For September 1999 issue

SCIENCE IN CRIME DETECTION

How do we preserve the bite marks

‑Dr. Anil Aggrawal

A forensic pathologist frequently encounters bite marks during his practice. They may be present following a fight between adults or children, as part of a sexual or physical assault by an adult on a child, in rape or attempted rape where bites are likely to be noted on the breasts, and between homosexuals. The marks, single or multiple in nature, may be of varying degrees of severity, ranging from a mild marking of the tissues to deep perforation of the skin, and may be found (in order of frequency) on the breasts, face and head, abdomen, shoulder, upper extremity, buttocks, female genitalia, male genitalia, legs, ear, nose and neck. An American study in 1983 by Vale and Noguchi noted that the arms were the most frequent sites, followed by breasts, legs, abdomen, back, face and head, shoulder, buttocks, female genitalia, hand and fingers, chest, neck, nose, male genitalia, ear and foot.

We all know what bite mark is, but technically speaking what is it? Well, A bite mark may be defined as a pattern produced by human or animal dentitions and associated structures in any substance capable of being marked by those means. It is more commonly associated with marks in the skin, but is also of forensic odontological importance in foods and other inanimate objects.

One may be wondering why the study of bite mark is so important. Well, bite marks are said to be very very unique. No two persons are said to have the same bite marks. They could be almost as unique as fingerprints. This immediately brings to one’s mind the possibility of using them to nab suspects. Suppose a person has raped and murdered a female, and during his sexual frenzy has bitten his victim’s breasts. When the body of the dead woman is brought to the forensic expert, he can frequently examine the bite marks on the breasts and tell who the culprit was.

(Editor: Please reproduce figures 001 and 002 here. Captions to these photos are given in the end)

Forensic specialists who specialize in deducing facts about crime by studying mouth and mouth parts are called forensic odontologists. Odontology actually means “study of teeth”, and when teeth are studied with the specific purpose of deducing crime related facts, such a study becomes forensic odontology. This author has been practicing forensic odontology for quite some time now, and in this installment I hope to share some of my experiences with our intelligent readers.

When called to examine suspected bite marks it is first necessary to ascertain whether the mark could have been caused by teeth. If the answer is in the affirmative, is the bite mark human, and if so, is it caused by an adult or child? It is not unknown for victims to bite themselves, either to bring a false accusation or draw attention to themselves, or they may have had their hand or arm forced into their mouth to silence them during an assault. The position of the bite mark is carefully assessed by the forensic odontologist with this in mind, and if the position is such that it would have been possible for the victim to bite the area, then it is necessary to take impressions of the victim's dentition in addition to carrying out an examination of the bite marks.

Bite mark examination is the one aspect of forensic odontology requiring an immediate response by the forensic odontologist. The marks fade rapidly, both in the living and the dead, changing appearance in a matter of hours; delay in examination may result in loss of valuable evidence. The forensic odontologist is also responsible for the examination of the dentition of those suspected of bite mark perpetration.

Examination of the victim

The victim may be adult or child, living or dead.

The live adult

The examination may be carried out in a police surgeon's room in a police station, a designated rape crisis centre, a hospital, a dental surgery or the victim's home.

On receipt of a request, immediately we give instructions that the victim should not wash until after saliva swabbing has been undertaken. It must be remembered that saliva sticks to the area bitten, and we can often deduce many important things about the culprit merely from his saliva. For instance, we can find out his blood group from the saliva. We also request that a police photographer attends at the same time as the forensic odontological examination is carried out. Unfortunately, in our country, the importance of very early examination of bite mark evidence is not always appreciated by investigating authorities, and the forensic odontologist may not be called until the next day or later. The photography may have already been undertaken; if this is so, it is advisable to have repeat photography under the supervision of the forensic odontologist. Correct 1:1 photography is essential for dental evidence to be of any value. By 1:1 photography I mean that the size of the areas depicted in the photographs should be same as that of the real areas.

Examination witnesses

Before carrying out an examination, we generally ensure that a police officer is present if we are acting for the prosecution, or a legal representative for the accused if we are acting for the defence. Where the person to be examined is a female, a male examiner ensures that a female witness is also present. We record the names and occupations of all the witnesses present and, in the case of police officers, their rank and serial number. We also note the day, date, time and place of the examination, which is necessary for legal and court purposes.

Procedure

Some victims will be very distraught and will require a calm, reassuring approach by the dentist. First of all we, introduce ourselves to the victim, and explain our function and the procedures to be carried out. This makes the patient a little easier. The reasons for these procedures need not be discussed.

We then obtain signed permission from the victim authorizing these procedures and have the signature witnessed. This is required for legal purposes. Suitable forms, carried in the kit, may be worded as follows:

I, [name of victim] hereby give permission for [examiner's name] to examine me and take any necessary dental impressions, saliva samples and photographs. I understand that these measures are being undertaken in connection with a police investigation.

Signed

Witness

Date

We take comprehensive notes, commencing with a full history of the events, including a note of the approximate time that the bite was sustained and the position of the bitten part at the time of the incident, whether the bite occurred through clothing, and if so, the number of layers and the nature of the materials. A full description of the injury follows, with details of abrasions, lacerations, bruises and swelling. We also include a sketch of the marks indicating the direction of anatomical landmarks.

