Bite Marks in Abuse and Neglect Casework

Bite marks on living victims are among the most perishable evidence in abuse casework. Bruising evolves over hours, the oval or circular pattern that initially reads as a clear arch can spread and blur within 24 to 48 hours, and in children the smaller body surface means the injury may be partly hidden by clothing or in areas not easily seen during a standard clinical examination. The examiner who first observes the mark must document it completely, because no subsequent examination can recover what is lost.

In post-mortem cases the situation is reversed: the injury is preserved but the context is gone. A pathologist cutting into what looks like a contusion on a child's trunk may find the subcutaneous bruise pattern of upper and lower arches only when the skin is excised and held up to transillumination. The lesson in both living and post-mortem cases is the same: patterned circular or oval contusions on a child should prompt systematic bite mark evaluation, not assumption that the injury has a benign cause.

The intercanine width, measured between the cusp tips of the upper canines, averages between 25 and 30 mm in primary (deciduous) dentitions and between 33 and 42 mm in permanent adult dentitions. The gap between these two ranges is the diagnostic space that makes age estimation from a bite mark possible. A mark with an arch width greater than 30 mm in a child victim argues strongly against another child as the biter and points toward an adult perpetrator. In practice, the 3 cm threshold (30 mm) is widely cited in the forensic odontology literature as the working cutoff.

Applying the rule requires two things: a clearly defined arch on the mark and a reliable measurement. Photography with an ABFO No. 2 scale placed at the level of the skin surface is the standard first step. If the injury site is curved (such as on an arm or buttock), a rigid flat scale will introduce perspective distortion. Three-dimensional photogrammetry or an impression material poured over the wound site corrects for that distortion and is preferred when a comparison is anticipated.

Every step of the documentation protocol exists because courts will scrutinise the original photographs, not just an expert's description. A badly lit, scale-free, or angled photograph of a bite mark has derailed more than one prosecution. The protocol's logic is consistent: document the mark in a way that allows any qualified examiner to measure and compare it independently of when the photographs are reviewed.

1. Swab before anything else
   Use the double-swab technique (wet swab first, then dry swab) across the entire bite area before cleaning, before application of any topical agent, and before detailed palpation. Seal and refrigerate swabs. Saliva degrades within hours on warm skin.
2. Photograph with scale and without
   Place the ABFO No. 2 scale flat at the plane of the injury. Shoot perpendicular to the skin surface to eliminate parallax. Take one series with scale, one without. Use standard white light plus alternative light source (violet/blue, 415-450 nm) to enhance subdermal bruising that is invisible under white light.
3. Measure and describe in writing
   Record overall arch width, individual mark widths, depth of indentations, colour (erythema, ecchymosis, laceration), and anatomical location referenced to fixed landmarks. Note the time of examination relative to the reported time of the incident.
4. Consider impression if post-mortem
   In post-mortem cases, or where the injury is in firm tissue, a polyvinyl siloxane impression of the wound provides a three-dimensional record for comparison with a suspect's dental cast. In living victims this is rarely practical and photography is the primary record.
5. Serial photography in living victims
   Photograph again at 24 hours and 48 hours. Some tooth marks become more distinct as haemoglobin products migrate outward. Others fade. The full photographic series, not just one snapshot, forms the evidentiary record.

The shift in bite mark casework over the last three decades was driven by DNA, not by improvements in dental comparison. Before molecular profiling, a bite mark could at best exclude a large portion of the population by blood type from ABO testing of dried saliva. Today, the same saliva deposit can yield a full short tandem repeat (STR) profile that identifies a suspect with a match probability measured in billions or trillions to one.

The double-swab technique was developed by Sweet, Lorente, Lorente, Valenzuela, and Villanueva and published in 1997 in the Journal of Forensic Sciences, specifically addressing the problem of thin, dried saliva on skin. The wet swab rehydrates and dislodges dried epithelial cells and buccal cells from the biter; the dry swab immediately absorbs the resuspended material. Studies have shown that the wet-then-dry sequence recovers significantly more DNA than a single swab alone. Recovery yield depends on time elapsed (the sooner, the better), the amount of saliva deposited, and the ambient temperature.

When DNA produces a profile, it provides the strongest and most defensible element of the bite mark analysis. When it fails (degraded sample, too little material, mixed profile), the investigation falls back on pattern comparison. Understanding this hierarchy, DNA first, pattern comparison as corroborative, is central to how courts in the United States, United Kingdom, Australia, and elsewhere are now approaching bite mark evidence.

Child abuse investigations are multi-agency, and the forensic odontologist fits into a larger clinical picture. A bite mark finding is presented alongside skeletal survey results, ophthalmological examination for retinal haemorrhages, metabolic screening, social work assessment, and the medical history. No single finding in isolation proves abuse; each contributes to a cumulative picture. The odontologist's job is to characterise the mark accurately and completely, not to render a verdict on the family.

• Multiple bite marks at different healing stages suggest repeated episodes of abuse rather than a single incident.
• Bite marks in the same location as other patterned injuries (cord marks, cigarette burns) suggest a pattern of deliberate infliction.
• A bite mark whose arch width and individual tooth positions are inconsistent with the child's own dentition or the dentitions of other children in the household narrows the field of investigation.
• Self-inflicted bites are possible (especially on the forearm and hand in older children) and differ from inflicted bites by their location, reach, and the fact that the pattern matches the child's own dentition.

Bite mark evidence has a troubled history in court. At least 33 people in the United States alone were wrongfully convicted partly on bite mark testimony before being exonerated by DNA. The Innocence Project catalogued these cases and pushed hard for methodological reform. The National Academy of Sciences 2009 report on forensic science and the 2016 President's Council of Advisors on Science and Technology (PCAST) report both found that the scientific foundation for unique identification from bite marks was not adequately established.

The ABFO responded with revised guidelines in 2018, tightening the language examiners are permitted to use. The phrase 'to the exclusion of all others' is no longer acceptable in ABFO-trained testimony. Opinions should be expressed on a scale ranging from 'excluded', 'inconclusive', and 'consistent with', stopping well short of unique source identification. The same caution has been adopted in the UK and Australia, where the Forensic Science Regulator and equivalent bodies have called for corroboration requirements.

Bite mark evidence should be used to narrow the field of investigation, not to close it. DNA, fingerprints, and other individuating evidence carry the identification burden.