Computer-based production of bite mark comparison overlays

Bite mark comparison protocols include measurement and analysis of the pattern, size, and shape of teeth against similar characteristics observed in an injury on skin or a mark on an object. The physical comparison of tooth position often depends upon transparent acetate overlays to detect similarities or differences between the teeth and the bite mark. Several methods are used to produce life-sized comparison overlays. The perimeter of the biting edges of the anterior teeth are usually recorded to produce facsimile images called hollow volume overlays. Some investigators hand-trace these outlines from dental study casts, or from bite exemplars produced in wax, styrofoam, or similar materials. Some use hand-traced outlines from xerographic images produced with office photocopiers that are calibrated to produce life-sized final images. Others use radiographic images and toneline photography of wax exemplars filled with radio-opaque materials, such as metal filings or barium sulfate. Dependence upon subjective input by the odontologist to trace these images manually is considered problematic. This is because the errors incorporated at any production stage are increased in the final product. The authors have developed a method to generate accurate hollow volume overlays using computer-based techniques. A PowerPC Macintosh computer, flatbed scanner, and Adobe Photoshop (a popular graphical interface application) are used to acquire, select, arrange and export detailed data from class and individual characteristics of a suspect's teeth to acetate film loaded in a high-resolution laser printer. This paper describes this technique to enable the odontologist to produce high-quality, accurate comparison overlays without subjective input.