Forensic Dentistry - Criticism of Bite Mark Analysis

Criticism of Bite Mark Analysis

Recently, the scientific foundation of forensic odontology, and especially bite mark comparison, has been called into question. A 1999 study, frequently referenced in news stories but difficult to actually locate, by a member of the American Board of Forensic Odontology found a 63% rate of false identifications. However, the study was based on an informal workshop during an ABFO meeting which many members did not consider a valid scientific setting.

An investigative series by the Chicago Tribune entitled "Forensics under the Microscope" examined many forensic science disciplines to see if they truly deserve the air of infallibility that has come to surround them. The investigators concluded that bite mark comparison is always subjective and no standards for comparison have been accepted across the field. The journalists discovered that no rigorous experimentation has been conducted to determine error rates for bite mark comparison, a key part of the scientific method.

Critics of bite mark comparison cite the case of Ray Krone, an Arizona man convicted of murder on bite mark evidence left on a woman's breast. DNA evidence later implicated another man and Krone was released from prison. Similarly, Roy Brown was convicted of murder due in part to bite-mark evidence, and freed after DNA testing of the saliva left in the bite wounds matched someone else.

Although bite mark analysis has been used in legal proceedings since 1870, it remains a controversial topic due to a variety of factors. DeVore and Barbenel and Evans have shown that the accuracy of a bite mark on skin is limited at best. Skin is not a good medium for dental impressions; it is liable to have a number of irregularities present before the imprint that could cause distortion. Also, bite marks can be altered through stretching, movement or a changing environment during and after the actual bite. Furthermore, the level of distortion tends to increase after the bite mark was made. Both studies suggest that for the bite mark to be accurately analyzed, the body must be examined in exactly the same position it was in when the bite occurred which can be a difficult if not an impossible task to accomplish. Bite mark distortion can rarely be quantified. Therefore, bite marks found at the scene are often analyzed under the assumption that they have undergone minimal distortion. Only limited research has been done in trying to quantify the level of distortion of a bite mark on human skin since the 1970s. The lack of research may largely be due to the fact that such studies are difficult to organize and are very expensive.

Bite mark analysis is also controversial because dental profiles are subject to change. The loss of teeth or the alteration of arch configuration through a variety of procedures is common in human populations. The onset of oral diseases such as dental caries has been shown to alter the arch and tooth configuration and must be taken into account when comparing a dental profile to the bite mark after a significant amount of time has passed since the mark was made.

While the methods behind collecting bite mark evidence at the scene are leading toward greater standardization, the methodology behind analyzing bite marks is extremely variable because it depends upon the preference of the specific odontologist. As discussed earlier, there are several methods used to compare bite marks ranging from life sized photographs to computer enhanced 3-dimensional imaging. These methods vary in precision and accuracy, and there is no set standard by which to compare or analyze them. The lack of analytical standards leads to a wide array of interpretation with any bite mark evidence. Some odontologists even disagree on whether or not a mark on the body is the result of a bite. Therefore, the interpretation of evidence lies largely on the expertise of the forensic odontologist handling the case.

One possible issue facing bite mark analysis is a lack of bite mark uniqueness in any given population. Bite mark analysis is based on the assumptions that the dental characteristics of anterior teeth involved in biting are unique amongst individuals, and this asserted uniqueness is transferred and recorded in the injury. However, there is very little reliable research to support these assumptions. A study performed by MacFarlane et al. supported the notion of dental uniqueness, but the study revolved around the visual assessment of a cast as opposed to the bite mark that could have been produced by the cast. In another study conducted by Sognnaes et al., the group tried to find uniqueness between the dental profiles of identical twins in an attempt to prove dental uniqueness in the general population. However, this study suffered from a small sample size (n=5), with the intent to extrapolate the data to the general population. They also used plaster of paris as the substrate to simulate skin, yet the two materials have very different properties. In a review conducted by Strom, he references a study conducted by Berg and Schaidt which suggested that at least four to five teeth need to be present in the mark to ensure its uniqueness and make a positive identification. However, this study was done long before many of the current evaluation methods, and this sheds doubt on how applicable these conclusions are today.

One of the most well known papers attempting to provide empirical data on bite mark uniqueness was done by Rawson et al. In this study, researchers determined that if five teeth marks can be matched to five teeth, it can be said with confidence that only one person could have caused the bite, and if eight teeth were matched to marks this would be a certainty. However, in this study the probabilities used to make this claim are based on the assumption that the position of each tooth was independent of all the others. This is probably unrealistic because there are a number of ways that the dental profile can be changed. For example, braces apply force to specific teeth, in order to shift the placement of multiple teeth.

One particular case that highlighted the lack of uniqueness in bite marks involved two suspects accused of attacking a man that had sustained a bite mark injury. Two separate forensic dentists, one representing the prosecution and one the defense, were brought in to analyze the mark. They reported conflicting results. One found the mark to come from suspect A and the other said it was from suspect B. This disagreement resulted from the fact that even though the two suspects had dental features making them unique, the bite mark itself was not detailed enough to reflect them. Therefore, the mark could have reasonably come from either of the men. The equivocal outcome demonstrated in the case emphasizes the difficulty in proving uniqueness.

Most of the controversies facing bite mark analysis are due to the lack of empirical supporting evidence. When searching the entire MedLine database from 1960–1999, only 50 papers in English were found that related to bite mark analysis. Of these 50 papers, most of which were published in the 1980s, only 8% came from well designed experimentation providing empirical data. The lack of research has led to the continued use of a few outdated and limited studies to support the validity of bite mark analysis. This brings into question whether or not there is enough scientific support for bite mark analysis to be employed in court.

There have been several instances when forensic dentists have made claims, accusations, and guarantees supported by bite mark evaluation that have been proven incorrect through other forensic sciences. DNA analysis has shed some light on the limitations of bite mark analysis because often the DNA from saliva surrounding the area of the bite mark proves to be a more reliable form of identification. In the case of Mississippi vs. Bourne, the DNA of a suspect excluded them from the crime after a dentist claimed the bite marks on the victim matched the defendant’s teeth. DNA sampling has been included as a task for a forensic odontologist. For a crime scene investigator, taking DNA samples is as common as taking pictures of the scene. In the case of State vs. Krone, the defendant was sentenced to death, which was overturned. Then Krone was later reconvicted and given life in prison. Both convictions were based largely on bite mark evidence, but ten years later DNA evidence surfaced that identified the real killer and Krone was set free.