Topic 11 - Wound ballistics and tissue simulants

Question 1

how much does a typical bullet weigh?

Answer 1

A bullet is a very small piece of metal, typically weighing 3 – 20 grams or 45 – 300 grains.

Question 2

what does tissue damage come from?

Answer 2

Tissue damage comes from the energy that the bullet
carries, the bullet design and properties of the tissue that is impacted.

Question 3

where are the majority of civilian firearm wounds inflicted?

Answer 3

The majority of civilian firearm wounds are inflicted at close to medium range (0 to 25 m), usually with handguns.

At very close range it is possible to analyse not just the tissue damage, but also propellant deposits and burn patterns

Question 4

what do contact wounds always leave?

Answer 4

Contact wounds will almost always leave a mark or imprint. Even if the contact is light, the metal of the firearm on the skin will cause cell damage at the point of contact.

Question 5

what is tattooing in ballistics?

Answer 5

• Hot gunshot residues can embed into human tissues at relatively close ranges, the pattern of which can be analysed as previously discussed.
• The pattern left on the skin is referred to as ‘tattooing’ or ‘stippling’. * This can be easily mistaken for shotgun wounds by the untrained eye.
• Gunshot residues are recoverable from skin around wound for close
  range shooting (less than a few metres).
• The distribution and shape of the pattern could also give an indication
  of incidence angle.

Question 6

what is gas injection trauma - contact wounds?

Answer 6

• A contact shot to the head can lead to a distinctive star-shaped
  (stellate) splitting of the skin.
• The propellant gases are ‘injected’ between dermis and cranium but have nowhere else to go, so they escape to the external environment by bursting out of the skin surrounding the wound.
• Note the dark metallic combustion deposits in the wound (recoverable).

Question 7

what is a muzzle imprint - contact wounds?

Answer 7

• The high temperatures associated with the shot permit the burning of class characteristics related to the firearm into the skin for contact shots.
• This can lead to an imprint of the muzzle and fore-end around entry wound.
• Manufacturer markings may also be visible around wound.

Question 8

what are tell tale signs of an intermediate and long range wounds?

Answer 8

• No muzzle imprint or gas injection trauma.
• Minimal or complete absence of tattooing and GSR collection not possible.
• Without residue deposits, it is almost impossible to estimate range of shot accurately.
• If available then sonic and echo data could indicate position of shot.
• Penetration data alone cannot be used to appreciate the range as this is dependent on too many factors.

Question 9

what do shotgun wounds typically look like?

Answer 9

• Shotgun wounds typically exhibit multiple entry wounds, dependant upon ammunition type.
• Unless shot at very close range, there are usually no exit wounds.
• The wadding could become embedded in the wound or at least mark the skin near the wound area.

Question 10

what do skull impacts look like?

Answer 10

• Skull impacts will often result in a phenomena called ‘bevelling’.
• This is sometimes the only reliable way of defining entry and/or exit wounds.
• Shape of the bullet hole in the bone can suggest the angle of impact.
• i.e. “key-holing” from acute angle impacts as opposed to circular for perpendicular
  impacts.
• The bullet will often remain intact, albeit deformed, after a skull impact.

Question 11

what is a gutter wound?

Answer 11

• The projectile may impact the surface of the skull at a relatively shallow angle and not penetrate – a tangential impact.
• This can create a so-called “Gutter” wound leading to serious external damage.
• The internal damage can be variable, depending on the creation of secondary projectiles or energy transfer that can effect the brain material.

Question 12

what is a comminuted fracture?

Answer 12

• Acute angled impacts can be deflected within the cranial cavity,
  causing a comminuted fracture diametrically opposite point of entry.
• The bullet is recoverable by a pathologist or surgeon; rifling mark data
  should be intact.
• Comminuted fractures can be mistaken for exit wounds – termed a
  pseudo-exit wound.

Question 13

what is a skull cap and what is it susceptible to?

