Sharp Force Injury

Question 1

Define an incision

Answer 1

Clean division of the full thickness of the skin or tissue under the concentrated pressure of a sharp edged instrument
Key that the edge divides tissue not the tip

Question 2

What causes an incision

Answer 2

Sharp edge drawn along skin surface
The tissue divides as the pressure is focused on a narrow edge - clean division
As concentrated pressure the force doesn’t need to be great
Can be a knife, broken glass or any keen edge

Question 3

Describe the appearance of an incision

Answer 3

It will be longer than it is deep
May see underlying fat and muscle if deep enough
Usually a gaping wound - skin elasticity pulls it open
No bruising, abrasion or tissue bridges - clean cut
Will be profuse bleeding - cut vessels tend not to constrict/spasm

Question 4

What are dicing incisions

Answer 4

Many small incisions caused by shattering glass
Some are linear, some L shaped, some Y
Glass forms small cubes with sharp edges and corners which cause incisions when they impact the occupants

Question 5

Which features suggest an incision are self-inflicted

Answer 5

Accessible chosen sites (at blood vessels such as wrist or neck)
Clothes drawn aside - can see what they are doing
Associated “hesitation wounds” - superficial incisions to gain confidence
Handedness - right handed person usually injures themselves on the left
1 or 2 potentially fatal wounds rather than many
Repetition in wound track

Question 6

What are the common sites of self-inflicted incisions

Answer 6

wrist
cubital fossa
neck
All have good access to blood vessels

Question 7

Are self-inflicted wounds more likely to be blunt or sharp force

Answer 7

Sharp force

Less force and pain required to cause injury

Question 8

Why do incisions gape

Answer 8

The elastic properties of the skin pull the edges apart

Question 9

What does an up and down blood spray pattern suggest

Answer 9

Pulsatile pattern of blood spray which suggests an arterial bleed - usually a major one like carotid

Question 10

What are the features of assault incisions/stabs

Answer 10

Aimed at target sites, but scattered - victim may try and dodge so some miss or glance
Clothing is cut - usually kept in the way
Most are deep and forceful as perpetrator can carry on with same force for longer
Many of the wounds are potentially fatal
Associated defense injuries - ward off blows or grab knife

Question 11

Where are defensive wounds typically found

Answer 11

Flexor aspect of fingers or palmar hand if they’ve tried to grab the knife
Forearms if they raise arms to protect themselves

Question 12

When do defensive wounds typically occur

Answer 12

Tends to occur earlier in attack before victim is overwhelmed
They show that the victim was conscious and mobile at the time- active attempt to stop attack

Question 13

What are the differences between laceration and incision

Answer 13

Laceration are blunt force but incision are due to sharp edge

Laceration will have slight bleeding but incision are profuse bleeding

Laceration may have trace evidence in wound but incision have no trace evidence (unless blade breaks which is very rare)

Question 14

List some signs of incision by serrated blade

Answer 14

Small, curved and evenly placed lines/cuts at edge of incision or in glancing wounds
Lines will match the serrations (peaks and troughs)
Spacing between lines may change as knife changes angle during movement

Question 15

What is a chop wound

Answer 15

Variant of incision
Caused by a wide blade such as an axe
The width of the blade causes traction and abrasions at the margins as it crushes the surrounding skin

Question 16

What are the features of a chop wound

Answer 16

Incision with marginal abrasion and bruising
Will be longer than it is deep
Abrasions trailing away from the main incision may be caused by handle

Question 17

What is the definition of a stab wound

Answer 17

Penetrating injury caused by separation of the skin and tissues under the pressure of a sharp or blunt pointed instrument
Created with the tip of the weapon (usually a sharp point such as knife, needle or blunt instrument like screwdriver)
Not the edge - this is an incision

Question 18

What is the greatest dimension of a stab wound

Answer 18

It’s depth
It goes deeper into the body than its length on the body surface (often small on surface)
It is this internal damage that makes them so dangerous

Question 19

What kind of instrument causes a puncture style stab wound

Answer 19

Blunt, long and rigid

Such as a spike or screwdriver

Question 20

Describe how stab wounds occur

Answer 20

Sharp tip thrust into skin and tissues

Clean division of skin and tissues under concentrated pressure of sharp tip

Question 21

Why can’t you accurately estimate weapon dimension from resting wound

Answer 21

It will have changed shape as tissues move
The elasticity of the skin will cause the wound to gape wider but also shorten slightly

Can push the skin together to reconstruct the dimension - more accurate representation so should be done

Question 22

Describe the anatomy of a knife

Answer 22

Handle at top 
Tang - portion of blade within handle 
Guard - may leave mark 
Hilt 
Blunt edge is called the spine 
Ricasso is the thicker, unsharpened area at top of blade 
Kick - flick out between ricasso and blade 
Heel - first portion of cutting edge 
Blade - sharp, cutting edge 
Tip

Question 23

What is a bowie knife

Answer 23

Single edged knife with serration on the back of the blade

Question 24

What is a dagger

Answer 24

A double edged blade

Question 25

What determines the force required for a stabbing

Answer 25

Sharpness of tip of blade - main one
Speed of approach - quick jab more likely to penetrate than a slow sustained push
Anatomical site (chest easily penetrated)

