of by surgeons who align their incisions in this way to
promote healing and reduce scarring. Wounds aligned at right
angles to Langer’s lines tend to gape widely, and scar
prominently, because the natural lines of tension of the skin
pull the wound open. Wound gaping is also influenced by the
extent of damage to the underlying supporting fascia and
muscles.

When a stab wound in a corpse is gaping then the wound
edges must be re-approximated to reconstruct the original
shape of the wound, something that is easily achieved with
transparent tape. It is the dimensions of the reconstructed
wound rather than the original gaping wound that are of value
in predicting the dimensions of the blade. One purpose in
examining stab wounds is to establish whether potential
weapons could have or could not have produced the wounds.

The dimensions of a stab wound give some indication of the
dimensions of the blade of the weapon. If a stabbing with a
knife is straight in and out then the length of the stab wound
on the skin surface will reflect the width of the knife blade.
However, there are important qualifications which apply. The
skin wound length may be marginally (a few millimetres)
shorter than the blade width as a result of the elastic recoil of
the skin. If the knife blade has a marked taper and the entire
length of the blade did not enter the body then the skin wound
length may not represent the maximum width of the blade. If
the blade did not pass straight in and out but instead there was
some rocking of the blade, or if it was withdrawn at a different
angle from the original thrust, then the skin wound will be
longer than the inserted blade width. Consequently, the most
reliable assessment of blade width is made from the deepest
wound with the shortest skin surface length, because this
wound represents the greatest blade penetration with least
lateral movement. A single weapon can produce a series of
wounds encompassing a wide range of skin-surface lengths
and wound depths. This is often seen in a multiple stabbing
fatality and is consistent with the use of only one weapon.
However, it is rarely possible to exclude any speculative
suggestion of more than one weapon, and, by inference, more
than one assailant.

The depth of the wound gives an indication of the length of the
weapon. Clearly the wound track length may be less than the
blade length if the entire blade did not enter the body. Less
obvious is the fact that the wound track length may be greater
than the blade length. This may occur if the knife thrust is
forceful and the tissues are compressed, so that when the
weapon is withdrawn the track length in the now
decompressed tissues is greater than the blade length. This
tissue compression effect is most marked in wounds to the
anterior chest and abdomen, since the entire chest or
abdominal wall can be driven backwards by the blow. A small
pocket-knife, with a blade of about 2 inches or 5 cm, can
cause, in a slim person, a fatal stab wound to the heart or one
which penetrates the abdomen to transfix the aorta. An added
difficulty in measuring the wound track length at autopsy is
that the corpse is supine with the viscera in a slightly different
relative position to a living person standing or sitting, and with
the tissues dissected and displaced. For all of these reasons
the wound track depth should be used with caution in
predicting the blade length of the weapon. If by chance some
fixed bone, such as a vertebra, is damaged at the end of the
wound track, then the assessment of depth of penetration is
easier, but still subject to inaccuracy.

As well as providing an indication of blade width and blade
length, a stab wound may provide other useful information
about the weapon. Wound breadth on the skin surface is a
reflection of blade thickness and a typical small kitchen knife
with a blade thickness between 1 and 2mm produces a very
narrow wound. The use of a thicker-bladed weapon may be
readily apparent from the measured wound breadth on the skin
surface.

Most knives have a single-edged blade, with one keen edge
and one blunt edge to the blade. The resultant wound reflects
the cross-sectional shape of the blade and, with the wound
gaping, often appears boat-like with a pointed prow and a
blunted stern. Skin elasticity may distort the blunted stern
shape into a double-pronged fishtail. The thicker the blade of
the weapon, the more obvious is the blunting of one end of the
wound when contrasted with the other pointed end. Knives
with double-edged blades (daggers) are specifically designed
for use as weapons and produce a wound that is pointed at
both ends, but an identical double-pointed wound can be
produced by a thin, single-edged blade.

If the blade penetrates to its full length then the wound may
show features from the hilt and adjacent part of the blade. A
knife with a folding blade has a notch at the hilt end of the
sharp edge of the blade. This notch, or ‘kick of the tang’ (the
tang being the rectangular portion of metal from which the
blade was forged), leaves a rectangular or square abrasion at
the end of the wound if the blade penetrates to its full length.
A hunting-type knife, having a rectangular metal plate
between the blade and the handle, leaves rectangular
abrasions, which may be subtle, at both ends of the wound.
Any imprint from the hilt of a weapon is usually only partial
because the knife entry is most often at an angle. Dove-tail
abrasions originating from either side of the stab wound are
typical of a wooden-handled blade. A semi-lunar abrasion at
one side of a stab wound is typical of a pen-knife handle.
Other protrusions from the base of the blade or hilt may leave
abrasions characteristic of the weapon. Serrated blades or
blades with a saw-type component, as is seen in some hunting
knives, can leave parallel linear abrasions or incised wounds
on the skin in association with the stab wounds. Serrated
blades leave corresponding striations on cartilage and
occasionally the dermis. Mottled bruising associated with the
wound may be produced by the knuckles of the assailant’s
hand, and an imprint bruise-abrasion from the clothing reflects
a similar mechanism of forceful impact from the hand.

