Forensic Dentistry - Bite Mark Analysis

Bite Mark Analysis

Upon collection of dental evidence, the forensic odontologist analyzes and compares the bite marks. Studies have been performed in an attempt to find the simplest, most efficient, and most reliable way of analyzing bite marks. Factors that may affect the accuracy of bite mark identification include time-dependent changes of the bite mark on living bodies, effects of where the bite mark was found, damage on soft tissue, and similarities in dentition among individuals. Other factors include poor photography, impressions, or measurement of dentition characteristics.

Most bite mark analysis studies use porcine skin (pigskin), because it is comparable to the skin of a human, and it is considered unethical to bite a human for study in the United States. Limitations to the bite mark studies include differences in properties of pigskin compared to human skin and the technique of using simulated pressures to create bite marks. Although similar histologically, pigskin and human skin behave in dynamically different ways due to differences in elasticity. Furthermore, postmortem bites on nonhuman skin, such as those used in the experiments of Martin-de-las Heras et al., display different patterns to those seen in antemortem bite injuries. In recognition of the limitations of their study, Kouble and Craig suggest using a G-clamp on an articulator in future studies to standardize the amount of pressure used to produce experimental bite marks instead of applying manual pressure to models on pigskin. Future research and technological developments may help reduce the occurrence of such limitations.

Kouble and Craig compared direct methods and indirect methods of bite mark analysis. In the past, the direct method compared a model of the suspect’s teeth to a life-size photograph of the actual bite mark. In these experiments, direct comparisons were made between dental models and either photographs or “fingerprint powder lift-models.” The “fingerprint powder lift” technique involves dusting the bitten skin with black fingerprint powder and using fingerprint tape to transfer the bite marks onto a sheet of acetate. Indirect methods involve the use of transparent overlays to record a suspect’s biting edges. Transparent overlays are made by free-hand tracing the occlusal surfaces of a dental model onto an acetate sheet. When comparing the “fingerprint powder lift” technique against the photographs, the use of photographs resulted in higher scores determined by a modified version of the ABFO scoring guidelines. The use of transparent overlays is considered subjective and irreproducible because the tracing can be easily manipulated. On the other hand, photocopier-generated overlays where no tracing is used is considered to be the best method in matching the correct bite mark to the correct set of models without the use of computer imaging.

While the photocopier-generated technique is sensitive, reliable, and inexpensive, new methods involving digital overlays have proven to be more accurate. Two recent technological developments include the 2D polyline method and the painting method. Both methods use Adobe Photoshop. Use of the 2D polyline method entails drawing straight lines between two fixed points in the arch and between incisal edges to indicate the tooth width. Use of the painting method entails coating the incisal edges of a dental model with red glossy paint and then photographing the model. Adobe Photoshop is then used to make measurements on the image. A total of 13 variables were used in analysis. Identification for both methods were based on canine-to-canine distance (1 variable), incisor width (4 variables), and rotational angles of the incisors (8 variables). The 2D polyline method relies heavily on accurate measurements, while the painting method depends on precise overlaying of the images. Although both methods were reliable, the 2D polyline method gave efficient and more objective results.