Case report: Forensic 3D-match of hair brush and scalp abrasions revealing dynamic brush deformation

Forensic injury interpretation can be greatly enhanced by the use of 3D-surface documentation, also in conjunction with full body scans. For static juxtapositioning, a rigid shape transfer from weapon or injury causing object to the victim's skin or bone will be assumed for patterned injuries, upon which an object's shape directly explains the shape of an injury. Forensic pathology expertise may be essential in reading the resulting 3D shape match attempts, particularly when in due process of injury causation, deformation of either the object/weapon or victim's tissues occurred, causing a dynamic rather than rigid shape transfer.

In this case, a 3D match using manual forensic juxtapositioning of punctate and elongated striate forehead skin abrasions and a hairbrush yielded a partial but not perfect match. Qualitative medicolegal appreciation of the partial match result clearly indicated centrifugal brush bristle deformation after impact, causing a centrifugal abrasion pattern, thus having dynamically extended an initially stationary punctate abrasion and bristle end shape match. This was corroborated by both the elasticity of the bristles and the brush frame fracture. Presence and location of the fracture indicated a relatively high force between the brush handle on one hand and the bristles on the other.

This case shows how the application of targeted forensic pathology interpretation can elucidate what technically only is a partial 3D match and add re-constructive meaning. Generally, more research is needed in establishing just how technical sciences and specific medicolegal expertise are best integrated.