MAP 4 The Physics of Gunshot Injuries PDF

Lecture The physics of gunshot injuries Module Medical Applications of Physics Code MAP.4 Lecturer Prof Kevin McGuigan Learning Outcomes MAP.04.01 Differentiate between rifles and handguns Describe the factors influencing the severity of gunshot MAP.04.02 wound...

Lecture The physics of gunshot injuries Module Medical Applications of Physics Code MAP.4 Lecturer Prof Kevin McGuigan Learning Outcomes MAP.04.01 Differentiate between rifles and handguns Describe the factors influencing the severity of gunshot MAP.04.02 wound injury. MAP.04.03 Describe the factors influencing bullet design. MAP.04.04 Define precession and nutation. MAP.04.05 Explain laceration and crushing. MAP.04.06 Explain shock wave injury. MAP.04.07 Explain cavitation injury. MAP.04.08 Describe the mechanism of immediate wound infection. NOTE The following lecture discusses the mechanics of injury associated with gunshot wounds. However it should be noted that many of the types of injuries discussed are also associated with wounds caused by explosions either deliberate (ordinance, improvised explosive devices) or accidental (fuel, industrial, etc.). Energy is sometimes defined as the capacity to do Work on an object. The severity of gunshot wound (GSW) injuries are determined mostly by how much Kinetic Energy (KE) is transferred from the bullet to the victim. The work done by the bullet is in the form of “re-arranging” the anatomical structures. Kinetic Energy = ½mv2 The amount of work done depends on the KE of the bullet, which depends on many factors: Prof. K.G. McGuigan, RCSI 4 1. Rifle or Hand-gun? The volume of expanding gas created by detonating the propellant, accelerates the bullet for as long as it is confined within the barrel of the weapon Prof. K.G. McGuigan, RCSI 5 WW1 LE.303 Baretta 9mm FN SLR SW.44 magnum AK47 Prof. K.G. McGuigan, RCSI 6 Rifle cartridges usually contain much more chemical propellant than those for hand guns because rifles have thicker gun barrels & can withstand higher pressures. Prof. K.G. McGuigan, RCSI 7 2. If it is a handgun, is it a pistol or a revolver? Revolver Pistol Pistol hand-grip/spent-shell ejection systems often require smaller cartridges than for revolvers. Consequently, pistols often have lower muzzle velocities.  Kinetic Energy = ½mv 2 If bullet mass doubles, KE increases by a factor of 2. If bullet velocity doubles the KE increases by a factor of 22 = 4. Prof. K.G. McGuigan, RCSI 9 3. Design of Bullet Prof. K.G. McGuigan, RCSI 10  1 grain = 1/7000 U.S./UK pound (lb) 1 grain = ~65 mg Prof. K.G. McGuigan, RCSI 11 Hollowpoint or expanding bullets are designed so that they deform on impact. Their weak sides expand outwards so that they fully decelerate and cause more serious injuries. Prof. K.G. McGuigan, RCSI 12 Prof. K.G. McGuigan, RCSI 13 3a. Stability of the bullet in flight will have an effect on the accuracy of the weapon, the size of the entrance wound and also the amount of damage along the wound path. All bullets are aerodynamically unstable, but some are more unstable than others. Prof. K.G. McGuigan, RCSI 14  An unstable bullet is an inaccurate & unpredictabl e weapon “Yaw or Yawing” is periodic movement along the longitudinal axis of a projectile. Its complete turn beyond 90° is called “tumbling”. Spin provides gyroscopic stability effect to counteract yawing and maintain the bullet’s “nose-on” orientation, resulting in a complex spiral movement of the bullet’s nose, known as “precession”, Prof. K.G. McGuigan, RCSIwherein the bullet rotates 15 Precession & Nutation are caused by the interaction of the conservation of the bullet’s angular Prof. K.G. McGuigan, RCSI 