The cross-sectional shape of the weapon, particularly the back
of the blade, may be accurately reproduced if it passes through
bone, eg skull, pelvis, sternum or ribs. At the same time,
passage through bone may cause trace material from the blade,
eg. paint, to be scraped off and deposited in the wound. Stab
wounds in solid organs such as the liver retain the profile of
the weapon, and this can be visualised by filling the wound
track with a radio-opaque contrast material and taking an xray,
or by making a cast of the wound using modern dental
casting material.

Weapons other than knives may produce characteristic stab
wounds. Bayonets, which have a ridge along the back of the
blade and a groove along either side, to lighten the blade and
facilitate withdrawal of the weapon, produce wounds like an
elongated letter “T”. A pointed metal bar which is square in
cross-section typically produces a cruciate wound, whereas
one which is circular in cross-section, e.g. a pitchfork,
produces an elliptical wound. Similarly a triangular file will
produce a three-cornered wound. If the cross-sectional shape
of the weapon varies along its length, eg a screwdriver, then
the depth of penetration will affect the appearance of the
wound. A forked instrument produces paired stab holes at
different distances depending upon the angle of penetration. A
scissors leaves paired wounds, off-set and pointed towards
each other. A screwdriver or arrow wound may be
indistinguishable from a gunshot wound because all leave a
central defect with a marginal abrasion produced by the same
mechanism of an inward pushing with grazing of the skin
margin. Post-mortem drying turns the marginal abrasion dark
brown or black.

Relatively blunt instruments such as pokers, closed scissors
and files, tend to bruise and abrade the wound margins, a
feature not otherwise seen in stab wounds. The blunter the
object and the thicker its shaft then the more likely will the
skin surface wound become a ragged, often cruciate, split. In
cases where the wound appearance is unusual, it is helpful to
conduct experiments with a duplicate suspect weapon and pig
skin, in order to see whether the wound appearances can be
reproduced.

Stab wounds inflicted during a struggle, with knife thrusts at
awkward angles and with movement of both victim and
assailant during the attack, may show characteristics reflecting
this. Even so, it is rarely if ever possible to reconstruct the
positions of victim and assailant from the location and
direction of the wounds. A notch on the otherwise cleanly cut
edge of the wound is a result of withdrawal of the blade at a
different angle from the entry thrust. Exaggeration of this
effect leads to a V-shaped or even cruciate wound when there
is marked twisting of the blade or twisting of the body of the
victim. By contrast, clothing tends to twist with the blade so
that a linear stab hole in clothing may overlie a V-shaped stab
in skin. A linear abrasion (scratch) extending from one end of
the wound results from the withdrawn blade tip running across
the skin. A single stab hole on the skin surface may be
associated with more than one wound track through the
tissues, reflecting a knife thrust followed by continuing
struggle or repeated thrusts of the weapon without complete
withdrawal.

A commonly asked question in the courts is the amount of
force required to produce a specific stab wound. This is
usually a difficult if not impossible question to answer. The
sharpness of the point of the weapon is the most important
factor in determining the degree of force required to produce a
stab wound. In general, relatively little force is required to
produce a deeply penetrating stab wound using a sharply
pointed weapon, and the amount of force is easily overestimated.
The greatest resistance to penetration is provided
by the skin and once this resistance is overcome then the blade
enters the tissues with greater ease. In this respect an analogy
can be made with the stabbing of a ripe melon. The important
implication is that the depth of the wound is not a measure of
the degree of force applied. However, penetration of any bone
implies a significant degree of force; all the more so if the tip
of the blade has broken off and remains embedded in the bone,
something which is best identified by X-ray. Similarly a
significant degree of force may be inferred from the presence
of the hilt mark of the weapon on the skin surface, an
uncommon finding, or a wound track significantly longer than
blade length, suggesting forceful tissue compression during
the stabbing. Even so, the stabbing force may have been a
combination of both the thrust of the weapon and also any
forward movement of the victim, such as in a fall. This latter
proposition is commonly raised by the defence, and is rarely
possible to discount, in deaths from single stabs.
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