An exploration of sacral morphology using geometric morphometrics and three-dimensionally derived interlandmark distances

Introduction Accurate sex estimation is an important component of a biological profile in forensic anthropology. The pelvis is widely accepted as the most dimorphic osseous structure, and thus, this dimorphism is also reflected by the sacrum. Aim This study aimed to explore sacral morphology and to derive a practically applicable discriminant function formula for sex estimation. Materials A total of 20 three-dimensional landmarks were digitised on a sample of 200 sacra from a sample of South Africans of African descent (Black South Africans) with ages ranging between 20 and 90 years, equally distributed for sex. Methods Geometric morphometric methods were used to analyse sacral morphology and sexual dimorphism as it captures size-independent shape variation and three-dimensional morphology. Results Size-independent shape analysis revealed four sacral structures and metrics that account for most of its shape variation. When these were compared between the sexes, we found that sacral curvature pattern, rather than depth, differed between sexes and that males have greater anterior sacral heights. Females have larger alae relative to the body of S1. In addition, the anterior posterior breadth of the sacral canal is larger in males, as is the relative size and projection of the superior articular processes. Discriminant analyses of these data produced average accuracies of only 72.5%, but this improved to 84.5% when using novel interlandmark distances derived from the raw coordinate data. Conclusion Our results demonstrate that landmark-based techniques allow for a more nuanced understanding of structural variation. In addition, accuracies were achieved that surpass traditional metrics using an equal number of variables. These results contribute to our understanding of sacral dimorphism and will assist in forensic casework.