Is skin a reliable medium for recording bite marks?

With respect to bite marks on skin, two factors further complicate the process of bite mark comparison. First, skin is a poor medium for registration of bite marks. Second, photography of bite marks introduces further distortions.

Bite marks on skin

Scientific research has strongly challenged the claim that human skin is a reliable medium for registration of bite marks. The properties of the skin such as its viscosity and elasticity lead to the distortion of bite marks. It is these properties which determine how an indentation is formed during a biting incident and how the indentation subsequently reduces and disappears when the skin rebounds. One such study titled ‘Biomechanical Factors in Human Dermal Bitemarks in a Cadaver Model’ by Mary A Bush et al (2009) examined the severe distortions which occurred on bite mark injuries on cadavers. The study noted that the elasticity of the skin caused it to deform easily with movement of the body and the underlying tension. It demonstrated that the deformations in the bite differed depending on whether the bite was perpendicular or parallel to the langer lines i.e. the tension lines in the skin. Further, the movement of the body also distorted a bite mark by pulling it in the direction of the movement. This assumes particular relevance in the context of a crime where the victim and accused would be in a state of motion. The study also noted that using cadavers excluded the impact of edema, haemorrhage and inflammation which is seen in living tissues.

Distortions during photography

In addition to the fact that the skin is a poor registration material for recording of injuries, using photographs of the injuries to compare it to the dentition introduces further distortions in the process. While the act of creating a two dimensional image of a three dimensional injury introduces inherent distortions in the process, human errors in conducting photography adds to the unreliability.

Some of the common errors in bite mark photography are as follows:

• Camera lens is too close to the bite mark injury making it appear larger than its actual size.
• The camera is not perpendicular to the bite mark injury causing a parallax error. This causes a distortion in the true viewing length.
• Absence of scale to allow sizing of the bite mark injury being photographed. Another error could be photographing injuries when the scale is covering parts of the injury Therefore, it is important to photograph the injuries with and without scale.
• Poor lighting which can create shadows or distort colour.
• Absence of orientation photographs which show the full body. Such images aid in visualising the location of injuries.