Question 26

When might the tip of the knife break off

Answer 26

Can occur if it strikes bone

May embed in the bone - trace evidence

Question 27

Which areas are easy to penetrate with a knife

Answer 27

Chest

Skin is held taut over the ribs so doesn’t have much room to compress and resist penetration

Question 28

Which areas are difficult to penetrate with a knife

Answer 28

Abdomen
It is very compressible (has a lot of give) so more resistant to penetration
Requires more force

Question 29

Describe the forces involved in stabbing

Answer 29

Difficult to quantify force and is often overestimated (doesn’t actually take much force)
Skin and clothing are more resistant to penetration
Once you have broken this the underlying tissue penetrate easily - sudden give due to energy in compressed tissue and energy in assailant arms building up

Question 30

Why are partial thickness stab wounds rare

Answer 30

Once the skin in broken the tissues underneath are easily penetrated - knife pretty much slides in
The energy required to break the skin is the same as that required to get it in up to the hilt
Would have to deliberately remove force to only push in partially
Therefore wound depth doesn’t necessarily = force

Question 31

List the features used to describe stab wounds

Answer 31

Damage to clothing - do holes match up?
Site
Shape - elliptical or boat etc.
Size
Alignment - horizontal/vertical 
Direction - 3D direction 
Depth
Underlying damage - which structures have been affected
Effects - e.g. external blood loss, internal blood loss, damage to nerves

Question 32

How do Langer’s lines of skin tension affect stab wounds

Answer 32

Wounds parallel to LL only gape slightly and heal with small scar (go with tension) - surgeons use these lines

Wounds at right angles to LL gape widely and heal with large scar - line of tension pulls it out of shape

Question 33

How can pathologists use Langer’s line

Answer 33

They can pull the lines back into place to allow better visualisation of the true wound shape and therefore better estimation of knife
Measured an photographed in this position

Question 34

What is a stab wound in the middle of the back suggestive of

Answer 34

Homicide

Inaccessible for suicide or accident

Question 35

What is a V shaped stab would suggestive of

Answer 35

Suggests the knife has been twisted or moved within the body

Question 36

When can a stab wound become an incision

Answer 36

Stab wound can trail off into an incision as the knife is pulled back out at an angle
Blade ends up dragging across tissue

Question 37

How can stab wound length be extended

Answer 37

If the knife is rocked/tipped and comes out at a different angle it will make the wound longer
Length of wound would be greater than the width of the blade itself
Sometimes surgeons will use the wound as a start point for their incision which disguises original dimension

Question 38

What effect do pegs or bolts on a knife have on the wound

Answer 38

They will leave corresponding abrasions

Can help determine knife - common in flip knives

Question 39

How can scissor stab wound vary

Answer 39

The wound depends on how open the blades are

May have a small notch caused by overlap of blades

Question 40

What wound dimension does blade length correspond to

Answer 40

Wound depth

Question 41

What wound dimension does blade width correspond to

Answer 41

Wound length

Question 42

What wound dimension does blade thickness correspond to

Answer 42

Wound width

Question 43

Why might the stab wound length be less than the blade width

Answer 43

Skin is stretched on penetration
It can contract back when knife is removed making the wound shorter
Will also be shorter than the widest part of the knife if it is not fully inserted

Question 44

Why might wound depth be less than the blade length

Answer 44

If it is not fully inserted

Question 45

Why might wound depth be more than the blade length

Answer 45

If the tissue is compressed the wound depth can be longer as knife goes in further but then tissues will decompress once removed and rise back out
Common in the abdomen
Also organs may have been affected by the stabbing but fall back deeper when body is on back for examination which makes it artificially longer

Question 46

If you have multiple stab wounds which is the best indicator of weapon dimension

Answer 46

Typically they vary in length and depth so you use:
The deepest - as you get full blade length
The shortest - this suggest no rocking and extension

Question 47

How are wounds numbered on pathology photos

Answer 47

By region not in order of event

Question 48

Describe the wound shape left by single edged blade

Answer 48

They leave a boat shaped wound
Flatter at one side and pointed at the other (edge with the blade)
If back is significantly blunt it can cause tearing that makes it look fishtailed or may create another single point

Question 49

Describe the wound shape left by a double edged blade

Answer 49

Will leave an elliptical shaped wound

Sharp at both sides as both have blades

Question 50

What does a concentrated attack to the face suggest

Answer 50

It often aims to depersonalise and disfigure the victim

Question 51

What does direction of blood flow indicate

Answer 51

The position the victim was in when injured
Will move with gravity
Flows down when upright and off to each side when lying down

Question 52

What can the shape of a stab wound indicate

Answer 52

The cross sectional shape of the blade
Direction of insertion
Movement of the victim or assailant

Question 53

When do you get a hilt mark

Answer 53

When there is full penetration of the blade

May come from the kick (small rectangular abrasion), guard, the handle or the assailants hand

Question 54

What shape would a bayonet stab wound be

Answer 54

Shallow T shape

Question 55

Does the shape of the stab wound suggest the shape of the weapon

Answer 55

Typically - it may indicate cross sectional shape of weapon
May also show direction of insertion and movement of victim
Must however take into account tearing at edges or gaping due to tension lines

Question 56

What pattern would a stab wound made by a blunt instrument make

Answer 56

Would be bruised and abraded as well as a penetrating injury