Variations in the shape of the chin in South African using cone bean computed tomography scans

Abstract

In order to support identification of crime victims in South Africa, new methods need to be sought. If victim identification involving fingerprints, DNA or dental records are not possible, facial approximation is often the only alternative. In order to gain necessary background data for the identification of unknown individuals, e.g. for facial approximations, facial features of modern South Africans need to be investigated and shape-influencing factors identified. New imaging technologies have opened the possibility of including living, dentate individuals, as specimens in skeletal collections are often edentulous. The aim of this dissertation was to assess chin shape variation and the factors influencing it, in black and white South Africans.

In the first part, the mental eminence was assessed by applying a morphoscopic sex estimation technique, to test its applicability to 105 dry mandibles from the Pretoria Bone Collection, and to the respective micro-focus X-ray computed tomography (micro-XCT) scans, obtained at the Nuclear Energy Corporation South Africa. Fleiss Kappa, Cohen’s Kappa and Wilcoxon Signed-Rank tests were applied. Score frequencies and observer performance were analysed. In the second part, the chin shape was assessed by quantifying its morphology, applying geometric morphometric methods to 291 retrospectively collected CBCT scans. The scans were obtained for medical reasons from dental patients, at the Oral and Dental Hospital, University of Pretoria. The possible influences of ancestry, sex, age and allometry on the chin shape were tested, using MAN(C)OVA, 50-50 MANOVA, permutation tests and discriminant function analysis (DFA) on the x-, y- and z-coordinates of the anatomical landmarks.

The morphoscopic method to estimate sex on the mental eminence, originally applied to bone, was found to be applicable to micro-XCT scans as well, and observer performance did not vary greatly between the two modalities. However, an observer’s personal affinity to assess 3D images, the level of experience and tendency to over- or underscore in one of the two modalities cannot be excluded and should be individually tested. The chins of black females and white males had the highest probabilities of correct sex estimation. Ancestry, age and allometry were significant chin shape influencing factors in the complete sample. In addition, ancestry influenced the chin shape significantly within the sex groups, allometry within the ancestral groups. Sexual dimorphism significantly influenced chin shape in the complete sample on the bony menton and in the ancestral groups. Most results from both parts of the study concurred, except the influence of age.

With the increasing availability of imaging techniques in forensic anthropology, researchers are motivated to look for new, and validate existing, methods in 3D. By assessing the applicability of a morphoscopic sex estimation technique to micro-XCT scans, and by investigating the chin shape variation using CBCT scans, the present study contributed to the quantifiable biological profiling methods involving 3D imaging techniques in South Africa.

This study could encourage further research on all five traits of the morphoscopic method in bone and 3D surfaces, and of the soft-tissue shape of the chin in the same populations.