Human bite is usually semicircular or crescentic, caused by the front teeth (incisors and canines), with a gap at either side due to the separation of upper and lower jaw. The teeth may cause clear, separate marks or form a continuous or intermittently broken line. Bite marks may be abrasions, contusions or lacerations or a combination of any two or three.
In forcible bite, the appearance is of two ‘bows with their concavities facing each other, and a gap at each end. The sucking action reduces the air pressure over the center and produces multiple petechial hemorrhages, due to rupture of capillaries and small venules in the subcutaneous tissues.
If the human bite is forcible, the petechiae are confluent and produce a contusion. If the bitten area is irregular or markedly curved, only part of the dental arch comes in contact with the tissues. Rarely, the bite mark may be linear in pattern, due to the scraping of the skin by the upper incisors, causing parallel tracks. Faint teeth marks become visible when examined under ultraviolet light in a dark room.
In sexual bites, the teeth are used to grip during sucking; the resulting central or peripheral suck marks are seen as petechiae, producing reddening. In many such bites teeth marks are not seen. Love bites are usually seen on breast, neck, cheek, abdomen, arms, thighs and genitalia.
In the living, these marks are seen from one to twenty four hours after infliction. Swabs of the human bite mark should be taken immediately, using a swab moistened with sterile water.
A swab of control area adjacent to the mark, and a swab of victim’s saliva should also be taken using swabs moistened with sterile water. If there is a delay in sending the swabs to the laboratory, they should be kept in the freezer compartment of the fridge.
In child abuse, bite marks can be found anywhere on the body. Self-inflicted bite marks are usually seen on the shoulders and arms.
They are useful in identification because the alignment of teeth is peculiar to the individual. Human bite marks may be found in materials left at the place of crime, e.g., foodstuffs, such as cheese, bread, butter, fruit, or in humans involved in assaults, when either the victim or the accused may show the marks, usually on the hands, fingers, forearms, nose and ears.
The bite mark is fully photographed with two scales at right angle to one another in the horizontal plane. Photographs of the teeth are taken by using special mirrors which allow the inclusion of all the teeth in the upper or lower jaws in one photograph. The photographs of the teeth are matched with photographs or tracings of the teeth.
Tracings can be made from positive casts of a human bite impression, inking the cutting edges of the front teeth. These are transferred to transparent sheets, and superimposed over the photographs, or a negative photograph of the teeth is superimposed over the positive photograph of the bite. Exclusion is easier than positive matching.
A plastic substance, such as a rubber or silicone based medium containing catalytic hardener is laid over the human bite mark, which produces a permanent negative cast. Plaster of Paris also can be used.