Lecture 11 - Wound Ballistics and Tissue Simulants

Aims

• Understand how bullets cause injury, including the mechanisms of penetration, cavitation, and fragmentation.

• Recognize wound profiles related to projectile types, considering velocity, caliber, and construction.

• Understand key tissue simulants used in wound ballistics for accurate and reproducible testing.

• Appreciate how wound ballistics testing is carried out, including experimental design, data collection, and analysis.

Important Principles

• Bullet weight: Ranges from 3-20 grams or 45-300 grains, influencing kinetic energy and penetration depth.

• Tissue damage: Depends on bullet energy (kinetic energy KE = \frac{1}{2}mv^2KE = \frac{1}{2}mv^2), bullet design (shape, composition), and tissue properties (density, elasticity).

• Focus: Wound analysis includes entry and exit wounds, internal damage, and projectile trajectory.

• Close range: Distinctive marks such as powder burns, stippling, and muzzle imprints.

• High velocity: Gruesome injuries due to extensive tissue damage and cavitation effects.

Close Range and Contact Wounds

• Most wounds: Occur at close to medium range (0-25m), commonly involving handguns.

• Very close range: Requires analysis of tissue damage, propellant deposits, and burn patterns to determine proximity and angle of impact.

• Contact wounds: Leave a mark/imprint of the firearm on the skin.

• Firearm contact: Causes cell damage due to heat, pressure, and chemical exposure.

Tattooing (Stippling)

• Hot gunshot residue embeds in tissue at close range and can be analyzed to estimate distance and angle.

• Pattern on the skin is called Tattooing or stippling, characterized by punctate lesions.

• Residues recoverable from skin around wound for close range (less than a few meters), aiding in forensic reconstruction.

• Distribution/shape can indicate incidence angle, providing insights into the shooter’s position.

Contact Wound - Gas Injection Trauma

• Contact shot to the head can cause a star-shaped (stellate) splitting of the skin due to gas injection.

• Propellant gases inject between dermis and cranium, causing rapid expansion and tearing.

• Gases escape by bursting out of the skin surrounding the wound, creating a characteristic stellate pattern.

Contact Wound - Muzzle Imprint

• High temperatures burn firearm characteristics into the skin, leaving identifiable marks.

• Imprint of muzzle and fore-end around entry wound, indicating direct contact.

• Manufacturer markings may be visible, aiding in firearm identification.

Intermediate and Long-Range Wounds

• No muzzle imprint or gas injection trauma, making range estimation more challenging.

• Minimal/absent tattooing; GSR collection not possible, limiting forensic evidence.

• Difficult to estimate range of shot accurately, requiring advanced techniques.

• Sonic/echo data could indicate shot position, providing supplementary information.

• Penetration data alone insufficient for accurate range estimation.

Shotgun Wounds

• Multiple entry wounds (dependent on ammunition), reflecting the dispersal of pellets.

• Usually no exit wounds (unless very close range), as pellets lose energy within the body.

• Wadding could embed in the wound or mark the skin, providing additional forensic evidence.

Skull Impacts

• Bevelling often defines entry/exit wounds; internal bevelling at entry, external bevelling at exit.

• Bullet hole shape suggests impact angle; rounder holes indicate perpendicular impacts, elongated holes suggest acute angles.

• Key-holing from acute angle impacts; circular for perpendicular, indicating bullet yaw.

• Bullet often remains intact, albeit deformed, retaining rifling marks.

Gutter Wound

• Projectile impacts skull at shallow angle, causing tangential injury.

• Tangential impact causes serious external damage, potentially fracturing the skull.

• Internal damage varies, depending on secondary projectiles or energy transfer.

Comminuted Fracture

• Acute angled impacts can deflect within cranial cavity, leading to complex fractures.

• Causes comminuted fracture diametrically opposite entry (contre-coup fracture).

• Bullet recoverable, rifling marks intact, aiding in firearm identification.

• Can be mistaken for exit wound (pseudo-exit wound), complicating forensic analysis.

Skull Cap

• Skull cap (calvaria) often pops off after high velocity trauma due to pressure build-up.

• Due to pressure increase inside skull after high energy transfer, causing explosive disruption.

