Bite Mark Collection, Documentation and Comparison

Saliva deposited during a bite contains epithelial cells shed from the biter's oral mucosa and, in some cases, leucocytes. This cellular material can yield a full STR profile and is often the most individualising evidence in the entire bite mark examination. It is also the most fragile: handling degrades it, washing destroys it, and even placing a scale or impression material over the wound can mechanically remove cells.

The double-swab technique, developed by David Sweet and colleagues in 1997, is the standard protocol. A sterile cotton swab moistened with distilled water is rolled firmly over the bite mark perimeter and the wound surface. A second dry swab immediately follows to collect the remaining moisture and cells. Both swabs are packaged separately and submitted for DNA analysis. The wet-then-dry sequence maximises cellular yield compared to a single wet or dry swab.

Photographs are the primary documentary record and the medium on which the comparison overlay is placed. Scale placement error or camera tilt introduces false size or shape discrepancies that carry through into the overlay comparison. The ABFO photographic protocol exists because these errors have had consequences in real cases.

1. Orientation photographs
   Wide-field images establishing the body region and the location of the bite mark within it. No scale required at this stage.
2. Close-up with ABFO No. 2 scale
   The L-shaped scale is placed in the same plane as the wound surface. The camera is positioned perpendicular to the wound plane. Lens distortion correction is verified using the circular targets on the scale. Multiple frames are taken at this stage.
3. Alternate light source photography
   Oblique or UV illumination can reveal bruising patterns invisible under white light. Forensic light sources at 415 nm to 450 nm (blue-green) with orange barrier filters are standard for this step. Often done before and after impression-taking.
4. Serial documentation in living victims
   Repeat photography at 12, 24, and 48 hours captures bruising evolution. The mark seen at 24 hours is often the most defined. Photographs are time-stamped and linked to the case file.

In postmortem cases where the wound shows distinct three-dimensional tooth marks, an impression can capture detail that photography alone misses. Polyvinylsiloxane (PVS) impression materials, the same materials used in clinical dentistry, are applied to the cleaned and dried wound surface and allowed to set. The resulting negative is then used to pour a stone or resin positive model.

Tissue excision is a more invasive option reserved for situations where the bite mark is on a discrete area of skin that can be removed, preserved, and examined under stereomicroscopy. The excised tissue is fixed in formalin and sectioned, revealing the three-dimensional geometry of tooth penetration into the dermis. This technique provides the closest thing to an objective measurement of tooth contact geometry, but it destroys the surface for subsequent photographic documentation.

A suspect's dental cast is taken by a registered dentist or forensic odontologist, with appropriate legal authority (court order, warrant, or consent depending on jurisdiction). Upper and lower impressions are taken with alginate or PVS material and poured in dental stone. The cast should be taken as close as possible to the date of the offence; dental changes over time through tooth loss, restoration, and wear can reduce the relevance of a cast taken much later.

The cast is photographed, measured, and archived. From it, the analyst can derive the inter-canine distance (the principal arch width measurement), individual tooth widths, tooth heights, and the position of any rotations, restorations, or wear facets. These measurements and features form the basis of the overlay and direct comparison.

The overlay is the primary comparison tool. An analyst creates a life-size representation of the suspect's biting surfaces, places it over a life-size bite mark photograph, and assesses the degree of correspondence. How the overlay is made matters because different production methods introduce different errors.

Direct comparison, placing the physical cast directly over the bite mark photograph or wound surface, is also used but carries the additional risk that any pressure or contact with a wound surface contaminates both exhibits. It is usually done with photographs only, not with the cast touching the skin.

The ABFO conclusion scale gives analysts a framework for expressing the strength of a comparison. After the 2018 revision, the scale has three permitted conclusions: 'Excluded' (the suspect's dentition is inconsistent with the mark), 'Inconclusive' (insufficient information for a determination), and 'Not Excluded' (the dentition cannot be ruled out as the source). Positive identification language such as 'Probably made by' and 'Made to a reasonable degree of dental certainty' is no longer sanctioned under the 2018 standards.

A valid report names the overlay method used, specifies the features compared, records both the features that correspond and those that do not, and does not use language implying uniqueness. The research record, as codified in the 2009 NAS and 2016 PCAST reports, does not support 'only this person could have made this mark' conclusions.