A multifactorial approach to the estimation of sex using the facet joints of the spine

Abstract

This research aims to demonstrate how “form related to function” can be used to explain differences in the degree of zygapophyseal (facet) joint sexual dimorphism seen in populations with differing lifestyles. Whilst sex can be estimated with a reasonable degree of accuracy from the facet joints of the spine in populations with a high degree of sexual dimorphism, it can be problematic in samples that do not. Bone adapts to reflect the physiological strain placed upon it during life. Analysis of the relationship between bony adaptation to occupational stress and facet morphology identifies extrinsic factors that correlate to changes in facet size and to a lesser degree sagittal angle. Extrinsic factors are external biocultural influences on bone morphology (e.g. nutrition and physical activity). The number of discriminant functions that can be derived from a population with a low degree of sexual dimorphism is increased when these factors are included, increasing the potential to estimate sex. The strength of correlation and prevalence of extrinsic factors can also be used to measure the degree of physical activity undertaken by individuals and is indicative of gendered division of labour in the population under study.
Analysis of facet size and sagittal angle and the relationship and prevalence of extrinsic factors related to physical activity from skeletal material were examined from three contrasting samples. Two were from 18th century London with differing socio-economic status and the third a composite sample from three 5th -7th century Anglo-Saxon cemeteries located in southeast England. A comparative study of facet size and angle identified inter-sample differences in the degree of sexual dimorphism. Further comparison of differences in lifestyle as evidenced by activity patterns was carried out to assess the impact of extrinsic factors on facet remodelling. In particular, this thesis focuses on evidence of the gendered division of labour as manifest by femoral robusticity, humeral directional asymmetry, vertebral osteophytosis, and osteoarthritis with analysis of the individual diagnostic criteria of eburnation, pitting and osteophytes. A distinct difference in prevalence of these factors was observed in the Anglo-Saxon sample and most obviously in females when compared with the 18th century samples, indicating that there was a difference in intensity of activity undertaken by this group in comparison with the other samples in this study. This suggests that the lack of facet joint sexual dimorphism observed in Anglo-Saxons is attributable to the degree of physical activity undertaken by these females and the subsequent remodelling of the facets as a functional adaptation to the mechanical loading they were subjected to.
This research demonstrates that for some populations, sex can be estimated with reasonable accuracy from vertebral facet dimensions but for less dimorphic samples, inclusion of extrinsic factors related to physical activity when deriving discriminant functions increases the opportunity to estimate sex. Furthermore, analysis of inter-sample prevalence rates for extrinsic factors provides supporting evidence of different levels of physical activity between the samples.

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Identifiers

ORCID for Joanna Sofaer: orcid.org/0000-0001-6328-8636

ORCID for Sonia Zakrzewski: orcid.org/0000-0003-1796-065X

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