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Articles » Odontology
Bite Me: Teeth in Forensic Science
Peter Lee
Forensic medicine is the specialized area of medical practice in relationship between medicine and the law. More specifically, forensic medicine is the scientific use of medical and paramedical specialties, as well as various dental, psychological, biological, and chemical, and mechanical techniques, in investigating the causes of a person's death, disability, injury, or disease. It is used in the pursuit of justice in court proceedings and its applications range from child abuse to murder and death from violations of civil rights.
A forensic odontologist is an expert who can differentiate bite impressions from different sets of teeth. He can identify the person who made the bite in terms of age, sex, jaw structure, etc. The techniques used by odontologists are extremely low tech and highly variable. Unlike fingerprint evidence, they do not have to find a certain number of similarities in order to pronounce a match. The odontologist only has to be satisfied that the suspect's teeth made the mark. Depending on the number and nature of the similarities, the expert can pronounce anything from a possible match, to a near certainty. There is, however, no standard way of comparing marks and teeth, nor any national database to compare a bite against. And there have been few attempts to check the accuracy of the various methods of analysis.
The usefulness of a bite mark as evidence in court depends on how clear it is, and how many distinguishing features the dentist can make out. A bite mark may consist of a couple of tiny bruises, a clear pair of arches or even an entire round wound.
The basic procedure is to make casts of a suspect's teeth and from that create an image of the biting edges on a transparent sheet. Odontologists all have their own favorite method of creating this overlay. Some simply place an acetate sheet over the upturned cast and draw around the tooth edges. Some press the model teeth into wax to create an imprint, then draw around the craters left by the teeth. Others prefer to place their cast on a photocopier and draw their outlines from the copy. Still others prefer to flood the indentations in a wax mold with amalgam and alcohol, a radio-opaque mix that can be X-rayed to produce an image. The final step is to place the overlay on a photograph of the bite and compare the pattern of marks.
With the help of technology, odontologists can achieve more accurate overlays. They use a graphics tool called a magic wand. If you point the wand at a particular part of the image, it picks out parts of similar brightness. So by pointing it at the biting edge of one tooth, it will "lift" the biting edges of the whole image, and transfer their outline to a clear plastic overlay for matching in the traditional manner.
Another method of differentiating bites is by teasing out details from a mark left on the victim's skin. Photographs of the bite mark taken under ultraviolet light will bring out hidden details, particularly where teeth have penetrated, while infrared photography can reveal deeper marks caused by subsurface bruising and bleeding.
However detailed the bite mark may be, the only true objective evidence in a bite is the DNA left in it by the biter. While improvements in techniques for analyzing marks will make the opinion-based evidence more credible, the real revolution in forensic odontology will come from improvements in techniques for collecting and analyzing trace amounts of DNA.
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Gardner, Robert.
Crime Lab 101: Experimenting with Crime Detection. New York, Walker, 1992.
Saferstein, Richard.
Criminalistics: An Introduction to Forensic Science. New Jersey, Prentice Hall, 1990.
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