Ballistic Trauma

• •Projectile trauma usually results in:

  –Complete discontinuities

  –Displacement

  –Fracture lines

  –Catastrophic bone shattering

  •Various details may be discerned from projectile bone trauma:

  –Causative weapon type (limitations)

  –Characteristics of projectile

  –Approximate position of weapon in relation to victim i.e. trajectory

  –Estimated sequence of wounds

  
• Bullet vs Bone

  •Huge variety of firearms and ammunitions available (makes analysis and interpretation difficult)

  •A number of aspects directly determine the effect of a projectile on tissues (including bone):

  –Velocity

  –Mass

  –Shape/formation of projectile

  –Density composition

  –Yaw (angle of projectile travel)/Tumble

  –Distance

  –Rifling/barrel modifications

  •Tissue absorption of KE more important than KE of missile

  –Influenced by:

  •Elasticity

  •Cohesiveness

  •Density

  •Shape
• Size of Projectile

  •Measured in terms of caliber, gauge or number

  •Caliber

  –Diameter of bullet and barrel of gun

  –Caliber can be complicated as actual bullet diameter can deviate from manufacturer specifications

  •Gauge

  –Relates shotguns

  –Barrel diameters are measured in gauges and pellets expelled by number

  –Refers to the max weight of a lead ball that would fit down the barrel of the weapon

  •E.g 10-gauge shotgun would allow a ball weighing one tenth of a pound down the barrel
• Size of Projectile

  •Pellets

  –Solid balls of lead/steel/bismuth/copper

  –Pellet size is number, the higher the number, the smaller the pellet

  –Shotgun round is designed to expel 1 ounce of pellets independent of their size
• Projectile Construction

  •Construction refers to profile, internal composition and jacketing.

  •Bullet profile:

  –Sharp

  –Blunt

  –Hollow point

  –Soft tip

  –Tubular hollow point

  •Internal composition

  –Either solid lead (deformable) or frangible

  •Jacketing – metal covering on bullet

  –Entire covering – full metal jacket

  –Partial covering – semi-jacket

  –Jacketing reduces deformation and fragmentation

  
• Projectile Velocity

  •Velocity has the greatest effect on wounding power

  •Kinetic energy has a linear relationship with mass but is a function of the square of its velocity

  –Doubling mass doubles kinetic energy

  –Doubling velocity quadruples kinetic energy

  •Catastrophic wounds are more characteristic of high velocity projectiles

  
• Shotguns

  •Share many similarities to high velocity projectiles

  •Has a wide bore (calibre) firing either a solid slug or packed shot balls

  •Barrel is smooth with no rifling

  •Capable of high energy transfer over short range (effective range ~35-40m)

  •At closer ranges, before the shot column has had time to disperse, the tightly patterned shot produce a ‘billiard ball’ effect

  •Longer ranges produce ‘shot cone’ which may ‘pepper’ target

  
• Low velocity projectiles

  •Typical of handguns

  •Usually under 1800 feet per second (545 ms-1)

  •Number and magnitude of fractures reflects relative power of the weapon and projectile.

  •The point of rest on x-ray may or may not reflect the wound path.

  –Remember the viscoelastic properties of bone

  –Failure of the bone in a ductile or plastic phase will cause permanent distortion of the fragments.

  –Lead or other bullet materials may be wiped into the site of passage.

  
• High velocity projectiles

  •Velocity greater than 1800-2000 feet per second (545 to 610 ms-1)

  •Cavitation

  -Temporary and Permanent

  •High magnitude of damage

  •Bullet fragments are often minute, looking like a “snowstorm” on x-ray.

  •Bone fragments set into motion as secondary missiles

  
• Anatomy of Bullet Travel

  •At beginning of flight, long axis of bullet is parallel to flight path (trajectory)

  •Deviation from trajectory

  –May result is oblique impact with target

  –Causes non circular wound

  •General rule:

  –Trajectory/impact perpendicular to target = circular outline

  –Trajectory/impact not perpendicular to target = oblique outline

  •When expelled from gun, some bullets spin around their long axis

  •Spiral grooves (rifling) in gun barrel initiate spin

  –Cause bullet to go straighter for longer

  
• Anatomy of Bullet Travel

  •Penetrating v Perforating Wounds

  •Projectiles can penetrate bone at low velocity - 60m/s

  •Issues with interpretation can be caused by:

  •Ricochet

  •Intermediate targets

  •Projectile deformation

  •Projectile fragmentation

  •Formation of secondary projectiles:

  •Bone Fragments

  
• Effects of Bullets on Bone

  •When bullet contacts bone:

  •Wound is formed

  –Severity dependant on multiple factors

  •Fracture

  •Radiating

  •Concentric

  •Fragmentation

  •The effect of a bullet on a bone can be considered in four distinct categories:

  –Wound bevelling

  –Wound shape

  –Wound size

  –Fracture lines

  
•   Beveling

  •When projectile strikes bone, it deforms to a variable degree

  –Dependant on projectile construction (jacketed/non-jacketed)

  •Deformation causes the exit defect to be larger than entry defect

  •Defect adopts funnel shape

  –Funnelling is termed bevelling

  •Three categories:

  –Inward

  –Outward

  –Reverse

  
• Beveling

  •Inward bevelling

  –Observed at site of entry

  –External defect is smaller than internal defect

  –Inward bevelling is seen on the internal surface of the entry defect

  •Outward bevelling

  –Observed at site of exit

  –Internal defect is smaller than external surface of defect

  –Contrast to inward bevelling wounds

  •Reverse bevelling

  –Characterised by bevelling on the external surface of an entry defect IN ADDITION TO bevelling on the internal surface

  –Tangential entry

  
• Defect Shape

  •Generally categorised into one of four shapes:

  –Round, oval, keyhole, irregular

  •Defect shape dependant on:

  –Type of ammunition/weapon

  –Projectile construction (deformable?)