(Editor please include photographs 001 and 002 here. Captions to be included are given at the bottom)

Where the bite has occurred through clothing each layer of clothing is examined and photographed, the various materials being described in the notes.

We also check whether there is any relevant medical history that might have some bearing on the appearance of the mark, for example blood or connective tissue disorders or the administration of anticoagulants. Anticoagulants are drugs, which prevent the blood to coagulate. If a person is taking these drugs, then there will be more bleeding from a bite mark than normal. Similarly if there are blood or connective tissue disorders, then also there may be more bleeding.

Saliva swabbing are undertaken for secretory antigens if the area has not been washed. You may be wondering what the secretory antigens are. Well, these are nothing but ordinary blood groups which are excreted through saliva too. You might have heard people saying that his blood group is B+, O- and so on. Well, these blood groups refer to the presence or absence of certain proteins or antigens in the blood (+ denotes presence, - denotes absence). In about 85% of cases, these antigens are secreted in the saliva too, so just by examining the saliva, a forensic odontologist could tell the blood group of the person. Thus when a person has bitten a woman on, say, her breast, he would invariably leave his saliva also. And there is 85% probability that we can tell the blood group of the person. Needless to say, that this goes a long way for the police to include or exclude a possible suspect.

If the bite was sustained through clothing it is checked that this has been (or will be) sent for saliva tests. If the case is one of sexual assault involving fellatio (i.e. when the woman has been forced to commit oral intercourse on the assailant!), check that oral swabs have been taken for semen. Even after several hours and the use of mouthwashes and tooth brushing, spermatozoa have been recovered.

Supervising the photography is also an important task of the forensic odontologist. The victim should be asked to assume a posture in which the bitten area is in a position similar to that at the time of the bite, if this can be remembered. Self-adhesive scales are not be applied until the correct position has been assumed and initial photographs taken. Where the victim is unsure of the position of the bitten area at the time of the incident, several postures should be photographed as distortion of the tissues in different positions affects the appearance of the mark. Arrangements should be made for repeat photography to be undertaken 24 hours and 36 hours later, preferably by the same photographer. If for some reason it is necessary for the forensic odontologist to take photographs, note should be made of the camera used, lens, filters, flash, film and film speed, distance, shutter speed and aperture.

If the bite mark has perforated the skin, or there are obvious indentations, we consider taking an impression of the area. There are two methods which are relatively quick. The first uses rubber base impression material. The investigator shapes a double thickness of wire gauze to the approximate curvature of the bitten area using a similar area as a guide. A light-bodied fast set syringe material is used (the automatic twin tube mix is the best). The investigator then commences injecting in the actual bite, avoiding air blows, and covers the bitten area as rapidly as possible. He produces a layer approximately 5 mm thick covering the complete bite and the surrounding area. The wire gauze is immediately placed gently on the unset rubber base without using pressure which might lead to distortion. Additional rubber base is injected on to the gauze so that it becomes incorporated in the impression material, acting as a strengthener.

(Editor: Please reproduce figures 003 to 005 here)

The impression is removed when set and the back of the impression is marked with a felt-tip pen to indicate the direction of the head.

The impression is placed in a self-sealing exhibit bag and signed.

(Editor: Please reproduce figure 006 here)

This technique may also be used for impressions of food and other inanimate objects, and several models may be cast from the impression over a period of time as it remains dimensionally stable.

Some indication of the extensibility and flexibility of the skin in the area of the bite should be looked for, preferably by examining the region opposite to the bite and moving the skin gently over the underlying anatomy, which in itself will give a clue as to any modifying factors that might influence the shape of the mark. To estimate the change of shape of the mark on skin movement, a simple method is to place three dots on the skin forming a triangle with the mark in the centre of it. The distance between the marks can then be measured in various skin positions. These marks may be made after the other investigations, have been completed.

Finally, the investigator reviews the notes for any omissions; it may be many months before the case comes to trial, and he will be cross-examined on the procedures. He checks that all the exhibit bags are correctly labeled, and keeps the impressions in his bags after casting.

This is a brief glimpse into the work of a forensic odontologist. This is not generally known to a lay person, and I feel this short report will give the general reader a fairly good idea of what this new exciting work is all about.

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Captions to the photographs:

001:Human bite mark features: a,b, marks made by upper canines (canine is the third tooth from the centre). Between a to b, one can see the typical human catenary arch. c,e, marks made by lower canines. The lower arch can similarly be seen between the points c and e. Note that the lower arch radius is lesser than that of upper arch. d is an interruption in the lower arch, which represents either a missing tooth, or a tooth whose tip is below that of others. CLU is the center line of the upper arch, while CLL is the center line of the lower arch

002:Bite mark on the breast with slippage resulting in scrape marks. a, marks made by the incisal edges of the lower teeth. b, area of laceration (tears)

003: Taking a bite mark impression using rubber base impression material. The investigator covers the bite and surrounding area with a layer about 5 mm thick.

004: The investigator then places the wire gauze lightly on the unset rubber base.

005: After this additional rubber base on the gauze is injected.

006: Exhibit bag and identifying label

(Dr. Anil Aggrawal, M.D. is a Professor of Forensic Medicine at the Maulana Azad Medical College, New Delhi. His E-mail is dr_anil@hotmail.com, and his website is http://members.tripod.com/~Prof_Anil_Aggrawal/index.html)