A large slow projectile (Fig 7) will crush (permanent cavity) a large amountof tissue, whereas a small fast missile with the same kinetic energy (Fig 4)will stretch more tissue (temporary cavity) but crush little. If the tissuecrushed by a projectile includes the wall of the aorta, far more damagingconsequences are likely to result than if this same projectile "deposits" thesame amount of energy beside this vessel.Many body tissues (muscle, skin, bowel wall, lung) are soft and flexible--thephysical characteristics of a good shock absorber. Drop a raw egg onto acement floor from a height of 2 m; then drop a rubber ball of the same massfrom the same height. The kinetic energy exchange in both dropped objects wasthe same at the moment of impact. Compare the difference in effect; the eggbreaks while the ball rebounds undamaged. Most living animal soft tissue has aconsistency much closer to that of the rubber ball than to that of the brittleegg shell. This simple experiment demonstrates the fallacy in the commonassumption that all kinetic energy "deposited" in the body does damage.The assumption that "kinetic energy deposit" is directly proportional todamage done to tissues also fails to recognize the components of theprojectile-tissue collision that use energy but do not cause tissuedisruption. They are 1) sonic pressure wave, 2) heating of the tissue, 3)heating of the projectile, 4) deformation of the projectile, and 5) motionimparted to the tissue (gelatin bloc displacement for example).The popular format for determination of "kinetic energy deposit" uses achronograph to determine striking velocity and another to determine exitvelocity. A 15-cm thick block of tissue simulant (gelatin or soap) is thetarget most often used. This method has one big factor in its favor; it issimple and easy to do. As for its validity, the interested reader is referredto wound profiles shown in Figs 1-7. Comparing only the first 15 cm of themissile path with the entire missile path as shown on the profiles shows thesevere limitation of the 15-cm block format. The assumption by weaponsdevelopers that only the first 15 cm of the penetrating projectile's paththrough tissue is of clinical significance (64) may simplify their job, butfails to provide sufficient information for valid prediction of theprojectile's wounding potential. The length of bullet trajectories through thehuman torso can be up to four times as long as those in these small blocs.Even if this method were scientifically valid, its use has been further flawedby nearly all investigators who have included the M-16 rifle bullet in thoseprojectiles tested. This method assumes that the projectile's mass remainsconstant through both chronographs. The M-16 routinely loses one third of itsmass in the form of fragments which may remain in the target (see Fig 2). Thepart of the bullet that passes through the second chronograph screens weighsonly about two-thirds as much as the intact bullet that passed through thefirst set of screens. No provision is made for catching and weighing theprojectile to correct for bullet fragmentation when it occurs. The failure tocorrect for loss of bullet mass can cause large errors in "energy deposit"data (8).Surgeons sometimes excise tissue from experimental missile wounds that is, intheir judgment, nonviable and compare the weight of tissue excised with the"kinetic energy deposited" (65). A surgeon's judgment and his technique oftissue excision is very subjective, as shown by Berlin et al (66), who foundin a comparison that "One surgeon excised less tissue at low energy transfersand rather more at high energy transfers than the other surgeon, although bothsurgeons used the same criteria when judging the tissues." None of theseexperiments included control animals to verify that tissue the surgeon haddeclared "nonviable" actually became necrotic if left in place. Interestingly,all studies in which animals were kept alive for objective observations ofwound healing report less lasting tissue damage than estimated fromobservation of the wound in the first few hours after it was inflicted (43-47,67, 68). In a study of over 4,000 wounded in WW II it was remarked, "It issurprising to see how much apparently nonvital tissue recovered" (69).Anyone yet unconvinced of the fallacy in using kinetic energy alone to measurewounding capacity might wish to consider the example of a modern broadheadhunting arrow. It is used to kill all species of big game, yet its strikingenergy is only about 50 ft-lb (68 Joules)-- less than that of the .22 Shortbullet. Energy is used efficiently by the sharp blade of the broadhead arrow.Cutting tissue is far more efficient than crushing it, and crushing it is farmore efficient than tearing it apart by stretch (as in temporary cavitation).
Lethal Pressure Crush 1159
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