Section 1 Brief Introduction to Mechanical InjuryInjury is a response of human body to external stimulating factors including mechanical, electrical, heat, light and radioactive factors. Injury may cover three kinds of damages to human body: • disruption of the normal structure of tissues, • functional disorder and • psychological disturbance
Three factors determining the development of mechanical injury: • the nature of the objects or instruments causing the injury • the nature of the affected tissues • the force applied to the body
1. The weapons or instruments causing the injury Three kinds of instruments: the sharp instruments, blunt instruments and guns or firearms. (1)The sharp instruments The sharp instruments are those pointed or sharp-edged objects, such as knife, ax, dagger and scissors. (2)The blunt instruments The blunt instruments are those without point or sharp edge, such as stick or club, the bricks, stone and fist etc.
1.Abrasion An abrasion is a destruction of the skin which usually involves the superficial layers of the epidermis only. Abrasion is caused by friction of the skin against some rough on sharp surface resulting in the scraping away of superficial portion of the epidermis. Abrasions are of medico-legal importance as they indicate that some force has been applied to the body. Under certain conditions, the features of abrasions may suggest the nature, direction and cause of the force and, possibly, the purpose for which it is applied.
Abrasions are commonly found on the head and face and over bony prominence caused by blows from blunt instruments and from falls. Abrasions are often accompanied by other injuries such as bruises, fractures or internal injuries. They may appear either as crescentic marks or as relatively broad parallel grooves which tail away at their ends as caused by fingernails. Abrasions may be produced after death when a body is dragged away from the scene of a crime. It is advisable to cut off the abrasion and conduct the histological examination for determination of the antemortem abrasion.
2. Contusions Contusions, also called bruises, are injuries characterized by the effusion of blood into the tissue spaces. The extent and the degree of bruising depend upon the amount of force applied to the body, and upon other factors such as the structure and vascularity of the affected tissues. Because of many variables, it is impossible to determine the mount of force from the extent and the degree of bruising.
In certain circumstances, the external pattern of a bruise may correspond to the form of the object or weapon, which may be of medico-legal importance and can be used as an indication for judgement of possible weapon. When the body is struck by a round stick, linear parallel bruises separated by apparently normal tissue in the skin may occur, which is called parallel subcutaneous hemorrhage. The colour of bruises changes as the extravasated blood undergoes hemolysis, which are not constant and cannot be relied upon as an indication of their age.
3. Incised wound Incised wounds are caused by sharp weapons or objects such as knives, jagged portions of metal or pieces of broken glass. An incised wound is usually linear or spindle in shape. The walls of the incised wound appear smooth and the edges of the wound are clean-cut. There are usually no connecting tissues (bridges of the tissue) between the walls. The wound is often gaping.
Hemorrhage from the incised wound is usually profuse. • Bruising may or may not be present surrounding wound edges. • The breadth of the cutting edge of a sharp weapon cannot be determined from the width of the wound. • Incised wounds are commonly seen in cases of assault and homicide, which are usually multiple. • Defense wounds may be found on the forearms and/or in the palms of the hands. • Accidental incised wounds are seen in traffic injury.
Suicidal incised wounds are commonly seen in region of the wrists and the neck. Incised wounds are sometimes self-inflicted for the purpose of ending his or her life or bring false charges against other persons. • Sometimes, in the cases of suicide, some superficial, parallel cut wounds are found on the front of the left forearm, the front and outer side of the thighs and the side of the neck. Such cut wounds are called tentative incisions or hesitation marks.
4. Stab wounds Stab wounds are caused by long narrow instruments with pointed end. Stab wounds are described as penetrating when they pierce deeply into tissues and as perforating when they transfix tissues and cause exit wounds. The shape of stab wounds is usually linear or irregular according to nature of weapons. The depth of a stab wound is greater than length and width. When a sharp weapon such as a knife has been used, the external wound almost invariably takes the form of a split having two pointed extremities.
The external opening of a punctured wound may have a triangular or cruciate shape if it is caused by a weapon such as a triangular file or bayonet, or by a square-sectioned instrument such as a spike. The dimensions of the external opening of a stab wound may be smaller than the diameter or transverse dimensions of the weapon, as the elastic skin is often stretched during the process of penetration. On the other hand, the opening may be larger in cases where the weapon is withdrawn obliquely after penetration.
