Post-mortem Identification

The most common role of the forensic dentist is the identification of deceased individuals. Dental identification of humans occurs for a number of different reasons and in a number of different situations. The bodies of victims of violent crimes, fires, motor vehicle accidents and work place accidents, can be disfigured to such an extent that identification by a family member is neither reliable nor desirable. Persons who have been deceased for some time prior to discovery and those found in water also present unpleasant and difficult visual identifications. Dental identifications have always played a key role in natural and manmade disaster situations and in particular the mass casualties normally associated with aviation disasters.

Many people are familiar with the concept of dental identification; it is frequently mentioned on television news. But the nuances and complexities of the process are rarely understood. The central dogma of dental identification is that post-mortem dental remains can be compared with dental records, including written notes, study casts, radiographs, etc, to confirm identity. Clearly, individuals with numerous and complex dental treatments are often easier to identify than those individuals with little or no restorative treatment. The teeth not only represent a suitable repository for such unique and identifying features, they also survive most post-mortem events that can disrupt or change other body tissues.

The forensic dentist produces the post-mortem record by careful charting and written descriptions of the dental structures and x-rays. Once the post-mortem record is complete, a comparison between these and dental records can be carried out. A range of conclusions can be reached when reporting a dental identification. The American Board of Forensic Odontology recommends that these be limited to the following four conclusions:

• Positive identification: The antemortem and post-mortem data match in sufficient detail, with no unexplainable discrepancies, to establish that they are from the same individual.
• Possible identification: the antemortem and post-mortem data have consistent features but, because of the quality of either the post-mortem remains or the antemortem evidence, it is not possible to establish identity positively.
• Insufficient evidence: The available information is insufficient to form the basis for a conclusion.
• Exclusion: the antemortem and post-mortem data are clearly inconsistent.

Unlike fingerprints there is no minimum number of concordant features that are required for a positive identification. A single tooth can be used for identification if it contains sufficient unique features. Equally, a full-mouth series of radiographs may not reveal sufficient detail to render a positive conclusion. The discretion of identification lies with the odontologist who must be prepared to justify the conclusions in court, surely the ultimate in peer-review.

Dental Profiling

When dental records are unavailable and other methods of identification are not possible, the forensic dentist can often produce a "picture" of the general features of the individual.  This process is known as post-mortem dental profiling. A dental profile will typically provide information on the deceased's age, ancestry background, sex and socio-economic status. In some instances it is possible to provide additional information regarding occupation, dietary habits, habitual behaviours and occasionally on dental or systemic diseases. 

The forensic dentist will often work with a forensic anthropologists to help in identification of an individual or the development of a profile from remains. The determination of sex and ancestry can be assessed from skull shape and form. Generally, from skull appearance, forensic dentists can determine race within the three major groups: Caucasoid, Mongoloid and Negroid. Additional characteristics, such as cusps of Carabelli, shovel-shaped incisors and multi-cusped premolars, can also assist in determination of ancestry. Sex determination is usually based on cranial appearance, as no sex differences are apparent in the morphology of teeth. Microscopic examination of teeth can confirm sex by the presence or absence of Y-chromatin and DNA analysis can also reveal sex.

In developing a profile dental structures can provide useful indictors to the individual's chronological age. The age of children can be determined by the analysis of tooth development and subsequent comparison with developmental charts. Conclusions are usually accurate to approximately ±1.5 years. While eruption dates can be used in determining sub-adult ages, these are highly variable and the actual developmental stages of the teeth are more accurate. 

Other features can be useful in individualising a profile. The presence of erosion  can suggest alcohol or substance abuse, an eating disorder or even hiatus hernia while stains can indicate smoking. Unusual wear patterns may result from pipe stems, cigarette holders, hairpins, carpet tacks or previous orthodontic treatment. The quality, quantity and presence or absence of dental treatment may give an indication of socio-economic status or likely country of residence.

Because of the resistant nature of dental tissues to environmental assaults, such as incineration, immersion, trauma, mutilation and decomposition, teeth represent an excellent source of DNA material. When conventional dental identification methods fail, this biological material can provide the necessary link to prove identity. 

Bite Marks

Forensic odontologists may also work backwards matching the bite marks on objects found at a scene to a suspect. They develop the skill of comparing dental impressions taken from a person's mouth to bite-mark impressions on the skin (or possibly the bones) of a victim.  There are many factors to consider, including matching for striations, whorls, indentations, pitting, and abrasions, and often this is done through computer-enhanced photography.  They can also analyse bite marks on food in cases where a perpetrator might have taken a bite out of something in the victim's home and left it behind.  An example is given below. What experts seek are a sufficient number of points of similarity between the evidence and a suspect to be able to say with a reasonable degree of certainty that this is the perpetrator.

However, bite marks left on foodstuff offer a three-dimensional impression, which is superior to the two-dimensional impression often left on skin.  A bite might penetrate the skin, but often only leaves bruising—and sometimes the blood marks of a bruise are mistaken for the impression of a tooth. Conversely bite marks might be dismissed as simple bruising. Some bites are forceful enough to leave a good impression, others are not.   

The physical characteristics of both the bite mark wound and the suspect's teeth include

• the distance from cuspid to cuspid
• the shape of the mouth arch
• the evidence of a tooth out of alignment
• teeth width and thickness, spacing between teeth
• missing teeth
• the curves of biting edges
• unique dentistry
• wear patterns such as chips or grinding. 

All of these are examined in detail and than compared, preferably in a blind test in which the odontologist is not aware of which teeth impressions belong to the suspect.  At the very least, the injury pattern itself should be completely analysed first before looking at the data from the suspect. 

 Currently, there is no agreement among forensic odontologists about the individuality (uniqueness) of dentition or the behaviour of human skin during biting. Although these issues have never been proven scientifically, much research is currently underway in an attempt to prove the suspicion that each human dentition is unique. The sizes, shapes and pattern of the biting edges of the anterior teeth that are arranged in the upper and lower dental arcades are thought to be specific to that individual. 

The amount and degree of detail recorded in the bitten surface may vary from case to case. And, even if it is assumed that the dentition is individual enough to warrant use in forensic contexts, it is not known if this individuality is recorded specifically enough in the injury. In situations where sufficient detail is available, it may be possible to identify the biter to the exclusion of all others. Perhaps more significantly, it is possible to exclude suspects that did not leave the bite mark.

The complexity of bite marks on skin has made their use a contentious issue in some jurisdictions.