• Brain material pushed out radially (temporary cavity), exacerbating tissue damage.

Other Bone Impacts

• Bones vary in resistance; density and structure influence fracture patterns.

• Smaller bones (radius/ulna/clavicle): Shatter, fragmentation extends beyond damaged area, creating secondary missiles.

• Larger bones (femur/pelvis): Chip, deflect/deform bullet, altering trajectory.

• Bullet may shatter or drop its core, reducing penetration depth.

Low Energy Transfer

• Handguns: Lower projectile energy (500-800 Joules), causing localized damage.

• Creates "simple fracture" in bones (single break, two main pieces), easier to manage.

High Energy Transfer

• Rifles: Higher projectile energy (1800-7000 Joules), causing extensive fragmentation.

• Creates "multi-fragmentary fracture" in bones (multiple breaks, widespread fragmentation), increasing complexity.

• Creates secondary missiles, enhancing tissue damage.

Bone Impact - Summary

• Bullet jacket (rifled segment of bearing surface) is forensic evidence, retaining rifling marks for firearm matching.

• Unjacketed bullets deform heavily upon impact, losing structural integrity.

• Fragmented bullets complicate entry/exit wound identification, requiring careful analysis.

• Deflected bullets can re-enter the body, creating additional wound tracks.

Soft Tissue Impacts

• High velocity bullets (500-1500 m/s) over-penetrate at close to medium ranges (25-300 meters), causing significant damage.

• Examine wound track post-mortem to assess extent of tissue damage.

• Re-align tissue holes to determine bullet path, aiding in trajectory reconstruction.

• Use polycarbonate rods to follow the bullet path, visualizing the wound track.

The Permanent Wound Cavity

• Path of tissue destroyed by bullet transit, resulting in irreversible damage.

• Caused by crushing or tearing of tissues along the bullet’s path.

• Does not completely close; exposes internal tissues to contamination, increasing infection risk.

• Cavity size related to bullet caliber, influencing the extent of tissue destruction.

• Destroyed tissue usually excised to avoid necrosis, promoting healing.

The Temporary Wound Cavity

• Opens by the transit of the bullet through tissues, creating a transient space.

• Soft tissues damaged due to stretching and compression.

• Less elastic tissues deform permanently, contributing to long-term injury.

• Dampens any such effect, reducing the overall impact.

• Formed by projectiles imparting radial acceleration to the tissue, forcing it out laterally during penetration.

• The kinetic energy passed into the moving elastic tissue is transformed into strain energy, causing temporary deformation.

Factors Affecting Temporary Cavity

• Larger projectiles create larger cavities, increasing tissue displacement.

• Projectiles with higher drag create larger cavities, enhancing energy transfer.

• Extent of temporary cavity formation depends on tissue elasticity, the proportion of impact energy converted into radial acceleration, and the dimensions of the leading surface of the projectile inside the tissue.

Temporary wound cavity effects

• If the penetrated tissue is highly elastic, pulsing of the cavity may occur until all of the strain energy has been released and the temporary cavity ceases to exist.

• Initially, the temporary cavity is almost a complete vacuum before air entering through the entry site and water vapour created by friction between the projectile and the penetrated medium, flood the cavity.

• It is generally the case that the temporary cavity itself is not a serious wounding effect of penetration on ‘elastic’ tissues but is more so for ‘plastic’ materials.

Low Velocity, Large Caliber

• Associated with pistol calibers, usually unjacketed/semi-jacketed, causing blunt trauma.

• Small permanent cavity, limited tissue destruction.

• Cylindrical, regular temporary cavity, less disruptive.

• Bullets lose energy quickly, may remain in victim, simplifying forensic recovery.

High Velocity, Small Caliber

• Most rifle bullets, almost always jacketed, causing significant tissue damage.

• Large, irregular permanent wound cavity if the bullet breaks up, increasing complexity.

• Large temporary wound cavity, widest in the middle, exacerbating tissue disruption.

• Non-partitioned bullets may strip from core, separating components.

• Core may over penetrate, jacket remains, altering wound profile.

General Wounding Considerations

• Wounding ability assessment useful in understanding projectile effects.

• Empirical testing is more useful to understanding wounding complexities of a particular projectile.

