Wound ballistics

Background

• Wound ballistics is the study of projectile penetration of tissues
• Wounding potential of projectiles is complex matter as it involves interplay of velocity, projectile type/composition, and tissue characteristics
• Knowing the type of weapon and ammunition helps in gaining a better understanding of the potential injuries and complications but should always be concerned with individual wound rather than potential of the weapon/projectile
• A clinically relevant understanding of wound ballistics helps with a targeted physical assessment and appropriate treatment options

Mechanisms of Projectile Wounding

• Wounding effect of a projectile is produced by transfer of Kinetic Energy (KE) from the projectile to the tissues
  • Projectile does not exit the body then all its kinetic energy has been transferred to the tissues
  • Projectile that exits the body then only some of its KE has been transferred to the tissues
• Wounding effect is the result of the amount and location of crushed and stretched tissue from this KE transfer
  • The greater the KE the greater the wounding potential

Damage produced in three ways

1. Laceration and crushing - Disruption and destruction of tissue caused by the projectile as it crushes and punches a hole in the tissue. The result is localized area of cell necrosis.
2. Cavitation
   • Permanent cavity - The hole left by the passage of the projectile is proportional to the size of the projectile. This is the volume of space once occupied by tissue which has been destroyed.
     • Cavity is a function of the penetration and frontal area of the projectile which makes it.
     • Cross-sectional area of the projectile track is comparable to the presenting area of the projectile and its dimensions are roughly the same for all soft tissues
     • Size of the permanent cavity can be increased by yaw (deviation of bullet from straight path within tissue). As more tissue is in contact with projectile there is more tissue damage
     • Size of permanent cavity can be increased by projectile deformation and fragmentation as well.
       • Deformation is when a projectile changes it shape upon contact with tissue which increases the diameter of the projectile. The flattening or deformation can cause 4 to 6 times greater tissue contact thus more tissue damage. Example would be hollow point bullets.
       • Fragmentation is when a projectile breaks apart within tissue creating many smaller projectiles. Fragmentation is more devastating than deformation.
   • Temporary cavity - The transient expansion of the permanent cavity by stretching due to the transfer of KE during the projectiles passage. The walls of the permanent cavity are temporarily stretched radially outward.
     • Temporary cavity may be up to thirty times the diameter of the projectile with a lifetime of 5 to 10 milliseconds with pressures of up to 100 to 200 atmospheres
     • Damage is due to stretching of the tissue
     • Damage is analogous to localized area of blunt trauma surrounding the permanent cavity
     • If the pressure of the temporary cavity exceeds the elastic limit of tissue then the organ may be disrupted "burst" or "fractured" and a large permanent cavity may be seen. Dense organs which are relatively inelastic are most susceptible to this affect (i.e. liver) whereas organs with low density and high elasticity are relative protected (i.e. lung).
3. Shock wave - acoustic wave which travels through tissue preceding the projectiles passage. Magnitude may be 100 atmospheres but duration is 2 microseconds. No detectable harmful effect on tissues.

Wounding Power

Projectiles mass and velocity

• Velocity dominant determinant of kinetic energy recall KE = 1/2( m * v^2)
• Velocity almost to exclusion of mass is operative factor
  • Double mass only doubles the KE
  • Double velocity quadruples the KE
• Range affects the velocity at which the projectile strikes body tissue

Projectiles physical specifications

• Caliber and shape of projectile influence effect in tissues and the loss of KE. Larger the caliber the greater the loss of KE.
• Structure of the projectile influences deformation and break-up in tissue. Projectiles which fragment result in greater loss of KE. Projectiles which deform in tissue cause greater loss of KE. High velocity projectiles more likely to break up than low velocity projectiles.

Characteristics of tissue that has been penetrated

• Resistance or vulnerability to stretch damage depends mostly on tissue elasticity
  • Elastic tissue - skeletal muscle, skin, blood vessels may be pushed aside but then rebound
  • Inelastic tissue - bone or liver may fracture or burst
• Wound track - the longer the wound track the greater the loss of stability and the greater the deformation of the projectile with resultant increased loss of KE
• The denser the tissue through which the projectile passes, the greater the retardation of the projectile and thus the greater the loss of KE

Misconceptions About Wound Ballistics

• Exit wounds are larger than entrance wounds -- unreliable and seldom impacts care
• All projectile tracks must be fully explored -- elastic soft tissue generally heals uneventfully and does not require excision provided the blood supply remains intact. Temporary cavity effects are analogous to blunt trauma.

Triage and Treatment Decisions

• Treat the wound not the weapon/projectile
  • Treatment decisions based on type of wound, location, and amount of tissue disruption
• Document wounds completely avoiding temptation to label as "entrance" or "exit" 

See Also

• Gun shot wounds
• Blast injury