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Experimental Wound Ballistics

Medical Science Publication No. 4, Volume 1

EXPERIMENTAL WOUND BALLISTICS*

CAPTAIN EDWARD W. A. OCHSNER, Jr., MC

In the experimental laboratory we have been attempting to develop a standard wound preparation comparable to a lethal war injury. Such a preparation would permit one to: (1) study in detail, and under rigidly controlled conditions, the disturbed pathophysiology associated with wounding, and (2) to compare various types of therapeutic measures that may be applied to the wounded.

The animal we have used in our studies is the goat, chosen primarily because of its size, docility and relative availability.

The animals have been wounded by two methods, each of which is comparable to a slightly different type of war injury. In one preparation a high-velocity missile wound is produced by shooting the animal through the large posterior muscle mass of the upper hind leg with a steel sphere (53.3 grains), traveling at a speed greater than 6,000 ft./sec. The missile produces external wounds of varying size on the lateral and medial aspect of each upper leg, but even when the external wounds are very small, the damage done to the tissues within the legs is always extensive. The second preparation, i. e., the blast injury, is produced by a high-explosive charge called "tetryl" (2, 4, 6 trinitro-phenyl-methyl nitramine). A small cylinder (0.795 inch in diameter and 0.494 inch in depth) is taped to the lateral aspect of each upper hind leg. On top of and in direct contact with the explosive is a detonator cap and the charge is detonated by 0.7 watt of electricity. The typical wound that results is superficially much larger than that produced by the high-velocity missile and even though there is no exit wound on the medial aspect of the thigh, the tissue damage extends through the leg.

These are interesting preparations in that untreated animals never survive more than 35 hours (with an average survival time of about 12 hours). They are all the more interesting when it is realized that blood transfusions will not by themselves save these animals. It is hoped that these preparations will be useful in attacking some of the problems raised by the observation and study of the wounded in the field. The current studies on this preparation will not be reported


*Presented 20 April 1954, to the Course on Recent Advances in Medicine and Surgery, Army Medical Service Graduate School, Walter Reed Army Medical Center, Washington, D. C.


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at this time, since what I really intend to add to this conference is not what happens as a result of wounding, but what happens during wounding. A concept of the wound mechanics is essential for the interpretation of the pathophysiology and an evaluation of any therapeutic procedures. Fortunately, many of these biophysical events have been defined for us by use of spark shadow-graphs, microsecond x-rays and high-speed motion pictures.

The first principle to be understood is that in order for a wound to be produced it is necessary for a missile to have a minimum striking velocity, this depending both on the character of the missile and the character of the tissue. The first major event occurring during wounding is shown by figure 1, which is a spark shadow-graph showing a small missile striking an air-water interface from above. This shows the hydraulic shock wave which is an extremely high-pressure front that spreads out radially with the speed of sound in water (4,800 ft./sec.) from the point of impact of the missile. This is a physical entity and must not be confused with the physiologic syndrome of shock which might occur subsequently. It is currently believed that the shock wave causes no tissue damage except perhaps in the presence of a gas-filled viscus.

FIGURE 1. [Omitted]

SOURCE: Harvey, E. N., McMillen, J. H., et al.: Surgery 21: 218-239, 1947.


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The second and more important event is the development of a temporary cavity which forms behind the missile, beginning first as a cone-shaped space which can be seen as the black area in figure 1, but which is better illustrated by figure 2, which is a microsecond x-ray of a .45 caliber lead ball that has been fired into a 20 percent gelatin block from above. Twenty percent gelatin is used because it has the same consistency as muscle. It should be noted that it is the presented cross-sectional area of the bullet that forms the cavity and that most of the missile may float free in space never contacting the tissue. As the bullet flings tissue from its path, it imparts energy to it and thus creates secondary missiles out of the body substances. The cone-shaped cavity now becomes cylindrical, producing local forces of the same destructive nature as would have been produced by an actual explosion within the tissue. The displacement is only temporary, however, the tissues tending to snap back roughly into their former position and the cavity pulsating (that is, collapsing and re-expanding) a few times before finally disappearing. Figure 3 is a series of x-rays (exposure time-1 microsecond) taken at a varying number of microseconds after impact, illustrating these events. It also shows that the cavity continues to grow even though the missile

FIGURE 2. [Omitted]

SOURCE: Herget, C. M.: Wound Ballistics in Surgery of Trauma-Ed.: W. F. Bowers. J. B. Lippincott Co., Philadelphia, 1953.