  –Angle of trajectory

  –Angle of axis (of bullet)

  –Entry or Exit

  –Presence/absence of wadding

  
• Round Defects

  •Angle of trajectory and angle of bullet axis are perpendicular to the bone surface.

  •More likely observed in entry rather than exit wounds

  •Depends on jacketing

  –  Jacketing may result in round entry and exit wound

  •Opening of defect bevel smaller than exit

  
• Oval Defects

  •Occur when either:

  –Angle of trajectory is not perpendicular to the bone surface

  –Bullet is tumbling when it strikes

  •Results in <90o  angle between bullet axis and bone surface

  •More commonly observed on entry wounds

  
• Keyhole Defects

  •Usually caused by bullets grazing bone

  •Can occasionally be viewed in exit wounds

  •Connected to round wound is splayed triangular exit wound with outward bevelling

  •Most commonly observed in cranial vault

  •Can originate from any bullet type

  
• Keyhole Defects

  •A – bullet enters on tangent, fractures radiate away from entrance defect and bone is levered from the outer cortex

  •B – External view of entrance wound, external view demonstrating external bevelling opposite impact point

  •C – bullet exits on tangent, fractures radiate away from entrance defect and bone is levered from inner cortex

  •D – External view of exit wound, external bevelling along periphery

  
• Irregular Defects

  •Lack uniform outline

  •Result of bone shattering

  –Catastrophic force (e.g. high velocity projectile)

  •More characteristic of exit wounds

  –Can occasionally be observed at entry wounds – why might this occur?

  •Result of deformable bullets

  –Soft tipped or hollow point bullets

  
• Defect Size

  •Defect size is multi-factorial

  •Primary factors:

  –Wound type (entry vs exit)

  –Projectile characteristics (caliber, construction, velocity)

  –Bone characteristics (thickness)

  •Thicker bones = bullet deformation = larger entry wound

  •Exit wounds generally larger than entry wounds

  •Effect of intermediate targets

  •Larger caliber projectiles tend to exhibit larger defects

  –Exceptions
• Fracture Lines

  •Radiating fracture lines

  –Originate from site of impact and travel peripherally

  –May intersect lines of weakness within vault

  –Power can be dissipated by suture, foramen, another fracture line

  •Concentric fracture lines

  –Lines encircle (completely or partially entry point)

  –Fracture occurs in internal cortex then passes to external cortex

  –Power dissipated when radiating fracture encountered.

  
• Penetration mechanics

  •Primary Fractures

  •Plug and spall produced by the penetration process

  •Plug formation seen when thick bone fails in shear due to large radial tensile forces

  •Spalling produces the characteristic internal bevel

  –A tensile release wave immediately behind the intense compression wave from impact interacts with the reflected tensile wave at the free surface of the internal table, producing tensile forces exceeding the bone strength.

  –The result is a cone shaped defect.

  
• Secondary fractures

  •Radial fractures with point of origin at impact.

  –Large hoop stresses build up tensile strain despite the relief from the primary fracture.

  –The material fails in tension and large pie-shaped fragments are produced by the radial fracture lines.

  
• Secondary fractures

  •The radial cracks form very quickly, and may displace prior to the bullet exit.

  –Radial cracks may:

  avoid buttressed areas

  travel the line of least resistance along suture lines bifurcate in tensile stress fields

  •Radiating fractures from entrance wound can reach opposing side of vault before bullet can travel through the skull

  
• Tertiary fractures

  •Termed “concentric heaving fractures”.

  •Outward displacement of the fragments from increased intracranial pressures.

  •Beveling of concentric fractures is external, regardless of entrance or exit.

  •Curvature of skull may attenuate bevel.

  –Concentric fractures may terminate when they intersect radiating fractures

  –Propagating fractures may intersect existing fractures and stop OR may jump pre-existing fractures/sutures if there is sufficient force

  
• BFT vs Ballistic Trauma

  •Berryman and Symes (1998)

  •Blunt force produces internally bevelled tensile failure in concentric fractures

  •Ballistic produces externally bevelled concentric due to tension on intracranial surface produced by elevated intra-cranial pressure

  
• Projectile Defect Analysis

  •Initial description of defects:

  –Use appropriate anatomical terminology

  •Relative to anatomical position

  –Placement relative to fixed craniometric or anatomical points (if possible)

  –Size and shape of defects

  –Number and description of fractures

  –Presence and direction of beveling

  •Subsequent analysis may include:

  •Estimation of trajectory/direction of impact

  •Estimation of sequence

  
• Estimation of Direction

  •Based on shape and alignment of defects

  –Round – bullet axis perpendicular to bone

  –Oval – bullet axis at angle to bone

  –Keyhole – bullet grazing bone

  •Alignment of entrance and exit wounds

  –Position of beveling

  •Direction of the projectile may be determined from the secondary and tertiary fracture patterns.

  –This is especially valuable when the primary fracture is absent

  •Tangential (keyhole) entrance defects will be eccentric to oval in shape with some external bevel.

  –The internal aspect will be fully bevelled.

  
• Estimation of Sequence

  •Step 1: Distinguish entry from exit wounds

  •Step 2: Distinguish radiating from concentric fracturing

  •Step 3: Follow radiating fractures from origin to terminus

  –  If radial fracture terminates in another fracture, it came later than this fracture

  •Use the phenomena of intersecting fracture lines to help determine sequence.