Bitemark Evidence

By far, the most controversial subdivision of forensic odontology, if not the entire forensic science field, is that of the use of bitemark evidence. For reasons that are not at all understood, biting is involved in many cases of violent person on person crime. Sometimes it is the victim of the crime being bitten but there are also instances of the victim doing the biting. During these physical altercations, many patterned injuries may be discovered on the body of the victim. Occasionally one or more of the patterned bruises can be produced from teeth. These patterns can be analyzed to determine if they are bitemarks, and more specifically, if they were inflicted by an adult or a child. In rare circumstances, it may be possible to do a comparison between suspect dentitions and a bitemark in order to establish linkage. However, in no instance should bitemark evidence be used as the only physical evidence linking a suspect to a criminal act.

Previous studies have shown that there is variation of opinion between experts when tasked with determining exactly what a bruise pattern must look like in order to qualify as a human bitemark. One such study1 published at the 2015 American Academy of Forensic Sciences (AAFS) annual meeting showed a wide range of disagreement among ABFO diplomates. When the results of the study were reviewed, it was the opinion of the participants that the terminology used to determine if an injury was a bitemark was ambiguous and contributed to the poor results. Additionally, many participants could not state with confidence nor agree upon what criteria was required for a patterned injury to be defined as a bitemark.

In an effort to improve the techniques and establish conformity among experts, the ABFO has revised the terminology but has yet to initiate a follow up study to see if the issue is the language or the inability of the diplomates to actually determine when a patterned injury can first be labeled a bitemark. For bitemarks to be useful as evidence, it is a critical first step that an investigator be able to demonstrate with validity and reliability the ability to properly identify a patterned injury as a bitemark.

Bitemark evidence presents with unique issues when compared to other kinds of forensic evidence. In many sub-specialties of forensic science investigations, there are laboratory procedures that can be reliably employed to test, examine and quantify an unknown sample of evidence. However, in bitemark investigation, there is no established database or testing equipment to establish nor quantify probabilities of teeth linking to a bitemark patterned injury. Further, because of the violent nature of biting, it is not possible to design laboratory experiments to study the infliction of the biting injuries in a way that reproduces real-world biting as it occurs in true life or death struggles.

There is also no data available for quantifying the mathematical probability linking a specific suspect’s dentition to a bitemark in skin. Bitemark analysis differs from DNA and fingerprint analysis in that databases exist that are used to establish a numerical probability and error rate when linking a DNA sample or latent fingerprint to an individual. Other complicating factors affecting the interpretation of bitemarks include the subjective interpretation of the examiner, inter-examiner differences and investigator bias, all of which can be significant factors in the resultant opinion.

Another issue that can affect the investigation includes how to measure the distortion that occurs in the biting. The pinching of the skin by the teeth during biting is quite painful and induces a reactive movement away from the biter’s teeth. Motion induced in the skin by both the biter’s teeth and the person who is bitten add distortion to bitemark pattern injuries. At this time, it is not possible to measure and fully understand the distortion that occurs. Bites through clothing, partial bitemarks, double or multiple bites over the same area and avulsive bites can all affect the evidentiary value of bitemarks in legal proceedings.

Even with these limitations, there are bitemark injuries that can be useful for analysis and possible comparison to suspect dentitions. Depending on the individualizing characteristics of the suspect dentitions and the amount of information contained in the bitemark, it may be possible to differentiate between possible biters. Individualizing characteristics include differences in arch widths, tooth positional configurations and locations in the arch, missing or unerupted teeth, and other observable details the teeth transfer to the skin during biting. Beyond the actual bitemark pattern(s), the presence of severe bruising from having been bitten can demonstrate violent physical contact which may contribute supplementary evidence in a criminal act. Bitemarks sometimes also provide an excellent source for collection of salivary DNA which, when collected properly, is very useful as evidence.

The appearance of a bitemark injury can range from a high-quality pattern distinctly showing both easily identifiable arches and clear individual tooth markings, to diffused or healing bruise patterns that appear as fuzzy “smoke rings.” A high quality bitemark can be used to link to a suspect’s dentition as well as exclude other dentitions.

Terms relating or linking a dentition to a human bitemark are:

  1. Excluded as having made the bitemark,
  2. Not excluded as having made the bitemark, such that dentition or a similar dentition cannot be excluded as having created the bitemark, and
  3. Inconclusive, when there is insufficient information to support any conclusion whether or not the bitemark could have been caused by dentition.

It may be possible to analyze high quality bitemarks for orientation of the upper and lower teeth, the individual tooth position and alignment as well as relative arch size. Analysis of high quality bitemarks can also help determine if the bitemark is from a child or adult dentition. It is only the very rare case in which a comparison to suspect dentitions is done.