An investigation into the accuracy and reliability of skull-photo superimposition in a South African sample

Abstract

One of the aims of forensic sciences is to determine the identities of victims of crime. In some cases the investigators may have ideas as to the identities of the victims and in these situations, ante mortem photographs of the victims could be used and identities established through skull-photo superimposition. The aim of this study was to evaluate the accuracy of a newly developed digital photographic superimposition technique on a South African sample of cadaver photographs and skulls, from the Pretoria Bone Collection. Forty facial photographs were selected and for each photograph, 10 skulls (including the skull corresponding to the photograph) were used for superimposition. The investigator did not know which of the 10 skulls corresponded to the photograph in question. The skulls were scanned 3-dimensionally, using a Cyberware™ Model 3030 Colour-3D Scanhead scanner. Once scanned, the raw data for the skulls were ‘cleaned’ using Cysurf™ programme. The photographs were also scanned for superimposition in the 3D Studio Max programme. Superimposition in 3D Studio Max involves a morphological superimposition, whereby a skull is superimposed over the photograph and assessed for a morphological match. Superimposition using selected anatomical landmarks was also performed to assess the match. A total of 400 skull-photo superimpositions were carried out using the morphological assessment and another 400 using the anatomical landmarks. In 85% of cases the correct skull was included in the possible matches for a particular photograph using morphological assessment. However, in all of these cases, between zero and three other skulls out of 10 possibilities could also match a specific photograph. In the landmark based assessment, the correct skull was included in 80% of cases. Once again, however, between one and seven other skulls out of 10 possibilities also matched the photograph. When using the morphological and landmark assessments combined, 97.5% of correct skulls were included in the list of possibilities, but between one and seven false positives per case were found. This study indicates that skull-photo superimposition has limited use in the identification of human skeletal remains, but may be useful as an initial screening tool. Corroborative techniques should also be used in the identification process.