The depth of a punctured wound may be greater than the total length of the penetrating object or weapon due to compression of the tissues during the process of penetration. A single wound track is usually found in relation to a single external opening in a punctured wound. In certain cases where the weapon is partially withdrawn and then reinserted in another direction, two or more tracks may be found in relation to a single external opening. The skin surrounding the wound may be bruised by the haft of the weapon when a weapon such as a knife or a dagger is thrust into the tissue with considerable force
5. Fracture of the bones When a force is applied to a bone directly or indirectly and the bone is bent beyond the limits of its elasticity, it fractures. (1) Fractures of the skull i. The types of fractures of the skull • Fissured fracture of the skull • Depressed fracture of the skull • Penetrated fracture of the skull • Comminuted fracture of the skull
Fissured fracture of the skull • Fissured fracture of the skull may develop when inner or outer table fractures, or both inner and outer table fracture. • Patterns of fissured fracture may vary with length, width, and direction as well as the number of the lines of a fracture. • Fissured fracture of the skull is an indication of blows striking head. The number of lines of fracture is also an indication of the number and sequence of the blows.
Depressed fracture of the skull • When greater force is applied to the skull, depressed fracture of the skull may develop due to fracture of outer table or both outer and inner table. • The shape of such a fracture appears to be in the form of cone, circle or oval. • The fracture lines in a depressed-comminuted fracture tend to run radially from the central point at the apex, and at the periphery the fracture lines tend to run in a circular manner.
Penetrated fracture of the skull • When a small object strike the skull at a high speed, such as a bullet, the skull may be penetrated and penetrated fracture of the skull occurs. Comminuted fracture of the skull • When violent force is applied to the skull, some part of skull may break into pieces or fragments to form comminuted fracture of the skull. • Comminuted fracture may develop by impact of one blow or repeated blows.
6. Injuries of the intracranial tissues (1) Intracranial hemorrhage Injury to the meninges and their related vessels often result in intracranial hemorrhages, including extradural, subdural, subarachnoid or subpial hemorrhages. i. Extradural hemorrhages (a) Definition Extradural or epidural hemorrhages are those that occur between the inner surface of the skull and the outer surface of the dural mater.
(b) Causes of extradural hemorrhages • Extradural hemorrhages may be resulted from torn diploic veins or from ruptured venous sinuses or meningeal vessels. • The most important type of extradural hemorrhage is caused by the rupture of meningeal vessels, especially the middle meningeal vessels. • The middle meningeal vessels may be transfixed by a spicule of bone or they may be lacerated by the edge of a fracture.
ii. Subdural hemorrhages (a) Definition Subdural hemorrhages are those that happen between the inner surface of the dura mater and the outer surface of the arachnoid. (b) Causes of subdural bleeding (hemorrhage) Subdural hemorrhages may arise from tears in the dural venous sinuses or cortical veins, but the most common cause of the subdural bleeding is the rupture of bridging or communicating veins.
iii. Subarachnoid hemorrhage Subarachnoid bleeding is the commonest form of traumatic intracranial bleeding which occurs between the arachnoid and the pia mater, and usually arise from the rupture of bridging veins. In cases where the hemorrhage is the only form of the injury, the possibility of a spontaneous subarachnoid hemorrhage from natural causes must be excluded. An important cause of spontaneous subarachnoid hemorrhage from natural causes is the rupture of a congenital or berry aneurysm of one of the arteries of Willis circle.
(2) Posttraumatic intracerebral hemorrhage Hemorrhages into the brain arising directly from impacts are called posttraumatic intracerebral hemorrhage. Posttraumatic intracerebral hemorrhage usually develops near the surface of the brain. A single deep-seated hemorrhage is usually due to some disease process. Emotional excitement or physical exertion may precipitate an intracerebral hemorrhage in an arteriosclerotic and hypertensive subject.
Indications which assist in the differentiation of natural hemorrhages from traumatic ones include: (a) The age of the subject; (b) The site and the extent of the hemorrhage; (c) The presence of vascular lesions in the cerebral vessels; (d) Signs of cardiac hypertrophy and generalized arteriosclerosis.
Delayed posttraumatic cerebral apoplexy In posttraumatic intracerebral bleedings, there is an important pathologic entity named delayed traumatic cerebral apoplexy. This term was first created by Böllinger in 1891. He observed that, after a posttraumatic interval of days or even weeks, a hemorrhagic effusion was found in the substance. Since then many cases concerning with such a type of cerebral apoplexy have been reported. But the mechanisms of the condition are not well known.
(3) Cerebral contusions i. Definition Cerebral contusions are circumscribed areas of brain tissue destruction which are accompanied by extravasations of blood into the affected tissues. Contusions are found most commonly in the cortex of the brain, but they occur also in the deeper tissues. In the cortex contusions are often covered by a narrow zone of intact cerebral tissue, which appear frequently wedge-shaped and are surrounded by numerous petechiae.