17 Bullet precession reduces with distance from the weapon. Yaw, nutation and precession all combine to ensure that the bullet seldom presents minimum surface area at the point of impact (entry wound). Prof. K.G. McGuigan, RCSI 18 Friction caused as the bullet travels down the barrel, can melt lead (Pb). Rifle bullets are often covered with a metal (usually copper or nickel) jacket to prevent this. (“Full Metal Jacket”) Jacket often separates from bullet on impact causing secondary Metal jacket wound tracks/injuries. covering a lead core Prof. K.G. McGuigan, RCSI 19 High velocity rifle and/or jacketed bullets may pass straight through the victim. This means they don’t impart all their energy/momentum to the victim. They may also cause a secondary wound elsewhere. Prof. K.G. McGuigan, RCSI 20 Prof. K.G. McGuigan, RCSI 21 Extent of injuries is determined by the velocity of the bullet on impact. If the velocity is subsonic (below the speed of sound in air ~ 340 ms-1 ) then we usually only see Laceration & Crushing. Only bone & tissue in direct contact with the bullet (or bullet fragments) is damaged. Prof. K.G. McGuigan, RCSI 22 Prof. K.G. McGuigan, RCSI 23 Only fatal if a vital organ, or major blood vessel is along wound path. Prof. K.G. McGuigan, RCSI 24 If the bullet velocity is supersonic ( > 340ms-1) more serious, indirect injury may result from: 1. Shock Wave 2. Temporary Cavitation Prof. K.G. McGuigan, RCSI 25 Shock waves can cause injury even if there is no direct contact with the projectile. Prof. K.G. McGuigan, RCSI 26 Prof. K.G. McGuigan, RCSI 27 Prof. K.G. McGuigan, RCSI 28 Prof. K.G. McGuigan, RCSI 29 Solid tissues such as muscle, liver, spleen and brain are very susceptible to shock wave injury. Applying shock waves to enclosed liquids, within the body can cause dramatic failure of the container material (Bladder, Heart, Bowels, Eyes, etc.) because liquids are incompressible. (Hydraulic Effect / Pascal’s Principle) Prof. K.G. McGuigan, RCSI 30  Shock Wave Damage by bullet to an empty metal container Damage by bullet to a metal container that was full of water at the time. Prof. K.G. McGuigan, RCSI 31  Shock Wave Empty skull shot from 14m with a rifle. Skull filled with ballistic gel (tissue simulant), shot from 14m with a rifle. See Zapruder film of JFK assassination Prof. K.G. McGuigan, RCSI 32 2. Temporary Cavitation The short-lived Temporary Cavity is sometimes called the “Stretch” Cavity The Permanent Cavity that forms the final bullet track (wound Prof. K.G. McGuigan, RCSI path) is 33 Prof. K.G. McGuigan, RCSI 34 High shutter speed photo of Temporary Cavitation caused by a high velocity projectile passing through Ballistic Gel Prof. K.G. McGuigan, RCSI 35 The amount of cavitation damage is generally: directly proportional to the density of the tissue. Indirectly proportional to the elasticity of the tissue. Skin & lung are very resistant to cavitation. Prof. K.G. McGuigan, RCSI 36 Large nerves are often severely damaged by the gross displacement they undergo during the cavitation process, particularly in the limbs. Prof. K.G. McGuigan, RCSI 37  Illustration from Otto von Gericke's “Experimenta”, 1672, horses tried in vain to tear apart two partially evacuated brass hemispheres using an early air hand-pump Prof. K.G. McGuigan, RCSI 38 Tissue necrosis caused by shock wave & cavitation create perfect conditions for bacterial growth. Prof. K.G. McGuigan, RCSI 39 The combination of Shock Wave and Temporary Cavitation often results in indirect tissue damage up to 10cm away from the obvious wound track. Surgeons often have to remove tissue far beyond the obvious boundaries of the GSW. Prof. K.G. McGuigan, RCSI 40