Forensic odontology (which is also called forensic dentistry or bitemark evidence expertise) mainly involves the identification of an assailant by comparing a record of their dentition (set of teeth) with a record of a bite mark left on a victim.
Other uses in law for dentists include the identification of human remains, medico-legal assessment of trauma to oral tissues, and testimony about dental malpractice. The forensic dentist, however, is to an ordinary dentist what the forensic pathologist is to an ordinary physician. They are board-certified specialists who deal primarily with bite mark evidence.
Although one doesn't have to be a board-certified specialist to become an expert witness in this area, it does help to lend validity to any scientific opinions rendered. Courts have been remarkably generous in granting qualification to experts, regardless if they are a forensic dentist or a non-specialist dentist.
Identification by teeth is not new. It goes back as far back as 66 A.D. at the time of Nero. As the story goes, Nero's mother Agrippina had her soldiers kill Lollia Paulina, with instructions to bring back her head as proof that she was dead. Agrippina, unable to positively identify the head, examined the front teeth and on finding the discolored front tooth confirmed the identity of the victim.
During the U.S. Revolutionary War, none other than Paul Revere (a young dentist) helped identify war casualties by their bridgework. Teeth are highly resistant to destruction and decomposition, so dental identification can be made under extreme circumstances. It was used on Adolf Hitler and Eva Braun at the end of World War II, the New York City World Trade Center bombing, the Waco Branch Davidien siege, and numerous airplane crashes and natural disasters.
The U.S. has a fairly well-developed system of dental records system (the Universal System), so it's not surprising to find it used for the identity of remains or "Jane Doe" victims. You can also tell age solely by analysis of teeth - the Gustafson method (looking for six signs of wear) or the Lamendin method (looking at transparency of roots). With the Universal System, each tooth is assigned its own number from 1- 32 and the five surfaces of each tooth are also classified.
Bite mark comparison is fairly new, however, going back to the mid-1970s. Police investigators have always noticed that at some crime scenes, criminals seem to leave their bite impressions on food products, chewing gum, or more commonly on the skin of their victims, especially in cases of battery, rape, child abuse, and homicide. There are seven types of bite marks which can be classified by four degrees of impression:
1. hemorrhage - small bleeding spot
2. abrasion - undamaging mark on skin
3. contusion - ruptured blood vessel, bruise
4. laceration - punctured or torn skin
5. incision - neat puncture of skin
6. avulsion - removal of skin
7. artifact - bitten-off piece of body
Contusions are the most common type of bite mark, and incisions offer the best three-dimensional image of the teeth. When avulsions and artifacts can be combined, you've also got three-dimensional imaging. The Marx case involved very clear three-dimensional bites. The forensic science of analyzing degree of impression involves, the specification of "violence", and this kind of testimony can be taken as evidence of the defendant's state of mind, aggravating circumstances, or especially heinous behavior.
Bite marks on a live body also have different characteristics from those on a dead body, so a forensic dentist might be able to assist with things like time of attack and/or time of death. Generally, the better the bite mark, the better an expert can make a comparison. The Illinois appellate case of People v. Milone (1976) establishes this principle in that, to be admitted, dental evidence must be agreed upon by the scientific community as "good quality". This means that bite mark evidence usually meets the Frye standard, at least in this regard.
Forensic dentists are either called by medical examiners or police investigators. There's a movement underway to set up emergency call-out procedures in many jurisdictions. What has happened is that someone has discovered a bite mark while conducting an autopsy or viewing the body at a crime scene.
While the bite mark can be photographed or even excised from the decedent, it's better to let the forensic dentist get there as soon as possible. One of the first things they'll do is obtain a saliva sample from the bite. This, of course, can be done by others, but a forensic dentist can make sure sample extraction doesn't affect quality of the bitemark. Next, the forensic dentist will take photographs.
This is a technique shared by all experts, and it's fairly important to get the lighting, color, and camera angle right. A linear (ABFO) scale should be placed somewhere in the photograph. A camera angle of 90 degrees is best for a flat surface, while a 40 degree angle causes 25% distortion of a bite mark. Next, the forensic dentist makes multiple impressions, casts, or molds of the bitemark, and has access to a variety of materials in which to choose the right gum, rubber, plastic, or powder to make a cast.
Some forensic dentists used the Dorion method, which advocates the removal of bitten tissue for microscopic examination. Others use advanced techniques such as scanning electron microscopy, computer-enhanced digitization, and xeroradiology. Computerized bite analysis software also exists. Once a suspect is apprehended, the forensic dentist makes one or more impressions of the suspect's teeth, comparing them to the recorded bite marks, and if called to testify, renders an opinion of the probability of a match.
Courts have upheld the constitutionality of involuntarily taking a dental impression from a suspect, as they have for most biological specimens from suspects (Schmerber v. California). The theory behind forensic dentistry is that no two mouths are alike (even identical twins are different), and that teeth, like tools, leave recognizable marks.
A bitemark, however, is not an accurate representation of the teeth. A lot depends on the mechanics of jaw movement and use of the tongue. Inside the mouth, the lower jaw (mandible) is movable and usually delivers the most biting force. The upper jaw (maxilla) is stationary, holding and stretching the skin, but when skin is ripped or torn, the upper teeth are involved more deeply.
Most bitemarks usually show a curvature where the upper teeth, at least, made an impression. The skin is elastic, tending to slip along the upper teeth until it catches hold. Depending upon the type of victim, some bitemarks last for hours and others for days, but almost all bitemarks alter themselves as time elapses.
Photographing bitemarks at intervals over a set period of hours or days is part of the standard protocol recommended by the ABFO. Bitemarks also change or become distorted when the posture of the victim changes, so the forensic dentist might be able to assist with determination of how the perpetrator moved the body.
One issue is the problem of standard protocol. This appears to be a field in which several experts go their own way, creating their own methods. There's also a digital divide in that some dentists have access to sophisticated lab equipment and others don't. Blind reviews also tend to be either not done or reflect poorly on the profession. In a blind review, a sample of experts would be unknowingly given the same material to analyze and compare, but might come to different conclusions.
Experts have not followed a points of comparison approach, but instead have relied upon a probabilistic assessment of the confidence in their opinion approach. A moderate amount of literature can be found in peer-reviewed journals, but this field has a long way to go before many of its claims can be scientifically substantiated.