An exit wound from a projectile is often larger than its corresponding entry wound. This difference is rooted in the complex interaction between a projectile and biological tissue.
Understanding Projectile Impact on Tissue
When a projectile, such as a bullet, strikes biological tissue, it transfers its kinetic energy to the body. This energy transfer is the primary factor in wound creation. Upon impact, the tissue directly in the projectile’s path is crushed and displaced, forming a direct channel. The immediate response of the body involves the displacement, compression, and stretching of tissues as the projectile forces its way through. This initial interaction creates the pathway for the projectile’s passage.
The Role of Cavitation in Wound Formation
A significant concept in understanding tissue damage is cavitation. As a projectile moves through tissue, it generates a pressure wave that radially flings surrounding tissue away from its direct path. This outward movement creates a temporary cavity, which can be significantly larger than the projectile’s diameter, sometimes up to thirty times its size. This temporary cavity exists for a very short duration, typically 5 to 10 milliseconds, rapidly expanding and then collapsing as the elastic tissue attempts to return to its original position.
While the temporary cavity is transient, its rapid expansion and collapse can cause considerable damage to tissues located far from the projectile’s direct track. If the pressure within this temporary cavity exceeds the elastic limit of the tissue, it can lead to tearing, rupturing, or even bursting of organs, particularly those that are less elastic like the liver. The permanent cavity, in contrast, is the actual hole formed by the crushed and destroyed tissue directly in the projectile’s path, representing the volume of tissue permanently destroyed.
Factors That Magnify Tissue Damage
Several characteristics of a projectile can significantly amplify the extent of tissue damage it inflicts, thereby influencing the size of the exit wound. Projectile velocity is a major determinant of kinetic energy; doubling the velocity quadruples the kinetic energy transferred. Projectile mass also contributes to kinetic energy, though its effect is linear compared to velocity’s exponential impact.
The projectile’s shape and stability as it moves through tissue also play a role. If a bullet yaws or tumbles, meaning it deviates from a straight, nose-forward path, it presents a larger surface area to the tissue. This increased surface area results in greater energy transfer and more tissue disruption. Fragmentation, where a projectile breaks into multiple pieces inside the body, further exacerbates damage. Each fragment acts as a secondary projectile, creating multiple wound channels and widening the area of destruction.
Why the Exit is Larger
As the projectile travels, it not only creates a permanent and temporary cavity but also accumulates displaced tissue, bone fragments, and other debris. This collection of material, combined with the outward pressure generated by the expanding temporary cavity, pushes forcefully against the tissue as it approaches the exit point.
The tissue at the exit point has already been stretched and weakened by the internal expansion of the temporary cavity from within. When the projectile and the accumulated debris finally breach the skin, they do so with a combined force and increased volume. This leads to a larger, often more irregular, and sometimes stellate, or star-shaped, exit wound. The maximum cavity diameter, and thus potentially the largest exit wound, typically occurs at the point of maximum loss of kinetic energy, which often coincides with maximum yaw or fragmentation.