How Forensic Dentistry Works

Adults typically have 32 teeth: four incisors, four canines, eight premolars, 12 molars and four wisdom. Since there are multiple teeth in each group, each individual tooth needs its own designation. There are dozens of methods for labeling teeth in use, but the three most popular are the Universal Numbering System, the Palmer Notation Numbering System and the FDI (Fédération Dentaire Internationale) World Dental Federation Notation.

In the United States, most dentists use the Universal Numbering System. In this system, each of the 32 adult teeth is assigned a number. Number one is the upper-right third molar, while number 32 is the lower-right third molar. The 20 deciduous, or baby, teeth are designated by the letters A through K or the number-letter combination of 1d through 20d.

Some teeth, like molars, have multiple surfaces. Each of these surfaces has a name. The center of the tooth is the biting surface, known as the occlusal. This surface has two elements: the cusps, or raised parts, and the grooves, or indentions. The mesial surface of the tooth is toward the front of the mouth, while the distal is toward the back. The side toward the inside of the mouth is called the palatal surface on the upper jaw and the lingual on the lower jaw. The tooth surface facing the cheek is the buccal. So if you get a filling on the distal of number 15, that means it's on the surface facing the back of the mouth on your upper second molar (or 12-year molar).

When you visit the dentist for a checkup, the dentist makes a notation about each tooth to show variations (e.g., chips), and dental work such as fillings, crowns and bridges. The dentist also includes observations about the health of your teeth, like receding gums or signs of periodontal disease. Most dental visits involve taking sets of X-rays, which can also show work not easily seen, like root canals.

In the next section, we'll look at how forensic dentists use these records to identify teeth.

Tooth enamel (the outer layer of teeth) is harder than any other substance in the human body, which is why teeth remain long after all other body parts have decayed. In addition, teeth can withstand temperatures of more than 2,000 degrees Fahrenheit (1,093 degrees Celsius). And even though teeth that have been through especially intense heat are very fragile and may shrink, they can be preserved with lacquer and used for identification as long as they are handled very carefully.

Because teeth are so tough, and because there is no database of teeth similar to those for fingerprints and DNA, forensic dentists are often employed to identify the dead, especially fire victims. If a corpse is intact, the dentist will likely work in a morgue and expose the jaws surgically for examination. Even if only a few teeth are available, a forensic dentist can still make a positive identification using dental records. While the best comparisons come from X-rays, notations on someone's tooth chart can tell the dentist if the teeth are the same when X-rays aren't available.

When there is an incident involving multiple deaths, forensic dentists receive a list of possible victims and compare available dental records with the teeth to find a match.

Identifying an individual by their teeth without dental records is much more difficult. However, it's possible to glean some information without them. Since teeth grow an average of 4 micrometers per day, for example, a forensic dentist can estimate someone's age based on the length of their teeth. It's also possible to distinguish ethnicity from the teeth. To wit: Some Asians and Native Americans have incisors with scooped-out backs.

In addition, things like broken teeth, missing teeth and gold crowns might be recognized by the deceased's friends and family members. And teeth can indicate things about the person's lifestyle. The teeth of pipe-smokers and bagpipe players have a distinctive wear pattern. Dressmakers and tailors, who often put pins and needles in their mouths, may have chipped teeth.

Sometimes, forensic dentists can retrieve DNA samples from a victim's teeth by extracting the pulp from the center of the tooth. Unlike the enamel, pulp can be damaged by fire and other conditions, but it can also last for hundreds of years.

Dental identification is often the last resort when trying to identify a deceased person, and it isn't always possible — some people simply can't be identified.

Now we'll look at the other main aspect of forensic dentistry previously mentioned — bite-mark analysis.

Bite-mark analysis is extremely complex, with many factors involved in a forensic dentist's ability to determine the identity of the perpetrator. It's also typically used in conjunction with other types of physical evidence.

When an investigator sees something on a victim that even resembles a bite, a forensic dentist is called in immediately, because bite marks change significantly over time. For example, if the victim is deceased, the skin may slip as the body decays, causing the bite to move.

