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Chapter: Forensic Medicine: General traumatology

Mechanical trauma

When the body is exposed to mechanical violence, certain factors will determine the extent of the injury or trauma: This is also called the effective wounding or injury force.

Mechanical trauma

When the body is exposed to mechanical violence, certain factors will determine the extent of the injury or trauma: This is also called the effective wounding or injury force.


The effective wounding or injury force

The effective wounding force, that is the effective amount of force resulting in a wound, and which will determine its extent and appearance, depends on the following factors:

The amount of energy transferred to the tissue. Mechanical violence to the body is the result of the relative movement of the body in relation to another object, for example a knife or the road surface. It therefore represents the kinetic energy of the object, which is represented by the formula Ek = ‰mv2, available for acting on the body.

The duration of the application of the force. Cricket players know too well that it is much more comfortable and less painful to catch a cricket ball while the hand is moving in the same direction as the ball. As the ball is stopped over a longer distance as well as time interval, the loss of energy by the ball occurs more gradually. As the energy transfer from ball to hand thus takes longer, energy and therefore the effective force that can cause trauma, is all the time kept low and thus trauma is prevented. Scientifically speaking energy loss per time unit is lower than it would have been if the ball stops immediately, for example as in hitting a wall. The amount of force needed to cause trauma (or pain) is thus never reached.

The surface of application of the force. When the body is hit by a flat piece of wood (plank) or other material, energy loss occurs over a larger surface area compared with the force exerted on the tip of a knife or on its cutting edge. The amount of energy per surface area unit is therefore much higher in the case of a knife tip compared with the larger flat surface of a plank.

The behaviour of an object when it hits the body. If the object disintegrates at the moment of impact, energy is lost, and less energy is then available to cause injury.

The biomechanical features of the tissue. Certain tissue types, for example the skin, can stretch before it is injured. Other types such as bone and organs like the liver, do not have the same degree of elasticity. Fatty tissue also bruises easily. The degree of resistance to injury by the skin varies. The skin over the abdomen can undergo significant distortion before it tears; the scalp is more easily torn because it is trapped between the object of force and the underlying skull. Abnormal conditions can also influence the effect of violence. If a person bleeds easily it will increase blood loss and therefore the extent of contusions (bruises). Other diseases affecting organs, for instance enlarged spleens in malaria cases also exacerbate trauma.

Protective material. A helmet and thick leather clothes as well as thick hair covering the scalp will diminish trauma.


Appearance of a wound

The appearance of a wound is determined of a number of factors:

·              The effective wounding or injury force, as described above.

·              The age of a wound. Certain changes occur after time  -   contusions undergo a spectrum of colour changes, from purple to yellow-brown. The edges of wounds are often swollen in the initial stage.

·              Therapeutic interventions. Suture material and other interventions change the appearance of the wound. In some cases the wound can even be excised, for instance where a plastic surgeon changes the appearance of a wound to achieve a better cosmetic effect.

·              Complications. Certain wounds, for example wounds due to animal bites, often become inflamed. The inflammation process can change the appearance of a small wound dramatically.

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