Mechanical injury – An injury is any harm, whatever illegally caused to any person in body, mind, reputation or property (Sec. 44, I.P.C.). A wound or injury is a break of the natural continuity of any of the tissues of the living body. Mechanical injury (wounds) are injuries produced by physical violence. Trauma is an injury inflicted by force on a living tissue.
Classification of injuries : Medical
Mechanical injury:
(A) Due to blunt force: Abrasions, Contusions, Lacerations, Fractures and dislocations.
(B) Due to sharp force: Incised wounds, Chop wounds, Stab wounds, Firearms, Firearm wounds.
Thermal Injuries:
(1) Due to cold: Frostbite, Trench foot and Immersion foot.
(2) Due to heat: Burns and Scalds.
Chemical Injuries:
(1) Corrosive acids. (b) Corrosive alkalis.
Physical:
Electricity, lightning, X-rays, radioactive substances, etc.
Explosions
Legal : (1) Simple, and (2) Grievous.
Medico-legal
Suicide, Homicide, Accident, Fabricated and Defence.
GENERAL PRINCIPLES of Mechanical injury
A wound is caused by a mechanical force which may be either a moving weapon or object, or the movement of the body itself. In the first case, the counterforce is provided by the inertia of the body, and in the second case by the rigidity of some stationary object against which he falls.
A combination of these two events is seen in most cases. Due to the impact between the forward moving force and the counterforce, energy is transferred to the tissues of the body, which causes a change in their state of rest or motion.
The human body contains many complex tissues which greatly vary in their physical properties, such as state of solidity, fluidity, density and elasticity, and because of this a change in the state of rest or motion of the body produced by a forceful impact does not affect the tissues uniformly.
Some of the energy is spent in moving the body as a whole, but most of the energy may cause non-uniform motion of localised parts of the body, due to which the affected tissues will be subjected to compression or to traction strains or to a combination of both.
All the body tissues, except those which contain gas, are resistant to compression, i.e., they resist force tending to reduce their volumes. Mechanical force does not cause compression of the tissue but causes their displacement and deformation, and traction strains are produced in the affected tissues.
Such strains may be due to forces causing simple elongation of tissues, but they may be due to more complex mechanism, such as bending, torsion or shearing.
A shear strain is a strain which is produced in a body by the forceful alteration of its shape but not its volume. It causes or tends to cause two parts of a body in contact with each other to slide relatively to each other in a direction parallel to their plane of contact. Because of the great variation in the resistance of the different tissues to traction, they rupture with varying ease, as their cohesiveness is exceeded.
The rigid tissues like bones resist deformation, but if the limits of their elasticity is exceeded fracture occurs. The soft tissues are plastic, and as such, mechanical force alters their shape, which is limited by the cohesion between the tissue cells, connective and vascular tissue frameworks and capsules of organs. Soft tissues rupture when they are stretched beyond the limits of their tensile strength.
FACTORS INFLUENCING Mechanical injury
1. Amount of energy discharged : Kinetic energy is measured in a moving object by:K.E= mv, where m = mass and v = velocity. An object with definite velocity and definite weight have a definite amount of energy. When the mass doubles, kinetic energy is also doubled. However, when velocity is doubled kinetic energy increases four times. This indicates that the velocity has far more influence on the energy compared to the mass of the object.
2. Time : The shorter the period of time needed for the transfer of energy, the greater the likelihood of producing damage. A punch with a fist if withdrawn quickly will produce more damage than one where the fist stays in place.
3. Area of transfer : If the area over which the force acts is smaller, the damage will be more than if the same force is applied on a broader surface.
4. Elasticity and plasticity : The less elastic and plastic the tissue, the greater the likelihood that a laceration will result (Hooke’s Law of elasticity).
5. Inertia of tissue : It is the tendency of a tissue struck by a force to move and its ability to stop its motion without causing disruption of tissue.
6. Hydrostatic pressure : A force, transmitted through a fluid containing tissue, such as stomach andbladder, will force the fluid away from the area of contact, resulting in tissue laceration provided by this force.
If a weapon deforms or breaks on impacting the body, the injury is less, because some energy is spent in deforming or breaking the weapon. The severity of the injury is less, if the body moves with the blow, as the period of time over which the energy delivered in increased. If the area over which the force is delivered is large, the injury will be less severe, because the force is dissipated.
A blow with a weapon having a flat surface will produce less severe injury than that due to narrow object, because of diffusion of energy over a larger area. If a blow is struck with a weapon having a projecting object, a much more severe mechanical injury is caused as all of the force will be delivered to the end of the projection.
A blow to a rounded portion of the body, such as the top of the head will produce a more severe mechanical injury than the same force delivered to a flat portion of the body, such as the back, where the force is dissipated because of contact with a larger area.
