The human pelvis is extensively relied upon by forensic anthropologists for age and sex estimations of unidentified skeletal remains. However, because subadult individuals are an underrepresented demographic in skeletal collections there is a lack of ontogenetic knowledge of the sub-elements (ilium, ischium, and pubis) and the overall pelvic complex. Subsequently, there is a concomitant lack of understanding of the onset of sexually dimorphism and/or age-related size and shape changes. The purpose of this research is to provide a detailed analysis of modern human pelvic growth using in situ pelvic landmarks, which can yield size (interlandmark distances) and shape (geometric morphometric) variables to understand broad ontogenetic patterns of the pelvis and more specific patterns associated with age and sex estimation of the pelvis. It is expected that the size and shape of the pelvis stabilizes at a younger age than previously assumed and subsequently the pelvic phenotype has an increased utility in the subadult biological profile, specifically age and sex estimation.
The current sample was queried from the United States sample of the Subadult Virtual Anthropology Database (SVAD) and the full body CT scans were generated prior to autopsy at the University of New Mexico Health Sciences Center, Office of the Medical Investigator between the years of 2010 to 2017 and are available via the New Mexico Decedent Image Database (NMDID). The final sample size was 807 individuals (females=324; males=483) with ages between birth and 21 years. Both chronological age and developmentally derived life history stages (LHS) were used in the analyses, which facilitates an ontogenetic framework to be used when evaluating growth patterns. The developmental LHS included infancy, childhood, juvenile, adolescent, and adult and were determined by dental and skeletal markers. Thirty-four pelvic landmarks on the left and right segmented surface models lead to 68 landmarks per individual. Unilateral and bilateral interlandmark distances (ILDs) were used in sex-specific and pooled-sex nonlinear regression models, including Loess regression and Multivariate Adaptive Regression Splines (MARS). Geometric morphometric analyses were performed to understand shape variation through ontogeny. The unilateral ILDs were tested for the utility of pelvic metrics in skeletal sex estimation using two distinct methodologies—linear discriminant function analysis (DFA) and random forest modeling (RF). Lastly, the utility of pelvic metrics for skeletal age estimation were assessed using the Mixed Cumulative Probit (MCP).
Both unilateral and bilateral measurements demonstrated sexually dimorphic growth in the dimensions associated with the true pelvis. The lack of dimorphic size relating to the false pelvis region and anterior-posterior measurements suggest these should be the focus of age estimation methods, particularly prior to fusion of the sub-elements or juvenile LHS. The vertical acetabular diameter is the earliest variable to stabilize in size and to show sexual dimorphism (~8 years of age). This suggests the biomechanical requirements for locomotion through stabilization of the hip joint is prioritized during early pelvic growth, followed by changes to the pelvic midplane and outlet relating to needs for sexual reproduction—particularly transverse expansion of the true pelvis (birth canal).
Although DFA models produced higher accuracies for skeletal sex estimation than RF models, the results are potentially invalid due to the violation of assumptions relating to normality and homogeneity of variance. The results of 85-87% accuracy achieved by the RF models are valid and still meet the 85% criteria proposed in forensic anthropology. Pelvic metrics can yield accurate and reliable skeletal sex estimations for individuals within the juvenile, adolescent, or adult LHS such that individuals potentially as young as 5 to 7 years of age can be included for accurate sex estimation.
Measurements of length across the unfused ilium, pubis, and ischium, and iliac breadth produce the most accurate and precise age estimations using the MCP. Age estimation using univariate pelvic metrics should be assessed prior to acetabular fusion, which is prior to size stabilization and the onset of sexual dimorphism both of which impact the precision of age estimates. If forensic practitioners are presented with an unfused pelvis, they can proceed with an age estimation using pelvic metrics and applying univariate MCP models. If there is active or complete fusion of the acetabulum, skeletal sex estimation using random forest model can be accurately performed. (Publisher provided abstract.)