The dentist first analyzes the bite to identify it as human. Animal teeth are very different from human teeth, so they leave very different bite-mark patterns. Next, the bite is swabbed for DNA, which may have been left in the saliva of the biter. The dentist also must determine whether the bite was self-inflicted.

Forensic dentists then take measurements of each individual bite mark and record it. They also require many photographs because of the changing nature of the bites. Bruising can appear four hours after a bite and disappear after 36 hours. If the victim is deceased, the dentist may have to wait until the lividity stage clears (the pooling of the blood), when details are visible. The bite photography must be conducted precisely, using rulers and other scales to accurately depict the orientation, depth and size of the bite. The photos are then magnified, enhanced and corrected for distortions.

Finally, bite marks on deceased victims are cut out from the skin in the morgue and preserved in a compound called formalin, which contains formaldehyde. Forensic dentists then make a silicone cast of the bite mark.

Forensic dentists use several different terms to describe the type of bite mark:

Since several different types of impressions can be left by teeth, depending on the pressure applied by the biter, the forensic dentist notes these as well. A clear impression means that there was significant pressure; an obvious bite signifies medium pressure; and a noticeable impression means that the biter used violent pressure to bite down.

The movement of a person's jaw and tongue when they bite also contributes to the type of mark that is left. If the victim is moving while being bitten, the bite will look different from that inflicted on a still victim. And typically marks from either the upper or lower teeth are most visible, not both.

A forensic dentist can tell a lot about the teeth of the biter based on the bite mark, too. If there's a gap in the bite, the biter is probably missing a tooth. Crooked teeth leave crooked impressions, and chipped teeth leave jagged-looking impressions of varying depth. Braces and partials also leave distinctive impressions.

Once investigators have identified a suspect, they obtain a warrant to take a mold of their teeth, as well as photos of the mouth in various stages of opening and biting. They then compare transparencies of the mold with those of the bite-mark cast, and photos of both the bite mark and the suspect's teeth are compared to look for similarities.

In January 2007, prisoner Roy Brown, who had been convicted of murder in New York in 1992, was set free. Brown was one of many prisoners who have been released after DNA analysis, not available or widely used during their trial, cleared them of their crimes. In Brown's case, bite-mark analysis was instrumental in his conviction. But DNA from saliva left on the bite matched with a different suspect. So what went wrong?

The bite mark in the Brown case showed six tooth impressions from the front teeth of the upper jaw, although he was missing two teeth at the time. The expert witness claimed that Brown could have moved the skin of the victim around when biting to make it appear that he wasn't missing any teeth. Although this testimony was not the only evidence used by the prosecution, it was instrumental in helping jurors reach a guilty verdict.

Just five years earlier, an Arizona man named Ray Krone was released from prison after 10 years of serving his murder sentence. The prosecution's witness claimed a perfect match between his teeth and a bite mark found on the victim. The witness stated that "a match is 100 percent" Krone was cleared after DNA belonging to another suspect was found on the victim's clothes.

With wrongful conviction exonerations on the rise, experts began studying the reliability of bite-mark analysis. In 2009, the National Academy of Sciences released a shocking report detailing the numerous problems with various forensic science technologies and techniques, including bite-mark analysis. While stating bite marks can sometimes reliably exclude suspects, the report noted no scientific studies supported the assertion that bite marks provide sufficient detail for positive identification.

Several years later, Drs. Iain Pretty and Adam Freeman of the American Board of Forensic Odontology (ABFO) conducted a study among ABFO-certified dentists concerning bite-mark analysis. An overwhelming number of participants couldn't even agree on whether they were looking at an actual bite mark. Freeman subsequently stopped practicing bite-mark analysis.

More recently, after the Texas Court of Appeals released Steven Chaney — a man wrongfully convicted for murder based on bite-mark evidence — the Texas Forensic Science Commission in 2016 called for an end to the practice.

Today, some critics feel that bite-mark analysis should be used only to eliminate, not to identify, a suspect. Others say that it's acceptable to state there is a probability that a suspect created the mark, but that it's important to clarify that bite marks can't be the only thing linking the suspect to the crime. Forensic dentist training as well as proper education of the jury are also factors.

For lots more information about forensic dentistry and crime stuff, check out the articles and links on the next page.