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has passed. The cavity concept will seem real, I am sure, if you will refer to figure 4, which is a microsecond flash picture of our high-velocity wound preparation taken 2,000 microseconds after impact of the missile.

[Omitted]
SOURCE: Herget, C. M.: Wound Ballistics in Surgery of Trauma-Ed.: W. F. Bowers. J. B. Lippincott Co., Philadelphia, 1953.

The maximum size of the temporary cavity is a function of the energy transfer and thus the velocity of the missile, the cross-sectional area presented by the missile, and the elastic properties of the tissue. Figure 5 shows the effect of cross-sectional area for .30


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FIGURE 4.

SOURCE: Biophysics Div., Med. Lab., Army Chemical Center, Maryland.

FIGURE 5.

SOURCE: Biophysics Div., Med. Lab., Army Chemical Center, Maryland.


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caliber bullet that was fired into a long gelatin block from the right. It began to tumble in the middle and presented its broadside to the left-hand side of the gelatin block.

The phenomena that I have mentioned have all been demonstrated to occur during missile wounding. What happens during wounding with a high explosive is less understood. It is known, of course, that one starts out with a solid material and ends up with a gas. The actual process of disintegration and the biophysical changes produced during this process need further elucidation. It is believed, however, that the same basic phenomena occur as in missile wounding. That is, there is the shock wave to begin with, and then the cavitational effect, perhaps myriads of cavities are produced as the solid charge bursts into small burning particles which enter the body as individual missiles until they burn up.

It should be easy to appreciate that the final wound seen by the surgeon may not reveal the total amount of tissue damage incurred at the time of wounding. After all, the permanent wound cavity represents only that tissue which has been permanently excavated or exteriorized. This can be demonstrated by figure 6, which shows the large radiolucent temporary cavity and the permanent cavity which is injected with radio-opaque material. It takes little imagination to appreciate that there is considerable tearing and crushing of tissues at a far distance from the permanent wound cavity.

In fact, in our standard wound preparations I have been impressed by the amount of damage done to vessels, nerves, and even bones (note the femur in figure 6)-although not directly hit by the missile. In the case of muscles, it appears that for many inches from the permanent wound, there may be damage in those portions of the muscle adjacent to the fascia, although the rest of the muscle appears normal. This suggests, as might be expected, that the temporary cavity spreads largely along the fascial planes, and it emphasizes the desirability of opening the fascia in the surgical treatment of these wounds.

As a result of the temporary cavity we can expect that not only destruction but contamination will be found far from the clinically presenting wound. In fact, Dziemian (1) has demonstrated that during the tension phase of the temporary cavity foreign material is actually sucked into the wound and not carried in by the missile. It must also be realized (fig. 7) that in the presence of armor, damage can be done by the temporary cavity even though the missile has been completely defeated. Proper offset of the body armor will, of course, eliminate much of this danger, but while saving wounds armor may introduce occult wounds and other problems for the field surgeon.


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FIGURE 6. [Omitted]

SOURCE: Herget, C. M.: Wound Ballistics in Surgery of Trauma-Ed.: W. F. Bowers. J. B. Lippincott Co., Philadelphia, 1953.


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FIGURE 7. [Omitted]

SOURCE: Herget, C. M.: Wound Ballistics in Surgery of Trauma-Ed.: W. F. Bowers. J. B. Lippincott Co., Philadelphia, 1953.

References

1. Dziemian, A. J., and Herget, C. M. Physical Aspects of Primary Contamination of Bullet Wounds. Mil. Surgeon 106: 294-299, 1950.

2. Harvey, E. N., Korr, I. M., Oster, G., and McMillen, J. H.: Secondary Damage in Wounding Due to Pressure Changes Accompanying the Passage of High Velocity Missiles. Surgery 21: 218-239, 1947.

3. Herget, C. M.: Wound Ballistics, p. 494 in Surgery of Trauma-Ed: W. P. Bowers. J. B. Lippincott Co., Philadelphia, 1953.