Creating guidelines for the approximation of the eye and periorbital regions of South Africans using cone beam computed tomography scans

Abstract

Introduction: South Africa is experiencing a rise in unidentified remains yearly, and due to poor access to healthcare and illegal immigration, standard identification methods often fail. The foundational data used to reconstruct South African faces are based on North American cadavers which result in inaccurate reconstructions. This study aimed to create guidelines for approximating the eyeball and periorbital structures for South African groups using cone-beam computed tomography (CBCT) scans. Methods: Retrospective CBCT scans of 206 South African adults (45 black females, 52 black males, 57 white females, and 52 white males) were included in the study, of which 187 had open eyes (32 black females, 51 black males, 52 white females, and 52 white males). Three-dimensional landmarks were placed manually on the hard- and soft-tissue renderings of the face and skull as well as the outline of the eyeball in 2D using the MeVisLab © v.3.0.2 software. The 3D coordinates of these landmarks were used to calculate the linear dimensions of the orbit, eyeball, palpebral fissure, and the position of the eyeball in relation to the orbital rim. Predictive equations and predictions based on proportionality for the position of the eyeball in relation to the orbital rim, eyeball and palpebral fissure dimensions were derived from these linear dimensions. The effect of sex and population affinity on the shape of the orbit and palpebral fissure was investigated using geometric morphometric methods (GMM). The second part of the study entailed the creation and validation of prediction models based on the relationship between shape of the bony orbit and palpebral fissures using an automatic landmarking method. Results: Sex and population affinity significantly affected the dimensions and shape of the orbit, eyeball and palpebral fissures. Black females had significantly smaller eyeballs which protruded more from the superior orbital margin than the other sex-population groups. Eyeball position, eyeball dimensions and palpebral fissure dimensions could be predicted with greater accuracy based on proportionality compared to linear regression formulae. Variations in the palpebral fissure dimensions in black South Africans were driven by shape, while it was driven by size in white South Africans. Population affinity had a greater effect on the shape of the orbital region and palpebral fissures than on the linear dimensions. The shape analysis based on the automatically placed landmarks concurred with this finding which resulted in the accurate prediction of the open eye from the underlying bony tissue with a small measurement error, especially when population affinity was added as a factor. Conclusion: When reconstructing South African faces, it should be noted that black South African females had significantly smaller eyeballs compared to all other groups, while the eyeball protruded more in the vertical plane in black South Africans than white South Africans. Black compared to white South Africans had a greater inner canthal distance and less upward slanting palpebral fissures. Instead of depending on normative dimensions, the palpebral fissure and eyeball position can be predicted based on the morphology of each individual without considering sex or population affinity. The open eyelids could be accurately predicted from the underlying bony tissue for the use in automatic facial approximations. These prediction results should be combined with prediction models of other facial features to create accurate, objective and less time-consuming automated facial approximations of South Africans.