• Shot placement is the largest factor affecting the chance of gunshot wound survival; impacts to vital organs are critical.

Stopping Power

• 'Stopping power' has no scientific basis, so never use it! It's a misnomer.

• Target stops only if the central nervous system is damages, or critical structure which induces massive bleeding:

• Impacts to the brain, spine, or heart cause immediate incapacitation.

• Impact anywhere else will only cause incapacitation only occur through eventual blood loss (exsanguination).

Ballistic Penetration Simulant

• "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."

Why Use Ballistic Simulants?

• To overcome ethical issues with testing on human tissues, ensuring humane research.

• To give comparative penetration data between different projectiles, aiding in weapon selection.

• To understand the potential wounding effects of projectiles, improving medical treatment.

• To give accurate and reproducible data, enhancing scientific validity.

Desirable Properties of Soft Tissue Simulants

• Non-cadaveric, non-biological, avoiding ethical and logistical challenges.

• Ethically sound, ensuring compliance with research standards.

• Readily available, easy to handle/store, facilitating practical use.

• Similarity in deceleration/deformation behavior, mimicking tissue response.

• Similarity in kinetic energy dissipation, replicating energy transfer.

• Kinetic energy dissipation measurability, allowing quantitative analysis.

• Extrapolation of cavity diameters, predicting tissue damage extent.

• Elastic behavior, simulating tissue elasticity.

• Reproducibility, ensuring consistent results.

• Able to be CT scanned, enabling non-destructive analysis.

Materials Used as Soft Tissue Simulants

• Water: Simple, but limited in mimicking tissue properties.

• Wet phone books: Layered structure provides some resistance.

• Strawboard: Inexpensive and easy to handle.

• Clay: Plastic material that records permanent deformation.

• Transparent gel candle: Allows visual observation of bullet path.

• Lead: High density for simulating bone impacts.

• Glycerine soap: Simulates temporary cavity formation.

• Hydrogels: Water-based gels with tunable properties.

• Synthetic polymers: Customizable for specific tissue properties.

• Cadavers: Most realistic, but ethically and logistically challenging.

Ballistic Gelatin

• Collagen extracted from bones/sinews of pigs, providing a consistent medium.

• Two formulations for soft body tissues:-

  • 10% gelatin @ 4 °C, mimicking muscle tissue.

  • 20% gelatin @ 20 °C, simulating denser tissues.

• Density is temperature sensitive, affecting penetration depth.

Ballistic Gelatin Production Method

• Measure gelatin powder (10% of end weight), ensuring correct concentration.

• Measure water component, maintaining the proper ratio.

• Add gelatin to cool water, allowing it to bloom.

• Add hot water, dissolving the gelatin completely.

• Mix with an electric paint mixer for 10 minutes to dissolve all lumps, ensuring homogeneity.

• Add 50 ml of propionic acid or 5 ml of cinnamon oil (bactericidal), preventing microbial growth.

• Remove excess foam, pour into mold, and wrap in polyethylene film, preventing contamination.

Soap

• Soap is a 'plastic' (non-elastic) material, retaining deformation.

• Allows observation of temporary cavity profile, visualizing cavity dynamics.

• Cavity size can be used to predict human tissue, providing estimates of tissue damage.

Leather

• Upholstery or chamois leather simulates human skin, replicating its structure.

• Collagen in skin represented by grain structure, affecting penetration resistance.

• Properties vary by body region and animal age, influencing ballistic performance.

Head Models

• "Skin-skull-brain" model includes multi-layered polyurethane bone structure, silicone skull cap, and 10% gelatin brain @ 4 °C, providing a comprehensive simulation.

• Shepherd model includes realistic geometry, polyurethane skull, drilled foramen magnum, 10% gelatin brain @ 4 °C, and silicone skin layer, enhancing anatomical accuracy.

Summary

• Range/point of shot determination possible from impact/GSR at short range, aiding in forensic investigation.

• Rifling mark data almost always recoverable, linking bullets to specific firearms.

• Know wound ballistic terms, facilitating effective communication.

• Be aware of key ballistic simulant systems and their uses, enabling appropriate experimental design.

• Understand how wound ballistics experiments are conducted, ensuring reliable data collection.