Quantification of variation in animal dentitions using 3D imaging tools

Currently there is no repository of information that can be used as a reference system for quantifying population variation in dentitions of biting animals such as those of humans and dogs. Three dimensional imaging tools provide images of bite-marks that are of high resolution, and the technology allows researchers to collect, store and share large numbers of 3D images of dentitions and bite-marks. This makes possible the compilation of images of the dentitions of biting animals in databases. Compilations of images would be made not in the vain hope of establishing the uniqueness of dentitions, but rather to quantify variation in landmark dental features. This may one day make possible probabilistic statements such as “only 0.3% of dogs and no full breeds other than German Shepherds have a maximum inter- canine distance of 40.10mm and a canine crown length of at least 17.25mm”. Such statements would qualify appropriately the probative value of evidence from a bite-mark analysis, and help to re-establish its credibility.

69 3.4.2 Contending with distortion using 3D imaging tools

To establish a match between a bite-mark and a suspect dentition correctly, it is critical to take into account the amount of distortion that may have occurred. Very little research has been conducted on distortion. There is an urgent need to investigate the various types of distortions that can be present in a bite-mark and the images of it, and to take steps to minimise them. Distortions associated with invasive impression-making techniques that are currently used in bite-mark analysis can be eliminated by 3D scanning, which is a non-invasive image-capturing procedure that requires no contact with the surface on which the bite-mark is registered. Distortions associated with 2D photographic techniques are eliminated also by using 3D scanning methods to record information in 3D space. The high resolution of the 3D scanners also facilitates image capture without the need of ABFO No 2 scales for reference as recommended by the ABFO (49). Tissue distortions defy bite-mark matching using current methods, but accurate recording of the positional location of key features in the bite-mark using 3D tools is a starting point, and in future sophisticated mathematical analysis of the relative location of the marks of key features may be able to establish a probabilistic match with a suspected dentition or at least eliminate other candidate dentitions. The ability of 3D imaging tools to record 3D images of bite-marks accurately, and the capacity to move images in 3D space, offers the future possibility of quantifying tissue changes such as stretching and displacement in three dimensions. The use of 3D imaging tools can assist in accurate modelling of time-related distortions by permitting recording of bite-marks in three dimensions at different time intervals. Neither of these is possible using 2D photographic methods.

3.4.3 Improving methodological rigour using 3D imaging tools

Because of their ability to be moved in 3D space, 3D images have the potential to facilitate measurements of a larger number of characteristics and landmark features than those possible by impression-making and 2D imaging techniques currently used in bite-mark analysis. Depending on the detail transferred in the bite-mark, this would allow the number and range of features required for a match to be established between a dentition and a bite-mark to be expanded. In these studies, the author has demonstrated that it is possible to measure and examine a large number of landmark dental features from 3D images (110 in human dentitions and 158 in dog-dentitions), with the potential to include more features if required.

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Using 3D scanners to record dentitions and bite-marks has the potential to remove or at least reduce random observer error, and to minimize bias. Previous studies have demonstrated that 3D image capture is more precise than the 2D photographic technique (60). Our own work has shown that the intra-oral 3D scanner can be used to record and measure dentitions reliably and accurately (61). The author has shown that there is minimal intra- and inter-observer error in the use by trained operators of either digital hand-held calipers or 3D scanning tools, but the difference is that 3D scanning minimizes subjective inputs and removes the possibility of bias from that source in the measurement process. The errors in matching bite-marks to dentitions have never been studied explicitly, but our proof-of-concept study (62) raises the possibility that bias can be avoided altogether by the use of 3D imaging technology. The process of matching 3D images of bite-marks with those of the suspect dentition has the potential to be automated. Automating the matching process may remove all requirements for subjective inputs from forensic odontologists, and hence minimize bias associated with bite-mark analysis. This proof-of-concept study also demonstrated that the accuracy of matching was conditional on which key dental features are recorded in the bite-mark, and laid a path towards establishing standards for accurate and quantitative assessment of the evidence in a bite-mark. 3.5 Recommendations

This section outlines research recommendations aimed at strengthening the scientific credibility of bite-mark analysis. Some of this research has been commenced by the authors of this paper (61, 62). However, there is more that needs to be done.