Injuries caused by physical violence to the body are known as Mechanical Injuries. These injuries can be caused by a blunt or a sharp force and the forensic medical examination of such injuries are an important section under forensic-medical traumatology. These injuries can be inflicted accidentally, or in situations at work, sports, private life, and so on.
Mechanical injuries can be in the form of contusion, abrasion, laceration, puncture wound, or incised wounds. There are a lot of factors and conditions that influence the naked-eye appearance of wounds, histological, histochemical timings of the wound, and the biochemical timing of wounds. Careful examination of injuries is required to determine the cause of death, manner of death, and to avoid false allegations.
Injuries can give an idea about the type of weapon used, the direction of impact, and the force of impact. Hence, the factors affecting an injury should be always kept in mind while examining an injury during an investigation. It will also help in understanding the time of infliction and age of the injury.
Factors Affecting Mechanical Injuries
1. Nature of Offending Object/Weapon
When a sharp object is used then the force is applied over the small area on the skin and hence a deep injury will happen as the object will penetrate deep inside. When a blunt object is used, the force will get dissipated over a larger area of the body.
The injury caused by a blunt force is comparatively less severe and less deep than the injury due to sharp force. A rigid weapon will produce greater damage than a plastic or flexible weapon. The damage will be very minimal if the weapon breaks on striking the body as the kinetic energy is lost.
2. Amount of Energy Discharged During Impact
The amount of kinetic energy will be doubled when the weight of the weapon is doubled and it will be quadrupled when the velocity of the moving mass is doubled.
This means that the relationship between energy and mass is linear while that between energy and velocity is exponential. This is why the velocity of the weapon plays a major role in the weight of the weapon.
3. Condition Under Which Energy is Discharged
If the time period of discharge of energy from blow/impact is increased then the destructive effect on the tissues of the target will be decreased.
4. Nature of Tissue Affected
- If the impact is made on the skin which is more pliable and less elastic, then the injury will be more. When the skin surface gets rubbed, the surface layer gets destroyed. However, the skin is resistant to traction force because of the firm cohesion of keratinized cells. The skin cells will get flattened and elongated when hit with a blunt force and so the severity of the damage will be less. The skin can readily split if it is struck with an object against rigid bone. This is why impact with the same force and same direction but on different body parts will cause different types of injury. The skin offers more resistance to stab wounds followed by muscles except for calcified cartilage or bones.
- The subcutaneous tissues act as a cushion for the body and are more plastic and pliable. But when they are struck with greater blunt force, the subcutaneous vessels get crushed and displaces fat droplets and tear the connective tissue framework resulting in contusion or laceration.
- Muscles of the body are elastic and plastic and can resist the impact to a greater extent. But when the force is more than the threshold limit of the muscles, then it will get avulsed, torn, ruptured, stretched, or crushed.
- If the force is applied to a body, it will first bend instead of completely breaking and will recoil to its original shape after releasing the force. But when it bends beyond its elasticity resulting in fracture, a twisting strain will produce spiral fracture and when the force acts on the greater surface area it will cause multiple fractures.
- Sudden compression of the chest may cause rupture of capillaries and small veins present on the face, neck, and shoulder from the retrograde displacement of blood in these veins. Violent compression can cause pneumostatic force causing damage like a blast injury.
Injuries have medicolegal importance because it helps in identifying the offending weapon, time of infliction, age of the injury, character, manner of injury, degree of violence and so on which can be helpful in the process of investigation.
While examining a wound the factors that can influence it should always be kept in mind to avoid any type of differential diagnosis and making wrong interpretations.