Mechanical Trauma
Mechanical trauma occurs when the force applied to a tissue, such as skin or bone, exceeds mechanical or tensile strength of that tissue. Mechanical trauma can be described as resulting from sharp or blunt force. Sharp force trauma refers to injuries caused by sharp implements, such as knives, axes, or ice picks. Significantly less force is needed for a sharpened object to cut or pierce tissue than what is required with a blunt object.
FIGURE 1 A gunshot wound to the head typically shows a small, clean entrance; note the stippling of gunpowder burns around the wound.
FIGURE 2 In protecting themselves from attack by a sharp object, victims often have wounds indicating their attempt to ward off their attacker.
FIGURE 3 Determining the size and shape of the weapon by the size and shape of the wound may be difficult. Here, the same-size wound could be caused by a knife of the same, smaller or larger size. Education, training, and experience are important for the forensic pathologist to make a proper interpretation.
Blunt force trauma is caused by dull or non-sharpened objects, like baseball bats, bricks, or lamps. Blunt objects produce lacerations, or tears in the tissue, typically the skin, whereas sharp objects produce incised wounds, wounds that have more depth than length or width. The size, shape, and kind of wound may allow the forensic pathologist to determine if a sharp or blunt object caused it. Judicious interpretations and caution are required because of the flexible nature of many of the body’s tissues and the variability of the violent force. For exam ple, a stab wound 1-inch wide, 1/8-inch thick, and 3-inches deep could have been produced by (1) a sharp object of the same dimensions, (2) a sharp object that is 1/2-inch wide, 1/8-inch thick, and 2-inches long that was thrust in with great force and removed at a different angle, or (3) a sharp object larger than the stated dimensions but was only pushed in part of its length, as represented graphically in Figure 3. Occasionally, the injury may be patterned and reflect the specific nature of the causative agent: For example, the threaded end of lead pipe used to beat an individual may transfer a set of parallel contusions with uniform spacing that corresponds to the pipe’s thread pattern. Death from blunt and sharp trauma results from multiple processes, but sharp trauma most commonly causes death from a fatal loss of blood (exsanguination) when a major artery or the heart is damaged. Blunt trauma causes death most often when the brain has been severely damaged. A contusion is an accumulation of blood in the tissues outside the normal blood vessels and is most often the result of blunt impact. The blood pressures the tissues enough to break small blood vessels in the tissues and this leaks blood into the surrounding area. Importantly, the pattern of the object may be transferred to the skin and visualized by the blood welling up in the tissues.
An extreme contusion, a hematoma, is a blood tumor or a contusion with more blood. Abrasions, scraping of the skin surface, while rarely fatal in themselves can often corroborate or help explain the circumstances surrounding death. Asphyxia is a type of mechanical trauma in which the body is deprived of oxygen. The brain is the most susceptible organ to asphyxia, and unconscious ness typically follows loss of oxygen flow in 10 s with irreversible coma resulting in a matter of a few minutes. Asphyxia can occur as a result of three main mechanisms. Suffocation occurs by covering the nose and mouth blockage of the major airways with a foreign object. Strangulation occurs by manual or ligature compression of the structures of the neck, often leaving characteristic physical evidence, such as the fracture of the hyoid bone and bruising. Finally, chemical asphyxiation occurs when the oxygen in the air is replaced by some other gas, such as carbon monoxide (CO).