Answer 13

• The skull cap (CALVARIA) is often susceptible to “popping off” following high velocity ballistic trauma.
• The is due to the large increase in pressure generated inside the skull following a high energy transfer to the brain material.
• The brain material is pushed out radially from the projectile’s path in a
  phenomenon known as the “temporary cavity”.
• In other incidents, this process could affect different bones in the body if they are adjacent to a temporary cavity being produced – e.g. in the leg.

Question 14

whats a low energy transfer? (HANDGUNS)

Answer 14

• Handguns tend to have a lower average
  projectile energy on impact and therefore
  have less energy to transfer upon direct
  impact with bone.
• Typically 500-800 Joules.
• This often creates a “simple fracture” in
  bones, particularly long bones – a single
  break leaving just two main pieces.

Question 15

what is a high energy transfer? (RIFLES)

Answer 15

• Rifles tend to present a higher average
  projectile energy on impact and therefore
  have more energy to transfer upon direct
  impact with bone.
• Typically 1800-7000 Joules.
• This often creates a “multi-fragmentary
  fracture” in bones, particularly long bones,
  with several breaks and widespread
  fragmentation.
• Also creates “secondary missiles”

Question 16

what are the soft tissue impacts of high velocity bullets?

Answer 16

• High velocity bullets (500 – 1500 m.s-1) will tend to over-penetrate at close to medium ranges: 25 – 300 metres.

Question 17

what is the permanent wound cavity?

Answer 17

• The permanent cavity is the most significant mechanism in the overall
  extent and severity of wounding.
• This is the path of tissue destroyed by the transit of the bullet and is
  formed by the crushing of soft tissues or tearing by shrapnel/fragments.
• This cavity does not completely close after the
  passage of the projectile and leaves the internal
  tissues exposed to atmospheric contamination.
• The permanent cavity size is most closely related to
  the bullet’s caliber.
• Destroyed tissue must usually be excised to avoid necrosis.

Question 18

what is the temporary wound cavity?

Answer 18

• The temporary wound cavity opens up by the transit of the bullet
  through tissues (but it’s effect is most visual in soft tissues).
• Though soft tissues will be damaged (often only slightly), natural tissue
  elasticity will cause this cavity to close (almost) immediately after the
  passage of the bullet
• Less elastic tissues are not able to withstand the process and deform permanently.
• Ignore myths concerning ‘hydrostatic shock’ waves causing fatal wounds. Human tissues are very elastic and dampen any such effect.

Question 19

characteristics of low velocity: large caliber projectiles

Answer 19

• Normally associated with pistol calibers and usually unjacketed or
  semi-jacketed.
• Small permanent cavity – about the diameter of the deformed bullet.
• Temporary cavity is kind of cylindrical and regular but does not extend
  far beyond the permanent cavity.
• Bullets tend to lose energy very quickly inside the body, so may remain
  in victim.

Question 20

characterisitcs of high velocity: small caliber projectiles

Answer 20

• Most rifle bullets fall into this category and are almost always jacketed.
• Large, irregular permanent wound cavity if the bullet breaks up.
• Large temporary wound cavity, widest in the middle.
• Non-partitioned bullets will tend to strip from the core.
• If this happens then core may over-penetrate and the jacket remains in
  victim.

Question 21

why do we not use the term “stopping power”?

Answer 21

• Much is made of the ability of a given round to “stop” the target.
• In reality the only way any bullet will stop the target is if it damages the
  central nervous system, or impacts a critical structure, the failure of
  which induces massive bleeding.
• In effect this means impacts to the brain, spine or heart are necessary
  to provide instant incapacitation.
• A hit anywhere else will slow the target, but loss of life or incapacitation
  will only occur through eventual blood loss (exsanguination).
• The term ‘stopping power’ has no scientific basis, so never use it!

Question 22

what is an IED?

Answer 22

an improvised explosive device

Question 23

what is a ballistic penetration simulant?

Answer 23

“any material, either biological or synthetic, that is able to give comparable and reproductive wound data in relation to the human body when penetrated by a projectile”.

Question 24

why do we use ballistic simulants?

Answer 24

• To overcome the ethical issues associated with testing on human
  tissues.
• To give comparative penetration data between different projectiles.
• To understand the potential wounding effects of projectiles.
• To give accurate and reproducible data (accuracy has not always been considered paramount over dramatic effects being visualised).

Question 25

what materials that have been used as soft tissue simulants?

Answer 25

• Water
• Wet phone books
• Strawboard
• Clay
• Transparent gel candle
• Lead
• Glycerine soap
• Hydrogels
• Synthetic polymers
• Cadavers

Question 26

what are the desireable properties of soft tissue simulants?

Answer 26

• Non-cadaveric, and if possible, non-biological so that any natural biological variation is avoided between test samples.
• Ethically sound.
• Readily available, easy to handle and stable during storage.
• Similarity in the deceleration and deformation behaviour of the projectile between the simulant and the living tissue type being simulated.
• Similarity in the kinetic energy dissipation between the simulant and the living tissue type being simulated..

Question 27

other desireable properties of soft tissue simulants?

Answer 27

• Kinetic energy dissipation measurability with reasonable accuracy.
• Extrapolation of temporary cavity diameter.
• Elastic behaviour similar to living tissue for observation and measurement of temporary cavity formation and tissue compression.
• Extrapolation of permanent cavity diameter.
• Reproducibility.
• Able to be CT scanned for visualisation of wound paths and to provide a permanent record from degradable simulants. Ballistic gelatin – a soft tissue penetrat

Question 28

what is ballistic gelatin?

Answer 28

• Ballistic gelatin in its raw form consists of collagen extracted from the bones and sinews of pigs by either acid or alkali hydrolysis.
• It is typically used in two formulations for the simulation of soft body tissues such as muscle:
• 10% gelatin @ 4 degrees C
• 20% gelatin @ 20 degrees C
• The density of ballistic gelatin is rather temperature sensitive, hence why different temperatures must be used to give similar penetration properties at different concentrations.

Question 29

how do you produce ballistic gelatin? (long method)

Answer 29

The method of gelatin block (10 kg) production according to Jussila is:

• Accurately measure the amount of gelatin powder (10% of end weight – 1 kg).
• Measure 50% of the water component (4.5 L), using purified tap water to ensure impurities and microbes are not present and set it aside.
• Measure the other 50% of the water component (4.5 L), using purified tap water, and heat it to just below boiling point.
• As the water in the kettle is close to temperature, add the gelatin particles to the cool water so they begin to swell.
• Add the hot water from the kettle, which should result in a mixture temperature of approximately 45o
  C and mix with an electric paint mixer for 10 minutes to dissolve all lumps.
• Add 50 ml of propionic acid or 5 ml of cinnamon oil (bactericidal).
• Remove excess foam from the top of the mixture, poor into the mould and wrap along with an identification card in polyethylene film to prevent
  evaporation whilst cooling and to stop any settling of aerobic microbes

Question 30

what material is commonly used as a skin penetration simulant?

Answer 30

Leather

• Most commonly used leathers to simulate human skin for ballistic testing are either upholstery or chamois leather.

Question 31

head models by Thali et al.

Answer 31

• The “skin-skull-brain” model by Thali et al. in Switzerland.
• Has a multi-layered polyurethane bone structure made up of the
  elastic, compact tabula externa and tabula interna, which sandwich a
  compliant and cancellous spongy diplöe centre.
• Uses a bespoke silicone skull cap and a
  10% gelatin brain @ 4 degrees C.
• Use of clay as a support during impact, which is
  analogous to the neck supporting the structure.
• Non-realistic geometry.

Question 32

head models by Chris Shepherd?

Answer 32

• More realistic geometry.
• Polyurethane single layer skull.
• Drilled foramen magnum.
• 10% ballistic gelatin brain @ 4 degrees C.
• Silicone skin layer (swimming cap).

Question 33

what are the difficulties in testing biological materials?

Answer 33

• Variability.
• Most standard methods cannot be used as materials are too soft.
• Sample preparation is often tricky.
• The materials are ‘viscoelastic’ and multi-axial testing is ideally required to include 3-dimensional microstructure of tissues.
• Dynamic properties are required for studying ballistic impact.
• Ethics!